BSc Industrial Microbiology Notes Study Material
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BSc Industrial Microbiology Notes Study Material
Various commercial products of economic value made by microbes are (i) medicines i.e. pharmaceuticals, including antibiotics, steroids, human protein, vaccines, and vitamins, (ii) organic acids, (iii) amino acids, (vi) enzymes, (v) alcohols, (vi) organic solvents and (vii) synthetic fuels. In addition to these, quite recently the potential of microbes could also be realized in (viii) recovery of metals from ores through bioleaching, (ix) recovery of petrol, and (x) single all protein production.
The term fermentation in industrial microbiology is used in a wider sense to include any chemical transformation of organic compounds carried out by using microbes and their enzymes. Production methods in industrial microbiology bring together raw materials (substrates), microorganisms (specific strains or microbial enzymes), and a controlled favorable environment (created in a fermentor) to produce the desired substance. The essence of an industrial process is to combine the right organism, an inexpensive substrate, and the proper environment to produce high yields of the desired product. (BSc Industrial Microbiology Notes Study Material)
Microbes possess a wealth of metabolic equipment that brings about diverse chemical transformations. This characteristic of microbes could be exploited in obtaining some valuable products of daily use. The cheap raw materials available in nature as waste may be converted into useful commercial products by the activity of microbes. Microbes thus serve a dual purpose. Firstly, they are good agents of disposal of these wastes, and secondly, the resultant end products of their breakdown are useful commercial products.
Industrial microbiology is an important area of applied microbiology. It refers to the use of microorganisms in commercial enterprises. Cheap raw materials are converted to valuable products through the metabolism of microbes. Microbes for this purpose could be exploited in different ways. For instance, this includes (i) the synthesis of fermentation products as acids, alcohols, or other organic compounds, (ii) the transformation of one compound into another desired type, (iii) the production of enzymes, antibiotics, or insecticides, or (iv) the use of microbes themselves as food. (BSc Industrial Microbiology Notes Study Material)
It is possible to distinguish different phases in the development of industrial microbiology. The first phase, relatively ancient, involves fermentations for the production of alcoholic beverages and traditional dairy products, and bread. These processes are non-septic and rely either on a large inoculum or on good housekeeping to prevent microbial spoilage of the product. They all also reflect the synthesis of a primary product of metabolism by the organism e.g. the enzyme rennin in cheese-making, lactic acid in curd, CO2 in bread, and alcohol in beers and wines.
The second phase was the development of large-scale relatively sterile culture facilities in which an inoculum is added. This included processes such as large-scale cultivation of yeast cells for the brewing and baking industries. Efforts were made to fermentation vessels and pipelines. Steam was often used for this purpose. The growth media were often by-products of other industries i.e. complex mixtures of molasses products come from butanol, and hence the industry that grew up was known as the fermentation industry.
The third phase involved the batch culture of microbes under aerobic conditions and an aseptic state. Strains of the microbes that are more efficient in antibiotic production were selected. Strains of Penicillium in current use produce as much as ten thousand times as much penicillium per unit volume of fermentation liquor than Fleming’s original isolate.
During the 1960s, the fourth phase, interest in the use of microorganisms as bulk sources of protein for animal and human food, the so-called single-cell protein, led to the exploitation of continuous culture methods on an industrial scale. This required the development of sophisticated fermentation equipment and complete aseptic conditions.
Finally, the most recent fifth phase has been biotechnology-a term often being used indiscriminately and fashionably. It can be defined as “the application of biological organisms, systems, or processes to the manufacturing and service industries”. There has been some tendency to equate gene cloning and recombinant DNA technology with biotechnology, though many other areas are included in it. Other important areas are the use of enzymes and immobilized cells as catalysts and sensors, the manipulation of cell physiology for the maximum production of metabolites, and the use of cell fusion to generate either monoclonal antibodies or new cell lines.
Microbes As Ideal Organisms
Microorganisms possess several characteristics, which make them ideal organisms for industrial processes. They possess a broad variety of enzymes making an array of chemical conversions possible. They have a relatively high metabolic activity which allows conversions to take place rapidly. They possess a large surface area for quick absorption of nutrients and release of end products. Moreover, they have a high rate of multiplication, as a 20-minute generation time for E. coli under ideal conditions. (BSc Industrial Microbiology Notes Study Material)
In an industrial process, a microorganism in fact represents a mini-chemical factory. The efficiency of this factory depends upon the liberation of a large amount of a single product that may be efficiently isolated and purified. The microbe should be easily cultivated and maintained and should have genetic stability with infrequent mutations. Its value enhances if it can grow on an inexpensive, readily available substrate that may be the by-product of other industrial processes.
For example, a large amount of whey is produced in cheese production. If there are other microbes that could convert the whey components to lactic acid, this will add to the overall profit of the cheese industry. (BSc Industrial Microbiology Notes Study Material)
Of the many species of microbes, relatively few possess the genetic information needed to produce economically useful products. Some microbial species are used for the production of commercial products.
After selecting the suitable microbe for the purpose, it is necessary to develop a fermentation process that optimizes conditions for desired microbial activity and that should yield the maximum amount of the product with the highest economic profit. The commercial process occurs in a stepwise manner, initially using small flasks, then small fermentors (under 10 gallons), intermediate-size fermentors (up to several hundred gallons), and finally, large-scale fermentors (thousands of gallons). The organic and inorganic composition of the medium, pH, temperature and O2 concentration are the main factors that are varied to maximize the efficiency of the production process.
Essential nutrients in the medium for microbial growth include C, N, and P sources shown below. The choice can be made on economic and biological grounds. (BSc Industrial Microbiology Notes Study Material)
Of all conditions for growth, aeration is the most important and difficult to maintain. Many industrial fermentations are aerobic and therefore it is important to achieve the optimal oxygen concentration to permit microbial growth with maximum product yield. The development of fermentors for the growth of obligately aerobic microbes in a broth (submerged aerobic culture) requires careful design in order to achieve optimal O2 concentration. Many fermentor designs have mechanical stirrers to mix the solution, baffles to increase turbulence and ensure adequate mixing, and forced aeration to provide the required oxygen.
Batch versus Continuous flow process
A fermentation process may be designated as a batch process that is analogous to inoculating a flask containing a broth with a microbial culture, or as a continuous flow process, analogous to that of a chemostat. In a batch process, a large tank with a volume of up to 100,000 gallons of medium is used. The tank is known as a fermentor since industrial microbiologists use the term fermentation for any aerobic or anaerobic process catalyzed by microorganisms. The medium is sterilized with steam or with a gas such as SO2.
Microorganisms, about 2 to 4% of the total volume are added to the fermenter. Oxygen is bubbled if the process is aerobic but omitted if anaerobic conditions are required. The microbial growth is allowed to take place for days, weeks, or months. Microbes are removed after fermentation is complete. The product is isolated from the materials in the tank. (BSc Industrial Microbiology Notes Study Material)
As compared to the batch processes, flow-through fermentors are more prone to contamination. However, flow-through design has the advantage of producing a continuous supply of the product for commercial distribution. In this method, the medium is added continuously to the tank to replace that which has been fermented. The isolation and purification of products from the medium is thus an ongoing process. An instrument called a chemostat is used to provide a constant flow, keeping microorganisms in the logarithmic phase of growth.
In some cases, a turbid state is used. This instrument measures the turbidity of the microbial population to indicate the level of growth. When a certain level is reached, it adds a new medium to the tank while withdrawing the spent material. The fermentation is thus maintained at a constant level. (BSc Industrial Microbiology Notes Study Material)
The use of immobilized enzymes is an attractive alternative for the production of the desired product. Microbial enzymes and/or microbial cells are adsorbed or bonded to solid surface support, such as cellulose. The bonded and thus immobilized enzymes act as solid-surface catalysts. The solution containing the biochemicals to be transformed by the enzyme is then passed across the solid surface.
Temperature, pH, and O2 levels are set optimal to achieve maximum rates of conversion. The use of immobilized enzymes makes an industrial process far more economical, as it avoids the expense of a continuous flow growth process and discards unwanted biomass. When whole cells rather than cell-free enzymes are employed care should be taken in maintaining the viability of the microbes during the process. This generally requires the addition of necessary growth substrates.
Recovery of product
Several methods are used for the recovery of the products. These include distillation, centrifugation, filtration, and chromatography separation. The recovered product is packaged and marketed.
1. Medicines (Pharmaceuticals)
The pharmaceutical manufacturing industry is a major source of employment for industrial microbiologists. Microbes have maximum application in medicines which include antibiotics, steroids, vitamins, and vaccines.
Here the emphasis will be on their source and production industry. There are thousands of antibiotics produced by microbes in nature. Of these, relatively few are produced commercially. The major antibiotics used in medicines and their microbial sources are given below:
Penicillin. An inoculum of Penicillium chrysogenum is produced by inoculating dense spore suspension onto a wheat bran-nutrient solution, incubated at 24°C for one week or so, and then transferred to an inoculum tank. Tanks are agitated with forced aeration for 1-2 days to have heavy mycelia growth for subsequent inoculation into a production tank.
The typical medium used for producing penicillin today has total glucose or molasses (continuous feed) -10%; corn steep liquor solids – 4-5%; total phenylacetic acid (cont. feed) – 0.5-0.8%; total vegetable oil (cont. feed) – 0.5%. Phenylacetic acid is the precursor used to form the benzene ring side chain of the penicillin G molecule. The pH of the medium after sterilization is approx. 6.0. It takes about 7 days to ferment.
After fermentation is complete, the concentration of penicillin reached maximal achievable levels, and the liquid medium containing penicillin is separated from the fungal cells, using a rotating vacuum filter. Fungal biomass is scraped from the surface of the filter drum, dried, and marketed as an animal feed supplement. Penicillin is recovered from the filtrate using various extraction methods.
The penicillin G thus obtained can be further modified to form various penicillin derivatives. The modification can be done chemically or by using microbial enzymes. For example, 6- aminopenicillins acid (6-APA) can be formed by fermentation, using bacterial acylase enzymes in an aqueous solution at 37°C. The same transformation of penicillin G to 6-APA by using chemical solvents, anhydrous conditions, and low temperature, can also be done in three steps.
Cephalosporins. Similar semisynthetic approaches can be used for the manufacture of other antibiotics. Cephalosporin C is made as the fermentation product of Cephalosporium acremonium. However, this form is not potent for clinical use. Its molecule can be transformed by the removal of an alpha-aminoadipic acid side chain to form 7-alpha aminocephalosporanica (7-ACA), which can be further modified by adding side chains to 10 clinically useful broad-spectrum antimicrobials.
Various side chains car added to as well as removed from both 6-APA and 7-ACA to produce biotics with varying spectra of activities and varying degrees of resist inactivation by enzymes produced by pathogenic microbes. Thus we have entered into the so-called third-generation cephalosporins, such as moxalactam, developed for the control of bacteria that produce enzymes capable of degrading penicillins and cephalosporins.
Streptomycin. Streptomycin and various other antibiotics are produced using strains of Streptomyces griseus. Spores of this actinomycete are inoculated into a medium to establish a culture with high mycelial biomass for introduction into an inoculum tank, with subsequent use of the mycelial inoculum to initiate the fermentation process in the production tank. The medium contains soybean meal (N- source), glucose (C- source), and NaCl. The process is carried out at 28°C and the maximum production is achieved at a pH range of 7.6-8.0.
High agitation and aeration are needed. The process lasts for about 10 days. The classic fermentation process involves three phases. During the first phase, there is the rapid growth of the microbe with the production of mycelial biomass. The proteolytic activity of the microbe releases NH3 to the medium from the soybean meal, causing a rise in pH. During this initial fermentation phase, there is little production of streptomycin. During the second phase there is little additional production of mycelium, but the secondary metabolite, streptomycin accumulates in the medium. The glucose and NH3 released are consumed during this phase.
The pH remains fairly constant-between 7.6 and 8.0. In the third and final phase, when carbohydrates become depleted, streptomycin production ceases and the microbial cells begin to lyse. pH increases and the process normally ends by this time.
After the process is complete, mycelium is separated from the broth by filtration, and the antibiotic is recovered. In one method of recovery and purification, streptomycin is adsorbed onto activated charcoal and eluted with acid alcohol. It is then precipitated with acetone and further purified by the use of column chromatography. (BSc Industrial Microbiology Notes Study Material)
The microbial biotransformation of steroids is very important in the pharmaceutical industry. Steroids are used in the treatment of various disorders and are also involved in the regulation of sexuality. The chemical synthesis of steroids is very complex because of the requirement to achieve the necessary precision of substituent location.
For example, cortisone can be synthesized chemically from deoxycholic acid but the process requires 37 steps, many of which must be carried out under extreme conditions of temperature and pressure with the resulting product costing over $200 per gram.
The most difficult is the introduction of an oxygen atom at the number 11 position of the steroid ring, but this can be accomplished by some microorganisms. The fungus, Rhizopus nigricans for example hydroxylates progesterone, forming another steroid with the introduction of oxygen at the number 11 position. The fungus Cnninghamella blakesleeana similarily can hydroxylate the steroid cortexolone to form hydrocortisone with the introduction of oxygen at the number 11 position. Other transformations of the steroid nucleus carried out by microbes include hydrogenations, dehydrogenations, epoxidations, and removal and addition of the side chains.
The use of such microbial biotransformations in the formation of cortisone has lowered the original cost over 400-fold, so that in 1980 the price of cortisone in the U.S. was less than 50 cents per gram, compared to the original $200.
In a typical steroid transformation process, the microbe, such as Rhizopus nigricans is grown in a fermentation lank using an appropriate growth medium and incubation conditions to achieve high biomass. In most cases, agitation and aeration are done to have rapid growth. After the growth of the microbe, the steroid to be transformed is added (as progesterone here) to the fermentor containing R. nigricans that has been growing for one day or so, and the steroid is hydroxylated at number 11 to form 11-alpha-hydroxyprogesterone.
The product is recovered by extraction with methylene chloride or other solvents, purified chromatographically, and recovered by crystallization. A large number of similar transformations are carried out to produce a great variety of steroid derivatives for different medicinal uses. (BSc Industrial Microbiology Notes Study Material)
[III] Human proteins
Genetic engineering has expanded the industrial applications of microorganisms including the production of human proteins. By using recombinant DNA technology, human DNA sequences that code for various proteins has been incorporated into the genomes of bacteria. By growing these recombinant bacteria in fermentors, human proteins could be produced commercially.
Human insulin, for instance, is produced by a recombinant E. coli strain and marketed as humulin. Other strains are used to produce human growth hormone, tumor necrosis factor (TNF), interferon (human recombinant beta interferon-trade name, Betaseron), and interleukin-2 (human recombinant interleukin-2, trade name- Proleukin). Humulin is used in the treatment of diabetes in individuals allergic to insulin harvested from cattle. The human growth factor is used in the treatment of dwarfism, and interleukin-2, interferon, and TNF are important components of the human immune system. (BSc Industrial Microbiology Notes Study Material)
Production of vaccines involves growing the microbes possessing the antigenic properties needed to elicit a primary immune response. Mutant strains and attenuated or inactivated virulent pathogens (without removing antigens) are used for producing vaccines. Prophylactic treatment of serious pathogenic viruses and bacteria could become possible only through vaccines. Viruses are grown in embryonated eggs or tissue cultures. The rabies vaccine, produced earlier in embryonated duck eggs with painful side effects has now been replaced by a vaccine produced in human fibroblast tissue cultures. (BSc Industrial Microbiology Notes Study Material)
Microbial fermentations are used for the commercial production of several essential animal vitamins. Some important vitamins and their microbial sources are shown below.
Vitamin B12 can be produced as a by-product of Streptomyces antibiotic fermentation. A cobalt salt is added to the fermentation reaction as a precursor to vitamin B12. The vitamin accumulates in high concentrations but is not toxic to streptomyces. It can also be produced by direct fermentation using Propionibacterium shermanii or Pseudomonas denitrificans. (BSc Industrial Microbiology Notes Study Material)
2. Organic Acids
Several organic acids including acetic, gluconic, citric, itaconic, gibberellic, and lactic acids are produced commercially through microbial transformations. (BSc Industrial Microbiology Notes Study Material)
Gluconic acid is produced by various bacteria, including Acetobacter spp, and several fungi including Aspergillus and Penicillium spp. A niger converts glucose to gluconic acid in a single enzymatic reaction. Submerged culture is needed. A niger is initially grown to form enough mycelial biomass. The latter part is enzymatic. A typical growth medium consists of 25% glucose, various salts, CaCO3, and a boron source. Fermentation is carried out at 30°C with aeration and agitation. Gluconic acid is recovered by adding calcium hydroxide to form crystalline calcium gluconate. Free gluconic acid can be recovered by adding the acid.
Citric acid is also produced by Aspergillus niger. Citric acid is used as a food additive, especially in soft drinks, as a metal chelating and sequestering agent, and as a plasticizer. It is essential to limit the growth of the fungus so that high levels of citric acid can accumulate. This is possible by having a deficiency of a trace element or phosphate in the growth medium. The medium contains molasses, ammonium nitrate, MgSO4, and potassium sulphate. Acid is added to lower the pH. Metals added to the medium are removed from the solution by cation exchange resins. (BSc Industrial Microbiology Notes Study Material)
Itaconic acid is used as a resin in detergents. The transformation of citric acid by Aspergillus terreus can be used for the commercial production of itaconic acid. The fermentation process involves a well-aerated molasses-mineral salts medium at a pH, below 2.2. At higher pH, this microbe degrades itaconic acid. Like citric acid, low levels of trace metals must be used to achieve acceptable product yields.
Gibberellic acid and related gibberellins are important growth regulators of plants. Commercial production of these acids helps in boosting agriculture. This acid is formed by the fungus. Gibberella fujikuroi (imperfect state, Fusarium moniliforme) can be produced commercially using aerated submerged cultures. A glucose-mineral salt medium, incubation at 25°C, and slightly acidic pH are used for fermentation. It takes normally 2-3 days. (BSc Industrial Microbiology Notes Study Material)
Lactic acid is used as a preservative in foods, leather production, and the textile industry. Various other forms of lactic acid are also used for other purposes – in resins as polylactic acid, in plastics, electroplating as copper lactate and baking powder, and in animal feed. Fermentation is carried out by Lactobacillus, Streptococcus, and Leuconostoc spp. The typical medium normally contains 10-15% glucose or other sugar, 10% CaCO2 to neutralize the lactic acid formed, and ammonium phosphate and nitrogen sources in the trace.
Corn sugar, beet molasses, potato starch, and they are often used as carbohydrate sources for fermentation; temp. of 45- 50°C and pH 5.5-6.5. Agitation is needed but not aeration. The process is complete within 5-7 days with approximately 90% of the sugar conversion to lactic acid. (BSc Industrial Microbiology Notes Study Material)
3. Amino Acids
Microbial production of lysine and glutamic acid every year has increased worldwide sales. Animal feeds are deficient in amino acids which are to be added to them. Lysine produced by microbial fermentation and methionine produced synthetically is used as animal feed supplements. In the microbial production of amino acids, only the desired L-isomer is formed, whereas their chemical synthesis produces a racemic mixture that requires expensive separation methods to remove the bioactive D-isomer half of the mixture.
Through the use of genetically-engineered strains of microbes, some of the limitations of fermentation processes have been overcome. The direct production of L-lysine from carbohydrates uses homoserine requiring auxotroph of Corynebacterium glutamicum.
The blocking of homoserine synthesis at the level of homoserine dehydrogenase results from feed-back inhibition of that enzyme and leads to the accumulation of lysine. Cane molasses is generally used as the substrate, pH near 7.0 by adding NH3 or urea. About 50 g/l of lysine can be produced in 2-3 days by use of this auxotroph of C. glutamicum L-glutamic acid and monosodium glutamate (MSG) can be produced by direct fermentation using strains of Brevibacterium, Arthrobacter, and Corynebacterium Cultures of C. glutamicum and Brevibacterium flavum are widely used for large scale production of MSG.
A glucose mineral salts medium, with periodic addition of urea, pH 6-8, and temperature of 30°C and well aeration, are used for fermentation. There are several methods for inducing leaky membranes that permit the excretion of the product from the cell. One way is to grow C. glutamicum in a medium with a suboptimal concentration of biotin or add fatty acids or detergents. These allow the secretion of glutamic acid by membranes. (BSc Industrial Microbiology Notes Study Material)
Enzymes have important applications in industry and these are produced by different microbes, mostly by fungi.
A generalized scheme for microbial production of the commercial enzyme. Enzymes produced for the industry include proteases, amylases, glucose isomerases, glucose oxidases, renin, pectinases, and lipases. Of these, proteases, glucoamylase, alpha-amylase, and glucose isomerase are produced extensively. (BSc Industrial Microbiology Notes Study Material)
Proteases attack peptide bonds of proteins. The largest application of microbial proteases is in the laundry, in modern detergent formulations. They are used for the removal of spots of milk, eggs, and blood. These are heal-stable. Proteases are largely produced by Bacillus licheniformis. Other alkaline proteases are being developed using recombinant DNA technology, to function over a wide range of pH and temperature. One such recombinant strain is Bacillus sp. Gx 6644, active for milk casein. Another strain Bacillus sp.
GX 6638 produces several alkaline proteases. Through this technology, a recombinant strain of a bacterium has also been developed. It produces the enzyme kerazyme, used for dissolving hair and opening hair-clogged drains. In the baking industry also these enzymes have important applications. Microbial proteases reduce mixing time and improve the quality of loaf. Fungal proteases are mainly obtained from Aspergillus spp. and bacterial from Bacillus spp. These enzymes are used as meat- tenderizer and in the leather industry for bating of hides.
Amylases are used for the preparation of sizing agents in the textile industry, preparation of starch sizing pastes for use in the paper industry, bread production, chocolate and corn syrups, and removal of spots in the laundry. There are various types of amylases -alpha-, -Beta– and glucoamylase. Aspergillus oryzae, A. niger, Bacillus subtilis, and B. diastaticus are principally used. The conversion of starch to high-fructose syrup utilizes amylases, i.e. in producing sweeteners. Various other enzymes produced by different microbes also have industrial applications. These are rennin used in cheese production and Mucor pussilus is used for its commercial production.
Fungal pectinases are used in the clarification of fruit juices. Glucose oxidase is used to remove oxygen from soft drinks, salad dressings, etc. (BSc Industrial Microbiology Notes Study Material)
Microbial enzymes are also used for the production of synthetic polymers. The plastic industry mostly uses chemical methods for producing alkene oxides used in the production of plastics. It is now possible to synthesize alkene oxides by using microbial enzymes and genetically-engineered strains would make commercial production feasible. The synthesis of alkene oxides from alkenes requires the sequential action of three enzymes: pyranose-2-oxidase from the fungus Oudmansiella Lucida, a haloperoxidase from the Caldariomyces, and an epoxidase from a Flavobacterium sp. (BSc Industrial Microbiology Notes Study Material)
Most organic solvents are synthesized chemically. But a few solvents can also be produced commercially by microbial fermentation. Ethanol, although produced by fermentation for beverages and gasohol, industrial alcohol for use as a solvent is mostly produced chemically. The production of acetone and butanol by fermentation was discovered by Chaim Weizmann, a Polish-born chemist working in England.
The microbial production of acetone and butanol uses anaerobic Clostridium spp. The fermentation process uses the conversion of starch to acetone by C. acetobutylicum. Another species, C. saccharoacetobutylicum is able to convert the carbohydrates in molasses to acetone and butanol. These bacteria first synthesize acids (acetic and butyric) which are then converted to acetone and butanol.
The solvents produced by fermentation are recovered by distillation. Glycerol is an important solvent in flavoring and food coloring and is also used in the production of explosives and propellants. Through fermentation, glycerol production in Germany was an important factor during World War I. Microbial production uses the addition of sodium sulphite to a yeast-ethanol fermentation process. Sodium sulphite reacts with CO2 to produce sodium bisulphite, which prevents the reduction of acetaldehyde to ethanol. Saccharomyces cerevisiae and bacteria such as Bacillus subtilis are used. (BSc Industrial Microbiology Notes Study Material)
Synthetic fuels produced by microbes should help meet the energy- crisis the world over. Useful fuels produced by microbes include ethanol, methane, hydrogen, and hydrocarbons. Right strains are able to do the job. For microbial production of fuels, waste materials such as sewage and municipal garbage are used as fermentation substrates. (BSc Industrial Microbiology Notes Study Material)
Ethanol production by microbes has become very popular in those areas where plant residues (agricultural and other wastes) are available in abundance. Brazil produces and uses large amounts of ethanol as automotive fuel. Gasohol, a 9: 1 blend of gasoline and ethanol, has become popular fuel in the USA. Despite some problems with the ethanol-fuel, several processes are employed for its commercial production. The most efficient microbes are Zymomonas mobilis (fermenting carbohydrates and producing alcohol twice as rapidly as yeasts) and Thermoanaerobacter ethanolicus, a thermophilic bacterium. Corn sugar and plant starch are used as substrates.
A two-step fermentation is used for the conversion of cellulose to ethanol, (i) conversion of cellulose to sugars, generally by Clostridium spp, followed by (ii) conversion of these sugars to ethanol by yeasts, Zymomonas or Thermo- anaacrobacter spp.
Methane produced by methanogenic bacteria is also another potential energy source. Methane is used for the generation of mechanical, heat and electrical energy Anaerobic decomposition of waste materials produces large amounts of methane.
Biogas, a mixture of different gases is produced by anaerobic microbes using domestic and agricultural wastes. The bulk (about 50-70%) of biogas is CH4 and other gases are in low proportions. These include CO2 (25-35%), H2 (1-5%), N2 (2-7%) and O2 (0-0.1%). In India, a large number of gobar gas plants are already in operation in rural areas. Leftovers of these plants are good food fertilizers also.
Animal waste is first hydrolyzed by hydrolytic bacteria. It is followed by acid formation by a group of acetogenic bacteria, which convert monomers into simple compounds like NH3, CO2, and H2. Finally, methanogens reduce acetate and/or CO2, to CH4. In India, cattle dung is the chief source of biogas. (BSc Industrial Microbiology Notes Study Material)
Other fuels include hydrogen which could be developed as a major fuel produced by microbes in the future. Photosynthetic microbes produce H. They are able to convert solar energy into a fuel that can be stored. The photoproduction of hydrogen is very attractive. Some higher molecular weight hydrocarbons are produced by some algae. However, a thorough understanding of the basic mechanisms of a microbial hydrocarbon formation and the formation of petroleum deposits should permit the development of genetically engineered microbes and fermentation processes to produce synthetic sources of petroleum hydrocarbons.