BSc 2nd Year Structure of Microorganisms in Microbiology Notes Study Material
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BSc Structure of Microorganisms in Microbiology Notes Study Material
The unit of measurement most often used for microorganisms is the micrometer-a unit of length equal to one millionth (micro-) of a meter i.e. 10-6 m. The word is symbolised as um-Greek letter u together with the abbreviation for meter, m. It is also equal to a thousandth of a millimeter i.e. 10-3 mm. Figure shows some common metric equivalents used in microbiology. It may be seen that the cells of the microbes listed here, range in size from the large, almost-visible protozoa, measuring 100 um to the incredibly small viruses at about 0.01 um, ten thousand times smaller. Mold filaments are fairly long and visible to the naked eye. However, their individual cells measure approximately 40 um by 10 um. Unicellular fungi, the yeasts, commonly have cells of 25 um in diameter. The largest bacteria are about 20 um in length while the smaller members of this group, the rickettsiae may be about 0.5 um long, and the chlamydiae, about 0.20 um.
The viruses are too small to be seen by conventional light microscope techniques. Their measurements vary from the large pox viruses at 0.25 um to the polio viruses at 0.02 um. Virologists use the term nanometer, a unit of length equal to one billionth (nano-) of a meter. A nanometer (nm) is a thousandth of a micrometer; therefore 0.02 um is equivalent to 20 nm. Nanometer is also the unit used in measuring the wavelength of radiant energy such as visible or ultraviolet light.
BSc Structure of Microorganisms in Microbiology Notes Study Material
The introduction of light microscope could make it possible to determine the overall shape of cells and their arrangement. Moreover, some internal structures could be seen by using phase-contrast or interference microscopy or by staining methods with the bright-field or fluorescence microscopy. Most of the stains in current use, such as basic dyes are relatively non-specific and thus gave little idea of the chemical nature or function of a cellular component. Some are more specific: fat-soluble dyes such as Sudan Black stains lipids, iodine colours staro granules and acridine orange fluoresces with nucleic acids. However, in small microbes like bacteria, the cell size (about 0.5 um diam.) is only slightly greater than the theoretical limit of resolution of the light microscope (about 0.2 um). The study of ultra-structure of cells, therefore, depended upon the transmission and stereoscan electron microscopy. Unfortunately one of the main problems in electron microscopy is in the interpretation of the chemical nature and function of the structures seen. There are only few specific stains available. Of course there may be potential in the current use of specific antibodies labelled with electron dense components like ferritin or colloidal gold, there are technical difficulties in their use.
Consequently, the primary method used for relating a given structure to function is to break open the cell and to fractionate the components which are then subjected to biochemical analysis. There may be some problems in rupturing a cell without denaturing the more sensitive components, particularly in bacteria with their tough walls and small size. However, during recent years methods of breakdown could be developed and most of the major cell structures have been isolated in a reasonably pure state in which they perform their functions normally. In bacteria, for instance, there are following two methods of cell breakdown.
BSc Structure of Microorganisms in Microbiology Notes Study Material
Mechanical disruption. Violent shaking of a bacterial suspension with glass beads causes rupture of the cell walls and liberation of the cytoplasmic contents. The walls can be concentrated by centrifugation and treated with appropriate solvents and enzymes to remove contaminating components. The wall isolated in this way retains the shape of original cell, and is also termed wall ghost. In some bacteria the wall can also be solubilised completely by the enzyme, Iysozyme, a component of many animal secretions and fluids.
Lysozyme treatment. Exposure of a sensitive bacterial species to lysozyme normally causes complete cell lysis. However, if the treatment is given in the presence of an osmotic stabiliser such as an isotonic sucrose solution, the cell assumes a spherical shape as the wall is solubilised.
The product after removal of wall is called a protoplast that is bounded by cytoplasmic membrane. The following points could emerge from such experiments :
(1) The wall determines cell shape since the protoplast is spherical irrespective of the original shape.
(2) The wall is responsible for the mechanical strength of the bacterial cell since a protoplast is very prone to mechanical or osmotic lysis while the cell is not.
(3) The cytoplasmic membrane and not the cell wall is responsible for the selective permeability of the cell surface; this remains unchanged in the protoplast. Further studies have shown that the protoplast retains most of the enzyme systems of the cell and is capable of regeneration and growth whereas the wall is not.
If a protoplast is subjected to controlled osmotic lysis by a gradual reduction in the external osmotic pressure, the cytoplasmic membrane ruptures liberating the cytoplasm and its components. The membranes and the components of the cytoplasm can then be isolated by differential centrifugation. These remain unaffected by mild lytic treatments.
BSc Structure of Microorganisms in Microbiology Notes Study Material
With the help of such methods, the structure and function of the components of apparently typical cells have been studied. It has become clear that there are two basic types of cell – prokaryotic and eukaryotic. The former are restricted to microorganisms (bacteria, including blue-green or cyanobacteria), whereas the latter occur in microorganisms (fungi, protozoa and algae) and in animals and plants. There are also viruses which have a simple acellular structure. The prokaryotes and eukaryotes share the common basic unit of life, the cell. All cells have the following essential components:
(1) DNA, as genetic information for replication.
(2) RNA, for protein synthesis.
(3) Enzymes, for catalysis.
(4) A membrane, to maintain the internal environment.
(5) Cytoplasm, as a solvent for the utilisation of food.
Thus, all cells share a common chemical composition, the common chemical activities of metabolism and a common physical structure of organisation, and together these give them potential of growth and self-replication.
Cells : The Basic Organisational Units of Living Systems
We consider briefly the properties of the structural and functional subunit of all living organisms – the cell. A cell is a self-contained unit separated from its surrounding by a membrane that serves as a limiting boundary. The membrane regulates the flow of materials into and out of the cell allowing the critical maintenance of the cell’s internal contents in a more highly organised state than the cell’s external environment.
Microorganisms can be classified on the basis of their cellular organisation. It may be seen that some microbes as viruses, viroids and prions lack cell walls (acellular). They will be discussed in detail with separate topic. Within their host cells, they are capable of acting as live systems beacause the host cell provides the essential boundary membrane. In contrast to these, cellular microbes, all other living organisms are composed of either prokaryotic or eukaryotic cells.
We would describe below in somewhat detail the various structures associated with typical prokaryotic and eukaryotic cells. These include the following:
(1) Structures involved with movement of cells – Flagella, Cilia.
(2) Structures involved in attachment of cells – Glycocalyx, Pili, Fimbria, etc.
(3) External structures that protect the cells – Cell wall, Capsule, Slime layers.
(4) Structure involved in regulatory movement of materials into and out of cells – Cytoplasmic (cell) membrane.
(5) Sites of energy transformation where ATP is generated – Cytoplasmic membrane, Internal photosynthetic membranes, Chloroplast, Chromatophores, Mitochondria.
(6) Structures involved in information flow in cells – Ribosomes.
(7) Cellular storage of genetic information – Bacterial chromosome, Plasmids, Nucleus and Chromosomes.
(8) Structures involved in coordinated material movement and storage – Exoenzymes, Endoplasmic reticulum, Golgi apparatus, Lysosomes, Microbodies, Vacuoles, Cytoskeleton, Inclusion bodies.
(9) Structures involved in survival during adverse environmental conditions – Endospores in bacteria, Various types of spores in eukaryotes.
Prokaryotic Microorganisms
We would prefer to refer all these organisms as bacteria. Although, prokaryotes are often subdivided, such a division appears to be artificial in the light of present state of knowledge about these microorganisms. Let us consider the range of prokaryotic microbes in terms of their structure.
Cell shapes. Various cell shapes occur in prokaryotes: spheres or coccoid bacteria, rods, helically grown cells (spirillum-type organisms), comma-shaped (vibrios), box-and plate-shaped, filaments, bacteria with stalks and prosthecae, and branched systems. There are three basic cell shapes: the sphere (coccus); the rod (bacillus); a cylinder with rounded ends; and the curved rod, either as one slight curve (vibrio) or as a spiral or helix.
Multicellular structures. Bacteria basically are unicellular. However, sometimes cell division and cross-wall formation are not followed by the separation of the daughter cells. In this way an undifferentiated multicellular structure is produced. The shape of this multicelled structure depends upon the planes of cell division.
The rods always divide in one plane, and if the cells remain attached a multicellular filament is formed.
In spherical cells, however, a variety of shapes can occur depending upon the plane of cell division. These are as follows:
(i) cells divide in one plane and remain predominantly attached in pairs, e.g. Diplococcus
(ii) cells divide in one plane and remain attached to form chains, e.g. Streptococcus.
(iii) cells divide in two planes to give plates, e.g. Pediococcus.
(iv) cells divide in three planes regularly to produce a cube, e.g. Sarcina
(v) cells divide in three planes irregularly producing bunches of cocci, e.g. Staphylococcus.
Spiral bacteria are predominantly unattached but the individual cells different species show striking differences in length and in tightness of spiral.
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In all these instances, the multicellular form is made up of separate individual cells. However, in actinomycetes multi-nucleate cells without cross walls produce branched mycelia of indefinite length which appear superficially similar to those of filamentous fungi. Various types of multicellular forms are shown in Figure.
Stalk. A few bacteria have a stalk by which they attach to a solid substratum using a holdfast at the tip. Upon cell division a flagellate cell is produced which swims around, eventually settling on a new surface where it forms a stalk in place of flagellum. This life cycle is very simple having an attached and a swarming phase. Example, Hyphomicrobium.
Buds. Most bacteria divide by symmetric division into two equal halves. However, a few as Rhodomicrobium divide by budding as in yeasts. The growth is polarised and cell division is asymmetric.
Motility structures. Motility may be by flagella, by gliding or by axial filaments. We shall refer to these structures later.
Spores and cysts. We shall describe them in the following pages.
Fine Structure of Prokaryotic Cell
The prokaryotic cells vary in size from a Mycoplasma (a sphere of about 0.12 um diameter) to a blue-green bacterium like Oscillatoria (a rod of dimensions as much as 40×5 um), but the majority have a diameter in the region of 1.0 um. The components of a typical prokaryotic cell are shown in Figure. These are as follows:
Cytoplasm or cytosol. The cytoplasm is sequestered within the cell membrane. Does cytosol has structure or is it just like a solution of enzymes and metabolites in a test tube? Whether cytosol in vivo is just a disordered solution (a “bag of enzymes”) or indeed highly organised (i.e. structure) is a matter of some debate. Though there is evidence that it has structure but the concept is still poorly understood. In electron microscope it is not organised. It is a concentrated contains a variety of enzymes, coenzymes and metabolites, perhaps in the form of an aqueous fluid or semifluid ground substance or matrix. The matrix is a complex mixture containing in solution a variety of inorganic ions, amino acids, some proteins, lipocomplexes, peptides, nitrogenous bases, sugars, vitamins, enzymes, coenzymes etc. Its main function is in intermediary metabolism and in providing an equable chemical environment for cellular activities.
BSc Structure of Microorganisms in Microbiology Notes Study Material
Flagella. Most motile bacteria possess long (upto 20um), thin (20 nm diameter) helical appendages called flagella. Unlike eukaryotic flagellum, the prokaryotic flagellum has no definite membrane. The flagellum is not visible using the light microscope without increasing its effective diameter by coating it with a suitable precipitate. In the electron microscope, negative-staining with phosphotungstic acid shows that flagellum is made up of identical (three or more in number) sub-units arranged (interwined) helically along the axis of the flagellum to give a hollow tube. These sub-units can be separated from each other by acidification and consist of protein molecules called flagellin. Each flagellin subunit is about 4.5 nm thick. The flagellum is attached at one end through the cell wall to the cell membrane by a special terminal hook, which in turn is attached to the basal body. The rings of basal body are attached to the cell wall and cell membrane.
Bacterial flagella are long projections extending outward from the cytoplasmic membrane that propel bacteria from place to place. In some, as Pseudomonas the flagella are polar flagella as they originate from the end or pole of the cell. In others, as Proteus, they surround the cell, and such flagella are called peritrichous flagella. A bacterial flagellum consists of a single filament composed of protein (flagellin) subunits. The characteristic hook structure of the flagellum allows it to spin like a propeller and thereby to propel the bacterial cell. Effectively, the structure allows the flagellum to spin like the shaft of an electric motor. Rotation of the flagellum requires energy which is supplied by a hydrogen ion gradient across the cytoplasmic membrane.
BSc Structure of Microorganisms in Microbiology Notes Study Material
The arrangement and number of flagella on a cell can be a useful criterion for identification and classification. The following four types of flagellation patterns are common in bacterial cells, as shown in Figure.
(a) Monotrichous. There is a single flagellum at one pole of cell.
(b) Lophotrichous. There are several or numerous flagella at one pole.
(c) Amphitrichous. The cell bears at least one flagellum at each pole.
(d) Peritrichous. There are flagella all over the surface of cell.
The flagellum rotates, being driven by a rotary motor in its basal body. The hook acts as a universal joint, and the flagellum acts similarly to a ship’s propeller, projecting backwards and driving the cell forward by exerting a viscous force against the aqueous medium.
The function of flagella is in locomotion and all naturally occurring flagellate bacteria are motile. However, some other less common types of motility are also shown by some prokaryotes. They have helical cells with axial filaments like flagella.
Specialised movement in bacteria
Chemotaxis
This is the mechanism for swimming toward or away from chemical stimuli, a behaviour known as chemotaxis. The chemosensors in the cell envelop, called binding proteins detect certain chemicals and signal the flagella to respond. Bacteria have a memory system that allows them to compare the concentrations of chemicals as they swim along, so that they effectively detect chemical concentrations over distances many times the length of a cell.
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To understand how chemotaxis works, we need to recognise that when bacteria move, they periodically change direction rather than reaching their destination by swimming in a single straight line. The straight line movements are known as runs, and the turns – which occur when the bacteria stop – are called tumbles or twiddles. At least in those with peritrichous flagella, the counterclockwise rotation of the flagellum results in a run and the clockwise rotation in a twiddle. The direction of flagella rotation, and hence the length of a run – that is, the amount of time before the organism stops and tumbles – is determined by the interactions of chemosensors in cytoplasmic membrane with attractants or repellents (i.e. the chemicals). An increasing concentration of attractant, for example, interacts with the chemosensors to decrease the frequency of tumbling, whereas a decreasing concentration of attractant causes increasing tumbling, and hence shorter runs. The same is true for the interactions with repellents. The net effect of this process, called a random walk, is a biased movement toward an attractant or away from a repellent based upon the relative proportion of running and tumbling.
Magnetotaxis
Some bacterial cells contain inclusion of iron granules known as megnetosomes, that permit them to orient their movement in response to magnetic fields, a phenomenon known as magnetotaxis. Bacteria can use these granules to navigate along the earth’s magnetic field. Some bacteria move predominantly north and others to south. Magnetotaxis allows some anaerobic bacteria to orient themselves so that they point downward into the sediment.
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Phototaxis
Some bacteria are able to detect and respond to differences in light intensity, a phenomenon called phototaxis. In some bacteria, this is similar to chemotaxis. Other bacteria form membrane – bound gas vacuoles that enable them to respond to light. Boundary layers of these vacuoles are not true membranes but are composed exclusively of proteins, which are hydrophilic as well as hydrophobic. In aquatic bacteria gas vacuoles provide a mechanism for adjusting the buoyancy of the cell and thus the height of the bacterium in the water column. Many aquatic cyanobacteria use their gas vacuoles to move up and down in the water column, depending upon light intensity levels, to achieve optimal conditions for photosynthesis.
Pili, fimbriae and spinae. The term pili was introduced by Brinton (1959) and fimbriae by Duguid et al (1955). Both are non-flagellar appendages of cell. They are extremely fine, non-flagellar appendages which look superficially similar to flagella. Pilis have been observed mostly in Gram-negative rods. They measure 3-25 nm in diam, and 0.5-20 um in length. Pilis are also made up of individual protein sub-units of pilin, arranged helically to form filament. However, they differ from flagella in the following respects:
(i) the filament is usually straight and is shorter than a flagellum.
(ii) the diameter is smaller (about 10 nm)
(iii) the function is not in motility.
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In some bacteria, there are longer, the f-pili on male cells that act as a bridge between conjugating cells. The smaller pili perhaps serve for attachment of cells to their substrates, e.g. pathogens to their hosts, for example Neisseria gonorrhea to cells of the human urinary tract. Both flagella and pili are antigenic and can be specific sites of attachment for bacteriophages. Pili are plasmid-coded and the plasmid is said to code for conjugation and resistance to antibiotics, colicin or hemolysin production. Fimbriae are involved in adhesion, between themselves as well as between them and the eukaryotes.
In some Gram-positive bacteria spinae have been reported. These are tubular, pericellular non-prosthecate rigid appendages made up of single protein moiety – spinin. They are said to help adjust cells to some environmental conditions as salinity, pH, temperature etc.
Many bacterial cells are surrounded by a specialised structure called glycocalyx that plays a role in attachment process. This is a mass of tangled fibres of polysaccharides or branching sugar molecules surrounding an individual cell or colony of cell. It may act to bind cells together, forming multicellular aggregates. Additionally, in some bacteria glycocalyx are also involved in attachment to solid surfaces. Some pathogenic bacteria adhere to animal tissues they invade via a glycocalyx. In aquatic habitats, bacteria seem to be held to rocks through the slime layers they secrete. An extensive polysaccharide slime, dental plaque enables bacteria to adhere to the tooth.
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Capsule. Some prokaryotes have a gel layer called the capsule surrounding the cell wall. It can be seen in the light microscope by negative staining with a dye as Indian ink. In electron microscope it normally looks as an amorphous shrunken layer. They are synthesised by cell membrane. The gel is usually formed of a polysaccharide (1-2% dry wt.) in water and there is a wide variety of different monosaccharide components joined in many different ways. Occasionally, there may be polypeptides also, as the peculiar polymer of the unusual D-glutamic acid found in anthrax-causing bacteria.
The capsule serves mainly as a protective layer against attack by phagocytes and by viruses; it may also help to prevent too rapid and lethal a loss or gain of water in the recurrent dehydration and hydration that occurs in many habitats. Finally, the capsule usually has an ion-exchange capacity which may aid in the concentration and uptake of essential cations.
Cell wall. The cell wall is the dense layer surrounding the cell membrane. The main function is to provide a mechanically strong bounding layer. Some, as Mycoplasma do not have a wall and thus exist only in restricted habitats.
The cell wall is not semi-peremeable membrane but it can act as a molecular sieve preventing large molecules passing through. In Gram-negative cells, some enzymes and metabolites are trapped between the cell membrane and the outer membrane of cell wall to form the periplasm. The wall components of the cell can be strongly antigenic.
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All peripheral layers including the cytoplasmic membrane, which enclose the cytosol (cytoplasm) are designated as cell envelope. The cell wall is structurally and chemically a distinct part of cell. The rigidity and strength of cell walls are mainly due to strong fibres composed of heteropolymers generally called peptidoglycans or mucopeptides, but also referred to as glycopeptide, muropeptide, glycosamino-peptide, mucocomplex, murein etc. These fibers form a relatively coarse (at molecular level), three-dimensional, tough meshwork rather than a solid structure. These fibres offer no resistance to inward movement of water, food as minerals, glucose, aminoacids. All wastes of cell can pass outward.
The peptidoglycans consist of alternating units of N-acetyl-glucosamine and N-acetyimuramic acid with B-1, 4-linkages. This backbone structure appears to be the same in all prokaryons studied. Linked to the muramic acid is a short peptide which varies in composition but always contains a minimum of three aminoacids, viz. alanine, glutamic acid and either diaminopimelic acid (DAP) or lysine. Glutamic acid and one of the alanines of peptidoglycans are in D-isomeric form rather than L-configuration which is characteristic of aminoacids found in proteins. Muramic acid, DAP and several D-aminoacids are found only in association with prokaryons. In this way one of the most important aspects of the chemical structure is the presence of unique monomeres in both the polysaccharide component (N-acetyl-muramic acid) and the polypeptide component (D-aminoacids and sometimes diaminopimelic acid).
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Structural rigidity of the peptidoglycans is achieved by cross-linking these polymers. The type and extent of cross-linkage varies among different species. In some bacteria, a short peptide (e.g. a pentaglycine) is used to link the tetrapeptide side chains extending from the muramic acid units, while in others, the terminal D-alanine of one tetrapeptide may be covalently linked to the DAP of an adjacent tetrapeptide. The peptidoglycans of Gram-positive bacteria are more extensively cross-linked than those of Gram-negative.
The peptidoglycans alone do not make the cell wall itself. In fact cell walls are structurally and chemically very complex. Electron micrographs of thin sections of typical Gram-negative bacteria often show a multilayered (as many as five) cell wall. The different layers as seen under electron microscope are the periplasmic space, the peptidoglycan layer, a more or less structureless zone (the “intermediate zone”), the outer membrane and lipopolysaccharides (Acker, 1977). The specific features of Gram-negative bacteria cell walls are the presence of (i) periplasmic space or also called periplasmic gel (Hobot et al, 1984), about 3-4 nm wide (ii) an intermediate layer, located outside the peptidoglycan and defined as a gap between peptidoglycan and outer membrane. According to Hobot et al (1984), intermediate layer does not exist and perhaps periplasmic gel might, also fill this space. (iii) fair amounts of lipoproteins and (iv) the outer membrane, whose three major components are proteins, phospholipids and lipopolysaccharides lines. In electron microscope this layer appears as two dense lines separated by a transparent zone i.e. exhibiting an aspect identical to a “unit membrane” or typical “bilayer”.
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Biochemical constituents of cell wall of E. coli, a typical Gram-negative bacterium are shown also in Figure, a schematic model of the cell wall.
Generally the cell walls of Gram-negative bacteria (Escherichia, Salmonella) are thin, more complex and multi-layered; the lipids including phospholipids and lipopolysaccharides make up to 80-85% of the total cell wall and remaining are proteins and peptidoglycans, the latter only single-layered. The cell walls of Gram-positive (Bacillus, Staphylococcus) bacteria tend to be thicker, amorphous and single-layered with 20-80% of the total cell wall composed of several layers of peptidoglycans. The remainder is made up of proteins, polysaccharides and teichoic acids (polymers made up of either ribitol or glycerol phosphates and aminoacids, glucose and N-acetyl glucosamine). Thus in Gram-positive cells, such as Bacillus and Staphylococcus species, the wall is about 50% peptidoglycans, and 50% teichoic acids or lipoteichoic acids (polymerised polyolphosphates), teichuronic acids, few proteins, lipids and neutral polysaccharides. The Gram-positive cell walls appear for the most part to be more homogeneous because distinct layers are not readily apparent and the peptidoglycan is distributed throughout the cell wall more uniformly in a three-dimensional network. Detailed cell wall structure of E. coli, a Gram-negative rod is shown in Figure. The structural and chemical characteristics of Gram-negative and Gram-positive bacteria are presented in Table and Figure.
Cell walls of archaebacteria
Cell walls of archaebacteria do not contain peptidoglycan (murein); rather their wall structures show great biochemical diversity. Some archaebacterial cell walls have pseudomurein which resembles peptido-glycan of eubacteria but contain N-acetyl-talosaminuronic acid instead of N-acetyl-muramic acid and lack the D-amino acids found in eubacterial cell walls. Other archaebacteria have walls composed of protein subunits; still others have walls with different biochemical composition. Although, variable in chemical structure, the walls are able to protect the cell membrane even in hot, acidic and saline environments where many archaebacteria live.
Regular surface layers (S-layers). These are present external to cell wall in all Gram-positive and Gram-negative bacteria even in those without peptidoglycans in their cell wall. Three main kinds of S-layers have been demonstrated in different cell envelopes of bacteria. S-layers are mainly composed of single homogeneous polypetides with carbohydrates occasionally as minor component. These layers have a predominantly acidic aminoacid composition. These layers may act as a barrier or molecular sieve controlling the movement of external and internal factors such as toxic macromolecules. These may also protect the peptidoglycan from action of lytic enzymes as lysozyme.
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Cell membrane. This is the boundary layer of the protoplast and is about 6-8 nm thick. In thin sections it appears a triple-layered structure consisting of two electron-dense (each about 2.5 nm thick) layers surrounding an electron-transparent one, a type of structure typical of most selectively permeable membranes in living organisms and called a unit membrane. The main chemical components are lipid and protein in about equal amounts in a fluid mosaic structure. This is composed of a bilayer sheet of phospholipid molecules, their polar heads on the surfaces and their fatty-acyl chains forming the interior. The protein components are embedded within the phospholipids, some spanning the membrane, some on one side, some on the other. The membrane is fluid in that components can diffuse laterally in its structure. It is also asymmetric, and by its activities it creates and maintains gradients across itself, so that the inside of the cell is very different to the outside environment.
The prokaryotic cell membrane is a principal structural component of the cell, that is obvious from its following possible functions.
(a) Selectively permeable layer. It allows the entry and exit of some molecules but not others. This is very important, because if this barrier is broken down, essential metabolites may pass out of the cell, resulting in its death. The transport of molecules across the membrane in either direction usually involves their specific combination with protein molecules called permeases which are built into the membrane structure. Due to this specificity of combinations, a large number of different permeases may occur in any single cell.
(b) Energy production. The membrane is the site of electron flow in respiration and photosynthesis leading to phosphorylation (i.e. conversion of ADP to ATP), and therefore is the site of the enzymes and carriers in these reactions.
(c) Extracellular polymer production. The final stages in the synthesis of some of the polymers in the cell wall, capsule and extracellular fluids are catalysed by membrane enzymes.
(d) Site of chromosome attachment. The single chromosome and the cell membrane are attached at a specific point at which replication starts. The first stage in nuclear division involves duplication of this attachment site, followed by a progressive bidirectional replication of the DNA by two replication forks.
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Cell membrane of archaebacteria. Archaebacteria have a cytoplasmic membrane with structures very different from true bacteria as well as eukaryotes and this feature distinguishes them from all other organisms. In extreme environment which they inhabit, unusual physiologically specialised membranes are needed for survival. In contrast to eubacterial and eukaryotic cell membranes that contain straight chain fatty acids linked to glycerol by ester linkages, the cell membranes of archaebacteria contain branched lipids. The lipids are diethers in which a glycerol unit is connected by an ether link to phytanols, the branched chains in which carbon atoms at regular intervals carry a methyl group. Moreover, glycerol has two optical isomers distinguished by the configuration of the molecule around the central carbon atom; the optical isomers rotate polarised light in opposite directions. Sulpholobus, an archaebacterium of high temperatures in acidic environments – has a cell membrane that contains long chain branched hydrocarbons twice the length of the fatty acids found in the cells membranes of eubacteria. The lipid chains are long enough to extend from one side of the membrane to the other giving it the appearance of monolayer while concealing its true bilipid structure. Similar unusual membrane structures occur in other archaebacteria of other extreme environments, including Thermoplasma living in high temperatures, and Halobacterium in habitats of high salt concentrations. The structure of these membranes makes them very resistant to conditions that would disrupt them and interrupt the function of a normal bilipid layer, enabling them to remain as semipermeable barriers in extreme environments.
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Intracytoplasmic membranes. The cytoplasmic membrane may not exist as a simple structure underlying the cell wall and following its contours. However, infoldings may occur which can produce complex internal structures and increase its surface area. Consequently, there occur a number of morphologically and physiologically differentiated intracytoplasmic membranous structures in different prokaryotes. All such structures may be grouped into two main categories.
(a) Chromatophores. These are pigment-bearing membranous structures of photosynthetic bacteria. They occur in all Rhodospirillaceae, Chromatiaceae and Cyanophyceae and vary in form as vesicles, tubes, bundled tubes, stacks, membranes or thylakoids (as in cyanobacteria).
(b) Mesosomes. Mesosomes were earlier called peripheral body or chondroid and have been seen in ultrathin sections of all Gram-positive and occasionally in Gram-negative bacteria. They have been assigned a number of functions by different persons from time to time. They have been defined as “vesicular, lamellar or tubular packets of membrane enclosed by the invaginations of the plasma membrane” and “as seen in thin sections of whole or plasmolysed cells the mesosome or its derived vesicles have all of the features of unit membrane profiles.” According to some others mesosomes are simply preparation artifacts. These are seen in chemoautotrophic bacteria with high rates of aerobic respiration, such as Nitrosomonas, and in photosynthetic bacteria such as Rhodopseudomonas, where they are the site of photosynthetic pigments. However, in electron micrographs of some bacteria, as Bacillus licheniformis, there are seen some localised infoldings of the membrane, which may perhaps be the effect chemical fixation on a specialised region of the cell membrane. Mesosomes a involved in septum formation, rather than being true structures in vivo.
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Ribosomes. They are seen in thin sections as relatively dense particles about 20 nm in diameter present as polysomes. Ribosomes can be easily obtained from ruptured cells by differential centrifugation on a sucrose gradient. They are made up to two sub-units of sedimentation constants 30S and 50S, which combine to give the characteristic prokaryotic 70S ribosome. Both components are made up of roughly equal amounts of RNA and protein. The 30S subunit contains one RNA molecule of 16S, and the 50S contains two, of 5S and 23S. Antibiotics as streptomycin and chloramphenicol specifically inhibit protein synthesis by ribosomes.
The relatively higher rate of multiplication in prokaryotic cells than eukarvotic ones is due to higher number of ribosomes per unit mass. In rapidly growing bacteria, the ribosomes may make up about 40% of the cell dry weight, chiefly as polysomes.
BSc Structure of Microorganisms in Microbiology Notes Study Material
Nuclear body. It is of primitive type and also referred to as nucleoid. It is possible to use coloured or fluorescent stains to delineate a central DNA-rich region in bacterial cells. From a lysed protoplast it can be isolated as a structure consisting of loops of supercoiled DNA together with about 10% RNA and 10% protein (mostly RNA polymerase). This nuclear body, an amorphous, lobular mass of fibrillar, intensely chromatinic material occupies about 10-20% of the cell volume. In the electron microscope this body shows a diffuse area containing fibrous material with no limiting membrane.
Double-stranded DNA forms a single circular chromosome, usually 1-2 mm in length in each nucleoid. The single molecule is about a thousand times longer than the cell itself. Thus, the thread is highly folded to give the bundles of fibres seen in thin sections. The chromosome of E.coli is best studied. This is by far the largest molecule to occur in any biological system. It is thus hard to conceive the behaviour of a molecule that is about one thousand times larger than the largest protein and that exists, moreover, coiled inside the cell several hundred times than itself. Unlike eukaryotes, there are no histones in the nuclear material. However, recently small basic proteins have been detected.
These have been given different names as HU, NS and DNA-binding protein II. They consist of two very similar polypetide chains, whose function is said to be similar to eukaryotic histones. Nuclear body exhibits no mitotic or meiotic phenomena. There is no nuclear membrane. Instead, there is an attachment between a specific point on the chromosome and the cell membrane at which replication starts. The first stage in nuclear division involves duplication of this attachment site, followed by a progressive bidirectional replication of the DNA by two replication forks, which meet at a point called the terminus. At each replication fork the nucleotides are added to each naked single strand. In E. coli growing at 370C replication of this length of DNA, almost 1000x the cell length, takes only 40 minutes. This part of the cell is called the C-period. The resulting two daughter chromosomes are then drawn apart by separation of these attachment points, and are induced to coil up to form the two new nuclear bodies.
BSc Structure of Microorganisms in Microbiology Notes Study Material
Chromosome replicates by semiconservative method, shown in 1958 in E.coli by Meselson and Stahl. The semiconservative method accounts for DNA replication, but it does not shed light on how a closed loop chromosome replicated. This problem perplexed microbiologists until 1962 when John Cairns and his co workers clarified some of the details of the process.
Cairns, experiments showed that DNA unwinds at a fixed point, whereupon an enzyme nicks the closed loop at a site called the origin. The two strands now separate, or “unzip” to establish a V-shaped replicating fork, as displays. Synthesis of DNA then occurs along the sides of the fork. In later years microbiologists established that along one side of the fork DNA is manufactured by continuous assembly of nucleotides, beginning at the origin. However, along the other side, the synthesis begins at the point of forking and proceeds in a discontinuous fashion. Here the DNA is synthesized in a series of segments that later join with the help of an enzyme called DNA ligase. The segments came to be known as Okazaki fragments, after Rejii Okazaki who discovered them in 1968. Each of the new DNA strands then combines with a parent strand as Meselson and Stahl postulated.
BSc Structure of Microorganisms in Microbiology Notes Study Material
A second type of DNA replication is called the rolling circle mechanism. This process takes place in bacteria undergoing the sexual mating process of conjugation. While one strand of DNA remains in a closed loop, an enzyme nicks the other strand. The broken strand then “rolls off” the loop and serves as a template for synthesis of a DNA strand complementary to itself. When the two strands combine, a double helix reforms again. Meanwhile the intact loop revolves 360 degrees and serves as a template for a strand of DNA complementary to itself. The loop combines with its new strand to form a second double helix. The bacterium now has two chromosomes, one of which will be used in mating. Sexual reproduction is rare and when it does occur it is unidirectional and incomplete.
Inclusion bodies. There occur a number of inclusion bodies, also called storage granules by some authors, in the cytoplasm. In the presence of an excess of an external energy source they may make up as much as 50% of the dry weight. Shively (1974) classified cell inclusion bodies into two major groups: (a) membrane-enclosed inclusion bodies, and (b) inclusion bodies lacking a membrane.
BSc Structure of Microorganisms in Microbiology Notes Study Material
(a) Membrane-enclosed inclusion bodies. These are enclosed by a non unit type membrane, that is 2-4 nm thick and perhaps made up entirely of protein. These inclusion bodies include chlorosomes, carboxysomes, magnetosome, gas vacuoles, poly-hydroxybutyrate (PHB) granules, sulphur globules and certain polyglucoside (glycogen) granules.
PHB granules occur in many microbes, including cyanobacteria. Sulphur globules are found in Thiorhodaceae and glycogen in Clostridium spp.
Chlorosomes were demonstrated by Cruden and Stanier (1970) in photosynthetic bacteria of Chlorobiaceae. These differ from chromatophores in being non-membranous, being simply enclosed by non unit proteinaceous covering. They contain photosynthetic pigments.
Carboxysomes have been demonstrated in chemolithotrophic bacteria and cyanobacteria by Murphy et al (1974). They are said to be involved in regulation of carbon metabolism.
Magnetosomes-term was given by Balkwill et al (1980) in the bacterium, Aquaspirillum magnetotacticum. Blakemore and Frankel (1981) studied magnetotactic bacteria in water bodies of Tasmania and Newzealand. The cells of these bacteria contain iron in the form of magnetite (Fe304). The intracellular magnet allows the cell to persue the most efficient aerotactic behaviour i.e. orientation along the geomagnetic lines of force eliminates the need for twiddling, long runs and short runs. This behaviour represents a mechanism for migrating down to and remaining in the sediments, avoiding the more oxygenated surface water.
BSc Structure of Microorganisms in Microbiology Notes Study Material
(b) Inclusion bodies lacking a membrane. These are polyglucoside (glycogen) granules, crystals and paracrystalline arrays and inclusions specific for cyanobacteria i.e. cynophycin granules and phycobilisomes.
Besides those included under types (a) and (b), there are also other inclusion bodies. One such common type are the intracellular refractile structures (R-bodies), demonstrated in bacteria by Lalucat and Mayer (1978). R-bodies are said to be similar to those reported in killer strains of Paramaecium.
Spores. Some prokaryotes, especially Bacillus and Clostridium species produce structures called spores. They have much resistance to environmental stresses such as heat, desiccation and radiation. They are the means of prolonged life of cell under unfavourable growth conditions. Characteristically, a single endospore is formed within a vegetative cell and on germination a single vegetative cell is again produced.
In the phase contrast microscope, spores appear as highly refractile bodies sometimes greater in diameter than the cell itself. Thin sections show under electron microscope a complex multilayered wall. The outermost layer of variable structure is exosporium, inside which there is a spore coat composed of several laminated layers of protein. Below this is the thick cortex containing a specific peptidoglycan and, below that the protoplast containing the most characteristic chemical component of the spore – a complex of calcium and dipicolinic acid which perhaps provides heat resistance. Spore formation occurs after a cell has gown and matured for some hours as a vegetative cell. In Bacillus spores are commonly formed in the centre of the cell and are elliptical, whereas in Clostridium, they are more oval in shape and often formed at the end of the cell or midway between the centre and end. Under conditions of limited supply of C, N, or P, certain Gram-positive rods (aerobic Bacilli and anaerobic Clostridia) and a few sarcinae and actinomycetes, form highly resistant, dehydrated forms, called endospores or spores. The bacterial spore is perhaps the most resistant living structure known to science due to its multiple coverings. These layers allow it to survive most toxic chemiclas and harsh physical conditions. Also a chemical, dipicolinic acid (DPA) helps to stabilize its proteins. DPA and large number on calcium ions provide resistance to heat. DPA is liberated as calcium dipicolinate during germination of spores.
BSc Structure of Microorganisms in Microbiology Notes Study Material
The surrounding mother cell from which spores are eventually released is called the sporangium. Like the seeds of higher plants, they have no metabolic activity. They are particularly adapted for prolonged survival under adverse conditions: they are relatively resistant to killing by heat, as well as by drying, freezing, toxic chemicals, and radiation. Though, resistant to heat, their main ecological role is probably the survival in the dry state (or in a nonnutrient medium). The bacterial spore is not a reproductive structure, as a single vegetative cell forms a single spore and a single vegetative cell will reappear when the spore germinates (cf. a fungus spore which serves both a resistant form as well as reproductive structure). Various stages of sporulation cycle are shown in Figure. The vegetative cell metabolises nutrients and multiplies in the vegetative cycle. After sometime it enters the sporulation cycle. The overall cycle includes four main stages:
(i) accumulation of cellular DNA to form axial filament and formation of transverse spore septum,
(ii) axial filament is then surrounded by the cell membrane to form forespore,
(iii) formation of resistant layers of coat material around the spore, completing the formation of spore, and disintegration of remaining cell to liberate the free spore, and
(iv) spore germination, when conditions become suitable, the bacterium emerging to resume the vegetative cycle.
BSc Structure of Microorganisms in Microbiology Notes Study Material
Spores are unusually dehydrated, impervious, highly refractile cells. They do not take ordinary stains (Gram’s, methylene blue). In the light microscope the first visible stage in sporulation is the formation of an area of increased refractility, the forespore at one end of a cell. The refractility gradually increases and the mature spore is completed in 6-8 hours. Blocks in DNA synthesis show that sporulat starts, like normal cell division, at a critical stage in a round of DNA replication. In electron micrographs the first detectable change is conversion of the compact nucleoid into an axial filament. The resulting movement of one chromosome to the pole of the cell triggers a specialised, asymmetrical cell division, with the ingrowth of a double layer of cytoplasmic membrane to form a subpolar spore septum (without peptidoglycan).
The peripheral zone of attachment of this double transverse septum moves toward the pole of the cell, and thus finally enclosing the chromosome and surrounding cytoplasm in a double membrane to form the forespore.
The specialised spore integument (envelope) is laid down between the two membrane layers of the forespore, which are initially extensions of the mother cell membrane but become differentiated in composition and function. Both facing surfaces correspond to the wall-synthesising surface of the parent cell membrane. In the maturation of the spore a large amount of material is laid down between the two membranes. The resulting thick envelope eventually occupies over half the spore volume, surrounding the protoplast (core). Several layers, successively initiated can be distinguished.
BSc Structure of Microorganisms in Microbiology Notes Study Material
(i) The innermost layer is the germ cell membrane, inner membrane or core membrane, or core wall-that surrounds the core.
(ii) Next is the thickest layer, the cortex, which contains a concentric laminated structure.
(iii) Outside the cortex is the densely stained coat. This may be differentiated into an inner coat layer and an outer coat layer.
(iv) Spores of some species, as Bacillus thuringiensis are further loosely shrouded in a delicate exosporium or exosporium basal layer.
The cortex contains many layers of spore peptidoglycan, much more loosely cross-linked than that in the vegetative cell. During germination this part of the peptidoglycan is rapidly autolysed and released. The coat is made of a keratinlike protein, rich in disulphide crosslinks. It constitutes about 80% of the total protein of a spore. The impervious protein coat is responsible for the resistance of spore to chemicals. The exosporium is a lipid-protein membrane, with 20% carbohydrate. It is not essential for survival, and its function is unknown.
BSc Structure of Microorganisms in Microbiology Notes Study Material
A characteristic feature of the spore is its huge content of Ca2+, normally accompanied by a roughly equivalent amount of dipicolinic acid which can chelate Ca2+.This acid may be as much as 15% of spore weight. It is located in the core. The core contains much less material than the vegetative cell. It has DNA of one chromosome (3×109 daltons), small amounts of all the stable components of protein-synthesizing machinery (including ribosomes, tRNAs, and accessory factors and enzymes), but no detectable mRNA. Aminoacids (and their biosynthetic enzymes) are absent, supplied early in germination by hydrolysis of a storage protein of low mol. wt, constituting about 20% of the total protein.
The overall process of converting a spore into a vegetative cell is often called germination. It is much faster than sporulation. Three stages can be distinguished; (1) activation-by agents as heat, low pH or an SH compound. (2) germination proper or initiation-various metabolites as alanine, dipicolinate, or inorganic ions as Mn2+ penetrate the damaged coat and initiate germination. (3) outgrowth- a gradual resumption of vegetative growth, with progressive increase in protein synthesis and start of DNA synthesis.
Eukaryotic Microorganisms
They exhibit a tremendous range of structure, function, behaviour and habitat. Most students would have the idea of the range of these features of eukaryotes from their studies of botany and zoology. It should be clear to them that the structure of eukaryotic cells is generally much more complex than prokaryotic cells. Eukaryotic cells are normally much larger, with a typical diameter ten times greater than that of the prokaryote (i.e. 10 um) and they show much diversity in Size and shape. The yeast cell may be taken as a typical of eukaryotic cells. We shall prefer to mention here only some of the components in any detail and an emphasis will be placed on those structures that serve to distinguish the cells fundamentally from those of prokaryotes.
Membranous structures. Within a eukaryotic cell, a variety of membranous selectively permeable structures occur, that give a complex multi-compartmental whole. Such structures are as follows:
(a) The plasma membrane. In its physical and chemical structure, the plasma membrane is very similar to the cytoplasmic membrane of prokaryotes. However, a difference is the presence of sterols in eukaryotic membranes. In eukaryotes also, this membrane has the same vital function as the prokaryotic cell membrane in being selectively permeable and of transporting specific solutes, but it does not have properties of respiration or photosynthesis. An additional feature, however, absent in prokaryotes, can be the ability by wall-less cells to ingest food in particulate form by phagocytosis or in liquid form by pinocytosis, in either case, a membrane-enclosed vacuole is formed within the cytoplasm. Such endocytosis is lacking in bacteria.
BSc Structure of Microorganisms in Microbiology Notes Study Material
(b) Endoplasmic reticulum (ER). The ER typically comprises more than half of the membrane in the cell. It is actually a sac, with the cytoplasm outside and its lumen inside. Its ramifications through the cell give a large surface area. In electron micrograph two form of ER are distinguished.
Rough endoplasmic reticulum. It is studded with ribosomes on its outer surface. This is the site of the synthesis of proteins destined for export from the cell, or to become wall components or to be stored in vesicles or vacuole within the cell, such as the lysosomes which contain digestive enzymes. As they are synthesised these proteins are secreted into the lumen of the ER, often becoming glycosylated to glycoproteins by membrane-bound enzymes.
Smooth endoplasmic reticulum. This lacks ribosomes. It is the site of phospholipid and sterol synthesis in the membrane, and the site of formation of membrane vesicles which are transferred to the Golgi apparatus for further processing.
(c) Golgi appartus. This is a series of membranous sacs, originating from the ER. It is a major site of biosynthesis of polysaccharides that are destined for export from the cell, such as pectin-like wall components in algae. Proteins and glycoproteins formed in the ER are covalently processed, for example by further glycosylation, while in the Golgi apparatus. The outer sacs of Golgi body then release these products in vesicles, some of which move to the plasma membrane, fusing with it, releasing their contents and adding to its surface area. Other vesicles are stored in the cell, for example digestive lysosomes.
BSc Structure of Microorganisms in Microbiology Notes Study Material
(d) Nuclear membrane. The nucleus is surrounded by membrane which is in continuity with the ER, and is thus topologically a membranous sac enclosing lumen. The inner-facing membrane has specific proteins on its surface which are said to hold the shape of the nucleus and to bind specific regions of chromatin. The nuclear membrane has pores, allowing transport of molecules in and out of the nucleus. Thus mRNA must be transported out and enzymes such as RNA polymerase must be transported into the nucleus.
(e) Vacuoles, lysosomes and peroxisomes. As mentioned above, phagotrophic microbes such as many protozoa engulf food particles by invagination of the plasma membrane. Digestion takes place by the fusion of a lysosome with this food vacuole. Vacuoles are formed in many cells. For example yeast, and appear to be involved in the accumulation and storage of metabolic intermediates and ions. The contractile vacuoles occurring in some cells, e.g. freshwater protozoa, function in osmotic regulation and in the excretion of waste products. Peroxisomes are membrane-bound packages of oxidative enzymes such as catalases. They are formed by budding off from smooth ER.
Thus, the above-mentioned membrane systems must be thought of in terms of their spatial and temporal relationships; they are common parts of a dynamic system of membrane flow and modification.
BSc Structure of Microorganisms in Microbiology Notes Study Material
(f) Cytoskeleton. In addition to several membrane-bound organelles, the eukaryotic cell has a cytoskeletal network, consisting of microtubules and microfilaments, that helps determine the ability of the cell to move and to maintain its shape. This network links the various components of the cytoplasm into a unified structure, the cytoplast, providing the rigidity needed to hold the various structures in their appropriate locations. The network runs throughout the cell, connecting membrane-bound organelles with the cell membrane. This provides support and movement to organelles, including the cell membrane. It provides the basis for membrane movement involved in transport of materials into and out of the cell by cytosis. Absence of cytoskeleton in prokaryotic cell does not allow cytosis.
Mitochondria and chloroplasts. These two organelles have many features in common, that are as follows:
(a) Both are concerned with energy conversions. Mitochondria are the site of oxidative phosphorylation, converting the energy of oxidative reactions into useful forms, especially ATP. Chloroplasts are the site of photophosphorylation, utilising light energy to produce ATP and NADPH2, and using these to fix CO2 to carbohydrate.
(b) Both are surrounded by two membranes. The outer one is relatively permeable and resembles the cell membrane, for example by having sterols. The inner one is impermeable and resembles the bacterial cell membrane.
BSc Structure of Microorganisms in Microbiology Notes Study Material
(c) Thus inner bioenergetic membranes have a large surface area. In the mitochondrion, this is the inner membrane which is extensively infolded to form cristae on whose surface are the respiratory chain enzyme complexes. In the chloroplasts within the inner membrane are stacks of membranous sacs (thylakoids) bearing the photosynthetic pigments and associated acceptors.
(d) The inner membrane encloses the matrix. This matrix in each case contains many enzymes; for example in mitochondria those for the TCA cycle, and in the chloroplasts the ribulose biphosphate carboxylase. The enzyme responsible for fixing CO. This enzyme may be much concentrated in the chloroplasts of many algae that it forms the most obvious object in the cell, the pyrenoid.
(e) They both have quasi-independence in the cell as they both have their own DNA and transcription and translation machinery. These resemble those of bacteria, as the DNA is several copies of a single circular chromosome. And the ribosomes are similar in size, make-up and sensitivity to antibiotics such as streptomycin and chloramphenicol. Their protein synthetic ability, however, is limited to only a few of their essential components. For example sub-units of cytochrome oxidase and ribulose biphosphate carboxylase. Other sub-units and essential components are coded for by the cell nucleus and translated on cytoplasmic ribosomes to be transported into the mitochondrion or chloroplast.
BSc Structure of Microorganisms in Microbiology Notes Study Material
(f) Following from (e), mitochondria and chloroplasts have to replicate their DNA and divide in pace with their parent cells. This can be prevented by specific antibiotics. So that treatment of the alga, Euglena with streptomycin prevents its chloroplasts dividing, changing it from a green photoautotroph to a colourless chemoheterotroph.
(g) Following from (e) and (f) has come the idea that eukaryotic cells arose by endosymbiosis. The putative fermentative prokaryote engulfing an aero bacterium and then a blue-green bacterium. This hypothesis is of course unprovable. But circumstantial evidence comes from the observation that endosymbiosis is a common phenomenon. For example some strains of Paramecium contain symbiotic bacteria and others contain symbiotic algae which divide in pace with their hosts.
The cytoplasm. The eukaryotic cytoplasm, like that of bacteria, contains many enzymes, metabolites and other solutes. But is also structured as it contains a cytoskeleton composed of microtubules, actin, myosin and other structural proteins. Cytoplasmic streaming is common, as seen by the mass movement of organelles within cells.
The nucleus. The nucleus is a definite structural entity surrounded by membrane and containing many chromosomes. The individual chromosome are made up of linear molecules of DNA compared with circular DNA of prokaryotes. Associated with the nucleus there is generally an RNA-containing body- The nucleolus which is a specialised structure responsible for ribosomal RNA synthesis. A function carried out by the single chromosome of the prokaryote nucleus. The eukaryotic chromosomes also differ in having their DNA complexed with basic proteins called histones to form chromatin. As the chromosomes contain much more DNA than does the prokaryotic chromosome. Each is replicated by many replication forks, acting in pairs from many sites of origin. This DNA doubling is followed by the complex process of mitosis designed to ensure an orderly partition of a complete set of chromosomes to each daughter cell.
BSc Structure of Microorganisms in Microbiology Notes Study Material
Sexual reproduction is common in eukaryotic microorganisms, and the consequent doubling of the haploid chromosome number then requires meiosis. This allows organisms to have alternate haploid and diploid life cycles.
Storage granules. The eukaryotic microbial cells contain a variety of inclusion bodies such as membrane-bound granules of starch, protein or lipid droplets.
The cell wall. There is much variation in shape, thickness and chemical composition of cell walls of eukaryotic microorganisms. Some protozoa apparently do not have walls. Their plasma membrane may have some additional strengthening to maintain cell shape and rigidity. The cell walls usually have a simpler structure than those of prokaryotes.
(a) Algae. Basic structure is usually maintained by microfibrils formed by intertwinning of long polysaccharide molecules, such as cellulose, mannans, xylans and pectins. In some algae there are silica or calcium carbonate in the walls.
(b) Fungi. The common structural polymer of the wall is chitin (B 1-4 poly-N-acetyl-glucosamine), often occurring with polyglucose built of B(1-3) ages compared with B(1-4) linkages of cellulose. The mechanical strength of the wall is maintained by cross linkages between microfibrils of chitin and the glucan matrix to give a thick and tough structure. Mannans are also common in yeast walls.
(c) Protozoa. There is a great variety of surface components, including structures built from protein, cellulose, calcium carbonate or silica.
BSc Structure of Microorganisms in Microbiology Notes Study Material
Flagella, cilia and locomotion. The eukaryotic microbes move usually by the action of flagella and cilia which have a structure quite different from that of the prokaryote flagellum. They are composed of a characteristic “9+2” arrangement of microtubules surrounded by a sheath. Which is an extension of the plasma membrane. They are powered by ATP. Eukaryotic microbes also move by amoeboid movement, as a result of cytoplasmic streaming in cells without a wall. Like prokaryotes, motile eukaryotes show movement towards or away from heat, light or certain chemical substances.
Unlike the bacterial flagella that rotate. The flagella and cilia of eukaryotic cells undulate in a wave-like motion to propel the cell. Eukaryotic flagella emanate from the polar region of the cell, whereas cilia, which are somewhat shorter than flagella, surround the cell. Both cilia and flagella are generally involved in cell locomotion. But cilia may also be involved in moving materials, such as food particles. The movement of flagella or cilia is based on a sliding microtubule mechanism in which the peripheral doublet microtubules slide past each other, resulting in bending of the flagella or cilia. The peripheral spokes of microtubular network contain a protein-dynein, which has ATP as activity. And is involved in coupling ATP utilisation to the movement of flagella or cilia.
Spores. Eukaryotic microbes form a great variety of spores, some produced in abundance for dispersal, others in small numbers for survival. The latter are generally thick-walled and resistant to harsh environmental conditions.
However, the cell chemistry of archaebacteria is very distinct from eubacteria as well as eukaryotes. The numerous and profound differences of organisation and function among the three cell types– eukaryotes, eubacteria and archaebacteria have now become fully recognised. These differences are summarised as follows:
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