BSc 2nd Year Microbiology Bacterial Diseases of Man Notes Study Material

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BSc Bacterial Diseases of Man Notes Study Material

Airborne Diseases

The airborne bacterial diseases are mainly those of the respiratory tract. They include important diseases as tuberculosis, diphtheria, whooping cough and primary atypical pneumonia. Usually these diseases are spread where people are crowded and living conditions are not proper.

[I] Tuberculosis

This is a disease of crowded populations, especially of lower socioeconomic groups. The causal organism is Mycobacterium tuberculosis. Often one member of a family is a source of infection to others.

The organism is a small rod that enters the respiratory tract and grows on the lung tissue. The symptoms are chronic cough, chest pains, high fever and a flow of thick expectorated matter called sputum. The latter may be rust-colored if the blood has entered the lung cavity. The incubation period is approximately 2-10 weeks for the primary infection, but six months may pass before the symptoms become fully recognised. In the infected lung, cells of macrophages, leukocytes and T-lymphocytes surround the parasite, and due to deposition of calcium salts and fibrous materials, a hard nodule-tubercle is formed. This may be visible in the patient’s chest X-ray: The tubercle may break apart and spread bacteria to liver bones and kidneys. The disease is then called miliary tuberculosis. The bacterium does not produce toxin, but due to rapid growth the tissues are consumed. Due to presence of fatty waxy materials in cell wall, the bacterium is much resistant to environmental changes. The organism is acid-fast, stained with acidfast technique, by Ziehl-Neelsen Carbolfuchsin stain. The cells stain red with this stain.(Microbiology Bacterial Diseases of Man Notes Study Material)

Bovine tuberculosis, caused by Mycobacterium bovis is equally dangerous in cows and humans. The organism is transmitted through milk. Bacille Calmette Guerin (BCG) is a preparation of an attenuated strain of M. bovis that is used in immunization programmes throughout the world. It is named for Albert Calmelle and Camille Guerin who developed it in the 1920s.

[II] Diphtheria

This is caused by Corynebacterium diphtheriae, first recognised in a material from a patients’ throat in 1883 by Edwin Klebs. The bacterium is club-shaped associated with leathery membranes, is Gram-positive with numerous metachromatic granules in the cytoplasm. The disease is a major health problem in India. The organism is inhaled in respiratory droplets and infects the upper respiratory tract near the tonsils. It grows rapidly and produces an exotoxin, the third dangerous chemical after botulism and tetanus toxins. The toxin interferes with the protein metabolism of cells as mucus, and leukocytes. The dead tissue accumulates forming a dirty grey pseudomembrane. The material is leathery and fibrous. In young children, there may be respiratory blockage.

In adults, the toxin spreads to blood stream causing damage to heart muscle, leading to cardiac weakness and heart failure. There may be paralysis. Mumpslike symptoms are common, and a severely swollen neck may be observed.(Microbiology Bacterial Diseases of Man Notes Study Material)

The disease is treated by antibiotics and antitoxins. Penicillins and horse serum toxins are used. Long-term protection is done by injecting diphtheria toxoid, that induces the formation of antitoxins. This is one of the DPT series usually given to infants.(Microbiology Bacterial Diseases of Man Notes Study Material)

[III] Meningococcal meningitis

This is caused by Neisseria meningitidis, a small Gram-negative diplococcus commonly called the meningococcus. The organism enters the body by droplets often from a carrier. It may pass rapidly through the mucuous membranes of upper respiratory tract into the blood stream. Here it multiplies quickly. Due to release of large amount of endotoxins, an endotoxin shock, called meningococcemia takes place. These events may occur within a short period of some hours of infection and substantial vascular damage may cause death. After the blood infection, the bacteria localise on the coverings of spinal cord and brain known as meninges. It causes severe headache and stiffening of the neck. There is also a hemorrhagic rash on the skin. The spots (capillary thromobses) begin as bright red patches, turn to purplish and finally blue-black. The patient is often confused and delirius, and death may occur in 50 per cent of untreated cases.

Earlier treatment is desirable. Sulfonamide drugs, rifampin, and ampicillin are often used. Diagnosis is based on physical symptoms, and observation of Gram-negative diplococci in centrifuged samples of cerebrospinal fluid.

Some cases are complicated by hemorrhagic lesions that occur in the adrenal grands and cause hormonal imbalance and physiological disturbances. This ondition, called the Waterhouse-Friederishsen Syndrome, may result from an allergic reaction involving immune complex formation and activation of complement.(Microbiology Bacterial Diseases of Man Notes Study Material)

[IV] Pneumococcal pneumonia

The word pneumonia in a broad sense includes a number of microbial diseases of the bronchial tubes and lung tissues. About 90 percent of bacterial cases are caused by Gram-positive diplococcus having a capsule and lancent-like pointed ends. This organism, commonly known as pneumococcus, was assigned in older literature to Diplococcus pneumoniae. However, in the latest edition of Bergey’s Manual, it is described as Streptococcus pneumoniae,

It exists in the upper respiratory tract, usually acquired from another carrier by respiratory droplets or contact. The patient becomes usually susceptible when exposed to viral infections, allergic reaction, extensive surgery, malnutrition, excessive smoking or depression of immune system. The disease is characterised by high fever, sharp chest pains and bacterial infiltration of the lung tissue. Due to destruction of alveolar walls, blood seeps into the lung spaces and rust-colored sputum is expelled. If entire lobe of lung is involved, it is called lobar pneumonia. If both sides are infected, it is called double pneumonia. Scattered patches of infection in bronchial tree yield bronchopneumonia. Penicillin is the common drug.(Microbiology Bacterial Diseases of Man Notes Study Material)

The bacterium produces several virulence factors-hemolysin, leukocidin and hyaluronidase. Due to presence of capsule, it resists phagocytosis. On the basis of several serological tests, about a dozen of variants have been identified, which cause human infections.(Bacterial Diseases of Man Notes Study Material)

The bacteria may be identified by isolating from the sputum and testing for alpha hemolysis on blood agar. The organism is also identified by bile-solubility test. A test known as the Neufeld Quellung reaction helps to establish the capsular type.

[V] Whooping cough (Pertussis)

This is a serious disease of young children. The bacteria grow in the lining of upper respiratory tract, causing disintegration of cells and their accumulation in the airways. Due to blockage of bronchi with mucus and debris, difficulty in breathing occurs leading to oxygen starvation or hypoxia. The high-pitched “whoop” on rapid inspiration results from the narrowing of tubes. A child may suffer ten to fifteen attacks of coughing during the day and generally falls asleep exhausted.(Microbiology Bacterial Diseases of Man Notes Study Material)

The causal organism, Bordetella pertussis, was first isolated by Jules Bordet and Octave Gengou in 1906 and became known as Bordetella-Gengou bacillus. It is a small, fragile, Gram-negative rod. The bacterium passes from one person to another by respiratory droplets. Early treatment with penicillin derivatives and erythromycin is recommended, as prophylactic measures. The vaccine is produced by killing Bordetella pertussis with merthiolate in a manner first described by Pearl Kendrick and Grace Eldering in 1939. The intact bacteria were then mixed with diphtheria and tetanus toxoids in the DPT preparation.

[VI] Primary atypical pneumonia (PAP)

The organism was located in the early 1940s by Monroe Eaton at Harvard University, as a tiny viruslike agent on agar media supplemented with blood. The organism was named as Eaton agent, and the disease described as primary (as was not due to any secondary complication of previous infection) atypical (as the amount of fluid in the lung and involvement of the alveoli was minimal as compared to typical bacterial pneumonia) pneumonia. By 1957 it was found that PPLO (pleuropneumonialike organism) of cows was identical with the Eaton agent of humans, and the name Mycoplasma pneumoniae was given to the causal organism.(Microbiology Bacterial Diseases of Man Notes Study Material)

Mycoplasma pneumoniae is one of the smallest of the free-living pathogenic organisms, measuring about 0.2 um in size, and lacks a cell wall, thus not! susceptible to penicillin. They are pleomorphic, passed by respiratory droplets in crowded conditions. This disease resembles viral pneumonia. There is fever, fatigue, dry cough due to lack of sputum. The disease, however, is not fatal and also called walking pneumonia. Antibodies produced during infection aggultinate human Type O red blood cells at low temperatures. This gave rise to a cold agglutinin screening test (CAST) used to test a patient’s serum, if the disease is suspected.(Microbiology Bacterial Diseases of Man Notes Study Material)

Foodborne and Waterborne Diseases

Some life-threatening diseases will be explored.

[I] Botulism

This is most dangerous of the food-borne diseases. The exotoxin of the bacteria is very powerful. For instance, an ounce of the purified material may eliminate the entire population of many countries. The bacterium, Clostridium botulinum exists as spores in the intestines of many fish, birds and barnyard animals as cows and horses, and also in human intestines. The spores reach the soil in manure, sewage and organic fertilisers and may be attached to harvested foods. When placed in anaerobic conditions as improperly processed cans or jars they germinate and multiply as Gram-positive rods which produce exotoxin.

Bacteria are of little consequence as they do not grow further in the body. However, this is the exotoxin which causes harm. It is a high molecular weight protein, which is activated by the enzyme trypsin in the intestine and is absorbed in the blood stream. Within hours the patient begins to feel paralysing symptoms.

There is blurred vision, impaired speech, difficulty in swallowing and chewing and respiratory distress. The limbs have little tone and become flably, a condition called flaccid paralysis. At the synaptic junction of the nerves and muscles, the toxin inhibits the release of the neurotransmitter, acetylcholine. Nerve stimulation to the muscle therefore ceases and the diaphragm fails to contract. Death by respiratory paralysis may occur within a day or so.

Antibiotics are of no value in botulism, as the patient is intoxicated rather than infected. Large doses of botulism antitoxin are given. There are at least eight types of Clostridium botulinum distributed in soils. In different animals they cause different types of botulism. These are fodder disease in cattle and limberneck in fawl (due to ingestion of toxin from silage and feeds), wound botulism in humans (due to active growth in dead tissue of wounds) and sudden infant death syndrome (SIDS) (due to germination of spores in infants’ intestines).

[II] Clostridial food poisoning

This is the second most common food-borne disease, caused by Clostridium perfringens. The symptoms are due to a toxin that increases the water secretion in the latter part of small intestine, leading to diarrhea. There are abdominal pains but no fever or blood involvement. The toxin may be produced under anaerobic conditions in canned foods as well as the intestine. Death is rare, and severe cases treated with antitoxin and penicillins.

[III] Typhoid fever

It differs from botulism or other food poisonings in that patient ingests bacteria rather than toxins and soon becomes infected rather than poisoned. The causal organism, Salmonella typhi is a Gram-negative rod that is very resistant to environmental conditions. Thus it may remain alive for long periods in fresh water and foods.(Microbiology Bacterial Diseases of Man Notes Study Material)

The chief source of disease is the human carrier. The recovered patient may continue to harbor bacteria which are shed in the feces for long periods. Typhoid Mary (1906) demonstrated this fact. Contaminated sewage is another source. When it mixes with drinking water due to broken sewer lines or natural disasters, epidemics may follow. Flies also transport bacteria from sewage to food or water. Since bacterium is acid-resistant it passes through stomach and enters small intestine, where it multiplies quickly. There it produces deep ulcers, constipation, and bloody stools are the indication of bacterial penetration to blood vessels of intestine walls. The fever rises gradually, and skin covered with bright red rose spots.(Microbiology Bacterial Diseases of Man Notes Study Material)

The disease was previously treated with chloramphenicol. However, resistant strains of bacteria have developed. At present amoxicillin and sulfamethoxazole-trimethoprim have been substituted. The bacterium has some preference for gallbladder, and antibiotics are not effective in this organ. In such cases gall bladder is to be removed. It should be noted that this bacterium is notable exception to the more caustic body fluid (the bile), which inhibits many bacteria otherwise in the body.(Bacterial Diseases of Man Notes Study Material)

Chemically killed bacteria are used in the vaccine for typhoid fever with about 100,000 dead organisms in each dose. The danger of the disease may be ascertained by testing the water samples for some indicator organisms as E. coli, a normal intestinal inhabitant. If these indicators are located in water (due to mixing of contaminated sewage with drinking water), it indicates that Salmonella may also be present and an epidemic may follow. The organism may also be identified by isolating it from urine, blood, stool or bone marrow cultures. In 1896, Fernand Widal devised the Widal test, an aggultination method in which typhoid fever antibodies are detected by mixing the patients’ serum with Salmonella cells and observed for clumping. These days extracts of flagella or cellular antigens of bacterium rather than cells are used for this test.

[IV] Cholera

There is severe dysentery, vomiting and violent cramps with much loss of fluid. Eyes become shrouded in grey and sink into orbits. Skin becomes dry, wrinkled and cold and muscular cramps occur in the arms and legs. There is continuous thirst but patient is unable to hold fluid. The blood is thickened and urine production stops. Without urine, meabolic wastes accumulate and tissues suffer toxic damage. The sluggish blood flow to the brain stops oxygen to this vital organ and person enters into coma and soon dies.

The causal organism, Vibrio cholerae, a Gram-negative rod was first cultivated by Robert Koch in 1883. The bacterium has two recognised biotypes: the classical – V. cholerae and the El. Tor biotype. The latter produces a hemolysin that disrupts sheep red blood cells, but the classical biotype does not. The El Tor also exists in carriers, not found in the classical form. The bacteria must omter the gastrointestinal system in large numbers since the stomach acid kills 1 of the bacteria. The surviving organisms enter the intestine and move about in the mucous coating. They adhere strongly to the tissue and produce toxins. The toxin consists of three chains of protein that stimulate the production of the enzyme, adenyl cyclase. This enzyme increases the level of adenosine monophosphate (AMP) which leads to abundant secretion of fluid by the intestine. The presence of infection also retards water absorption through the wall.

The critical treatment is to restore the water balance in the body. Intravenous injections of tetracycline are useful in control of bacteria. Immunization with preparation of dead bacteria provides protection for about six months. Prevention methods include sanitation, personal hygiene and care in food preparation.

Soilborne Diseases

[I] Tetanus

This is among the most dangerous of human diseases. The bacterium, Clostridium tetani exists in air, water, animal and human intestine and especially the soil. The bacteria enter a wound in very small numbers and produce second most powerful toxin known to science. The toxins provoke sustained and uncontrolled contractions of the muscles, and spasms occur throughout the body: the patient experiences a violent death.

The bacterium exists as spores in the intestines of many animals and man. However, intestinal infection does not occur. The organism is excreted in the feces to the soil, from where the spores are introduced to the anaerobic environment of dead necrotic tissue in a deep puncture. Wound contamination may also occur. Rusty nails are a threat because spores may cling to the rough edges of the nail.(Microbiology Bacterial Diseases of Man Notes Study Material)

The symptoms develop very rapidly. Within hours of entry, the spores revert to vegetative cells which produce several toxins including tetanospansmin, a high molecular weight protein. The toxin spreads through the blood stream and along the axons of the nerve cells to the nerve endings where it inhibits the removal of acetylcholine in the synapse. There is muscle contraction. There is muscle stiffness, especially in the jaw and swallowing muscles. Soon the back arches, the jaws clench and a fixed smile comes over the face. These symptoms give the disease its traditional name of lockjaw.

It is surprising that the bacterium is saprobe rather than parasite as long they live only in dead anaerobic matter. They are motile Gram-positive rods that may exist for many years in soil as spores. The disease incidence is high in India, where ear piercing is common and where umblicial stump of newborns is sometimes dressed with soil.(Microbiology Bacterial Diseases of Man Notes Study Material)

Immunisation to tetanus may be achieved by injecting toxoid consisting of formaldehyde-treated toxin that is precipitated with alum. This is used in the DPT programmes, and booster injections are recommended every ten years. For suffering persons antitioxins, to neutralise the remaining toxins, and penicillins are given.(Microbiology Bacterial Diseases of Man Notes Study Material)

Sexually-transmitted and Contact Diseases

A summary of sexually transmitted and contact bacterial diseases of humans is given in Tables 4 and 5 respectively. We shall explore here only gonorrhea, syphilis, (genital contact), leprosy (skin contact) and urinary tract infections.

[I] Gonorrhea

The causal organism is Neisseria gonorrheae, a small Gram-negative diplococcus named after Albert L.S. Neisser who first isolated it in 1879. The cells are double-bean shaped and is often called gonococcus. It is very fragile and may be easily killed by most antiseptics and disinfectants. It survives only for a brief time on a dry surface. Most transmission is during sexual intercourse. In the female the bacterium invades the epithelial susface of the cervix and urethra. After puberty the vaginal epithelium may resist infection. There is invasion of Bartholin’s glands and blockage of fallopian tube passageway by pus and scar tissue which may lead to sterility. There may be abdominal pain and a burning sensation on urination, and interruption of normal menstrual cycle. However, about 50% of female infections are asymptomatic so that disease may be carried and transmitted unknowingly.(Bacterial Diseases of Man Notes Study Material)

In males, primary infection is in the urethra, characterised by a thin, watery discharge followed by white thick fluid from the penis. There is pain on urination. When epididymis is infected, the flow of sperms is blocked, resulting in sterility. The symptoms are more acute than in females.

The bacterium may also infect eye through fingertip, towel etc. causing corneal infection – keratitis. In homosexuals, rectum and pharynx may become infected. The disease was earlier treated with penicillins. However, there have developed penicillin-resistant strains, which possess plasmids containing genes for the production of penicillinase (an enzyme which destroys penicillin). Such strains are called penicillinase-producing Neisseria gonorrhoe or PPNG. AT present spectinomycin with tetracycline is used.

[II] Syphilis

The disease is caused by Treponema pallidum, a spirochete that moves by axial filaments. This can penetrate skin only on abrasion, wound or hair follicle. Sexual intercourse (direct contact) is the most common form of contact. The disease occurs in three stages:

(a) Primary syphilis. The first stage characterised by the chancre, a circular, purplish ulcer with a raised margin formed at the point of entry of spirochete.  Generally, this occurs on the external genital organs – the penis, vagina, though lesions may appear on skin, lips, rectum or pharynx.

(b) Secondary syphilis. That occurs some weeks later when the organism has spread throughout the body. There are flat wart-like lesions filled with spirochetes over the entire body surface.(Bacterial Diseases of Man Notes Study Material)

(c) Tertiary syphilis. The third stage develops in one quarter to one half of the people. This stage develops after many years. Soft, granular lesions called gummas develop in the cardiovascular and nervous system or on the skin. They may result into paralysis.(Bacterial Diseases of Man Notes Study Material)

The spirochete multiplies very slowly in tissues. Penicillins may control disease satisfactorily at primary and secondary stages. However, gummas appear to be due to an allergic reaction. Treatment must be cautious at this stage.

The spirochete was first observed by Fritz Schaudinn and Evich Hoffman in 1905. It cannot be routinely cultivated in the laboratory. The organism is diagnosed by Treponema pallidum immobilisation (TPI) test. The patient’s serum is combined with spirochetes obtained from an infected rabbit’s testicle. If syphilis antibodies are present in patients serum, they will attach to spirochete and immobilise them. Another test is FTA-ABS, (fluorescent treponemal antibody absorption) test. Several other tests are used for syphilis. These are the VDRL and rapid plasma reagin (RPR) tests. There is also Treponema pallidum hemagglutination test (TPHA). This includes absorption of disrupted spirochetes into surface of sheep red blood cells, and addition of patients’ serum. If syphilis antibodies are present, they will induce agglutination of the red blood cells. The TPI, VDRL. FTA-ABS, TPHA and other tests are useful in early diagnosis of syphilis.

[III] Leprosy (Hansen’s disease)

This is a contact disease which lingers for years, degenerating the tissues and deforming the body. According to WHO, there are over 10 million victims of leprosy in the world, particularly in Southeast Asia, Africa and South America. There is a long incubation period of three to six years during which victims are not easily identified.(Microbiology Bacterial Diseases of Man Notes Study Material)

The causal organism is Mycobacterium leprae, a heat-sensitive organism which prefers nerves and cutaneous tissues. The disease does not generally affect the internal organs. There are two forms.

(a) Lepromatous leprosy. This is characterised by tumor-like growths called lepromas the skin and along the respiratory tract. When patches of infection occur on hands and feet, the skin may lose pigmentation.

(b) Tuberculoid leprosy. It involves the superficial nerves. Due to this muscles undergo atropy. There is disfiguring of skin and bones, twisting of limbs and curling in the fingers to form characteristic claw hand. Often the nasal cartilage and outer ear tissue degenerate. If death occurs, this may be secondary infection, but not due to leprosy alone.(Bacterial Diseases of Man Notes Study Material)

Mycobacterium leprae was first observed in 1874 by Gerhard Hansen Norwegian physician. The organism is, therefore, called Hansen’s bacillus and the disease, Hansen’s disease. The organism has not yet been cultivated out of live tissues. They are being grown since 1974 in the tissues of armadillo. The organism is an acid-fast rod belonging to the genus to which tubercle bacillus. The BCG used for immunization against tuberculosis has been found useful for this disease also. Antibiotics as rifampin, clofazimine and some sulfone drugs as dapsone are also used. Resistance to disease appears to be due to cellular immunity as in tuberculosis. There is a rapid screening test called the lepromin test or Mitsuda reaction. An extract from lepromatous tissue is taken and test performed as in case of tuberculin test. If the person has been sensitised by exposure to the disease, a nodule will develop in three to four days.

[IV] Urinary tract infections

A major cause of such infections is the Gram-negative rod, Escherichia coli. This travels from the intestine to the bladder via either the lymph channels or the urethra. The urine may have over 100,000 bacteria per ml during active phase. Infants, pregnant women and patients using indwelling catheters are mostly susceptible to disease. Bladder infections may be controlled with antibiotics. Other Gram-negative bacilli responsible for UTIs, are Klebsiella pneumoniae, Serratia marcescens, Alcaligenes faecalis and Pseudomonas aeruginosa.

BSc Microbiology Bacterial Diseases of Man Notes Study Material

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