Bacteria and its Etiological Agents | Microbiology

Here is a list of some of the bacteria and its specific etiological agents which causes diseases in man and animals. Also learn about its prevention and control.

Corynebacterium:

The members of the genus are slender, straight to slightly curved rods, with irregularly stained segments or granules. Frequently show pointed or club-shaped swellings at the ends. Snapping division produces angular and palisade (picket-fence) arrangements of cells. Gram-positive to variable, sometimes young cells and sometimes old cells being Gram-negative. Granules invariably Gram-positive.

Generally quite aerobic, but microaerophilic or even anaerobic species occur. Some pathogenic species produce a powerful exotoxin. The group is widely distributed in nature. The best-known species are parasites and pathogens on man and domestic animals. Several species are well-known plant pathogens; still other common species are found in dairy products, water, and soil.

Corynebacterium Diphtheriae:

Cells rod-shaped, varying greatly in size from 0.3 to 0.8 by 1 to 8 μ, occurring singly. Rods straight or slightly curved, frequently club-shaped at one or both ends. As a rule, rods do not stain uniformly with methylene blue but show alternate bands of stained and unstained material, and one or more metachromatic granules which are best shown by special stains. Non-motile. Gram-positive.

Disease Produced – The cause of diphtheria in man. Pathogenic for guinea pigs, kittens, and rabbits. The organism localizes in the throat, where it produces a powerful extracellular toxin which is absorbed into the blood stream and may cause death unless neutralized by antibodies known as antitoxin.

Diphtheria is an acute, febrile infection, generally of the nose, throat, and tonsils. The throat becomes considerably inflamed, especially the fauces, where a grayish false membrane is formed. The membrane may eventually spread to the entire respiratory tract. The toxic action of the organism results in the destruction of the superficial layer of cells.

This is followed by the exudation of a plasma-like fluid which clots and covers the surface of the injured mucous membrane with a tough elastic network of fibrin in which are embedded dead cells and bacteria. The toxin elaborated by the organisms produces an injurious action on the kidneys and muscles of the heart. Injury to the heart is probably the most important action of the toxin.

Carriers –  Convalescents usually harbor the organisms for 3 to 4 weeks, after which the bacteria gradually disappear. Three negative throat examinations are generally required before a convalescent is released from quarantine.

However, studies on the bacterial flora of normal throats have revealed the presence of a surprisingly high percentage of persons who habitually have the diphtheria organisms in their throats. These are chronic carriers and of no importance unless the organisms are of the virulent type. Whether they are or not can be determined in the laboratory by performing virulence tests on animals.

Diagnosis – A sterile swab is rubbed over the tonsillar region of the throat or the pharynx and then streaked over the surface of coagulated serum known as Loeffler’s medium. The swab is also rubbed over the surface of a glass slide and stained with methylene blue.

The culture is incubated at 37°C. for 24 hr., and the slide is examined under the microscope for the presence of characteristic barred and granular rods. The slide examination gives a preliminary idea of what to expect from the culture.

Since the culture on blood serum will show the presence of many kinds of organisms, it is necessary to streak some of the mixed growth over the surface of a solid medium contained in a Petri dish to obtain C. diphtheriae in pure form. An excellent preparation for this purpose is known as cystine-tellurite medium.

Typical colonies of C. diphtheriae on this medium are opaque and dark gray or black in colour. The organisms are capable of absorbing the potassium tellurite and reducing it to the coloured metal. Characteristic colonies are transferred to tubes of Loeffler’s coagulated serum medium, incubated at 37°C. for 24 hr., then tested for virulence.

Virulence Test – The method generally employed is the intracutaneous test, performed as follows: Two guinea pigs are used, one being the control. The abdomens of the animals are shaved. The control pig is given 250 units of diphtheria antitoxin intraperitoneally. The growth from a 24-hr. culture of the organism to be tested is emulsified in about 20 ml. of salt solution and 0.15 ml. injected intracutaneously into each pig.

If the culture contains virulent diphtheria bacilli, it will produce toxin which will have no effect on the immunized pig but will produce in the other pig a definite local inflammatory lesion in 24 hr. which becomes necrotic in 48 to 72 hr.

Schick Test –  This test is employed to determine the susceptibility of an individual to diphtheria. The test is performed as follows: Diphtheria toxin is diluted so that 0.1 ml. contains one-fiftieth of the minimum lethal dose (M.L.D.) required to kill a 250-gm. guinea pig in 96 hr.

One-tenth milliliter of toxin so diluted is injected intracutaneously in the arm. The same amount of heated toxin is injected in the other arm as a control. The injected areas are examined daily for several days. Usually the fourth day gives the most reliable readings.

Mycobacterium:

The genus includes a number of species which differ from the great majority of bacteria in containing a higher content of waxy or fatty substances. This material is stained with difficulty but when once stained, resists de-colourization with acid. Because of this fact, they are called acid-fast organisms. The members produce diseases characterized by the presence of nodules or tubercles in various organs.

Organisms grow as slender, straight, or slightly curved rods; occasionally slender filaments, but branched forms rarely seen. Non-motile. Aerobic. Non-spore-forming. Gram-positive.

Mycobacterium Tuberculosis:

Rods, 0.3 to 0.6 by 0.5 to 4 μ, straight or slightly curved, occurring singly and in occasional threads. Sometimes swollen, clavate, or even branched. Stain uniformly or irregularly, showing banded or beaded forms. Acid-fast. Gram-positive. Growth in all media slow, requiring several weeks for development. Contains mycolic acid, to which it owes its acid-fastness.

Disease Produced – The cause of tuberculosis in man, monkey, dog, and parrot. Experimentally very pathogenic for guinea pigs but not for rabbits, cats, goats, oxen, or domestic fowls.

Disease characterized by insidious onset with parenchymal pulmonary infiltration, recognizable by X-ray examination before constitutional symptoms or physical signs appear. Pleurisy almost always first symptom. Advanced disease accompanied by cough, fever, fatigue, and loss in weight.

Diagnosis – Suspected material such as sputum, urine, feces, cerebrospinal fluid, or stomach contents is examined for the presence of tubercle bacilli. Sputum may be smeared on a slide and stained by the Ziehl-Nelsen acid-fast technique. The presence of typical organisms is usually indicative of infection with the tubercle bacilli.

It is usually advisable to concentrate the tubercle bacilli before making laboratory tests. 

The infected material is treated with 3 percent sodium hydroxide to digest the sputum, pus, or other material with which the organisms are mixed. The digested material is then neutralized with acid and centrifugated.

Only the sediment is retained. The sediment may be used for – (i) preparation of smears for staining and direct microscopic examination, (ii) injection into animals, or (iii) inoculation of culture media, Animal Inoculation. A guinea pig is inoculated in the groin or the muscle of the thigh with some of the sediment.

Enlargement of the regional lymphatic glands occurs in 2 or 3 weeks, and the animal usually dies in about 6 to 8 weeks. On autopsy, the animal shows necrotic areas in the liver and spleen and enlarged lymph nodes filled with caseous material. The lungs and kidneys are rarely attacked.

Inoculation of Culture Media – Some of the sediment may be used for the inoculation of appropriate culture media. A variety of media may be used, consisting largely of egg, glycerin, and some dye to kill or inhibit the growth of contaminating organisms. The media are tubed, slanted, then sterilized by heat. During the heating process, the egg albumin is coagulated, producing a solid medium.

Use of Tuberculins – A large number of tuberculins are available. They are prepared in different ways but consist of filtrates of liquid cultures of the tubercle bacillus. They contain certain products liberated after the death and disintegration of the organisms.

The first tuberculin preparation, known as “Koch’s old tuberculin,” is prepared as follows: The organisms are cultivated in a slightly alkaline 5 per cent glycerin peptone broth for 6 to 8 weeks. The culture is concentrated in a water bath, heated to 80°C., until reduced to one-tenth of its original volume. The culture is then filtered to remove bacterial debris. The clear filtrate contains the tuberculin.

Tuberculins are used to test the sensitiveness of persons or animals to proteins of the tubercle bacillus. By injecting tuberculin intradermally, a positive test appears in 6 to 8 hr., reaches a maximum in 24 to 48 hr., and generally subsides in 6 to 10 days. It is characterized by a reddening of the skin about 1 cm, in diameter. The reaction is positive in those having active or healed lesions. Since most individuals have healed tuberculous lesions, the test is of limited value.

Source of Infection – Discharges from lesions or articles freshly soiled with discharges, the most important being sputum. Of less importance are discharges from the intestinal and genitourinary tracts, or from lesions of the lymph nodes, bone, and skin. Milk from tuberculous cattle.

Prevalence – One of the most common communicable diseases of man; endemic in practically all populations and races. Incidence and mortality rates declining. Mortality highest among infants and adults beyond middle age.

Mode of Transmission – Through discharges of the respiratory tract, less frequently through discharges of the digestive tract; by inhaling droplets expelled during coughing, sneezing, talking, or singing; by kissing; by use of contaminated eating and drinking utensils; by contaminated dust, flies, etc. Infection results usually from continued contact with an infected individual.

Incubation Period – Variable, depending upon type of disease, age, etc. Probably not less than 1 month, usually much longer.

Susceptibility and Immunity – Susceptibility to infection is general; higher in children under three years of age, lowest from three to twelve years, then relatively high for the remainder of life. Greater in undernourished, fatigued, and neglected persons than in well-fed and well-cared-for persons.

Natural immunity to disease generally negative.

Prevention and Control – Education of public to danger of tuberculosis, mode of spread, and methods of control. Avoid overcrowding. Improve working and living conditions. Isolation and treatment of infected individuals. Pasteurization of milk and milk products. Separation of babies from tuberculous mothers at birth. Slaughtering of tuberculous cattle. Tuberculous patients should be prohibited from handling foods for public consumption.

Disinfection of sputum and articles soiled with it, such as handkerchiefs, towels, paper, and eating and drinking utensils. Disinfection of rooms previously occupied by tuberculosis patients.

Vaccination of uninfected individuals with BCG (Bacillus Calmette-Guerin) may be useful. Cure based largely on rest, good wholesome food, fresh air, sunshine, and freedom from worry. Surgical, chemotherapeutic, and antibiotic therapy are all supplementary and cannot be substituted for the basic rest treatment.

Combination of streptomycin and para-aminosalicylic acid (PAS) is commonly employed in adults for 6 months to a year or more. Isoniazid either alone or in combination with PAS or streptomycin, or both, is also useful.

Mycobacterium Bovis:

Rods shorter and more plump than the human variety, measuring 1 to 1.5 μ. Short rods often mixed with longer forms. Cells may stain solidly or show a banded or beaded appearance. Less easily cultivated than human species. Acid-fast. Gram-positive.

Disease Produced – The cause of tuberculosis in cattle. May be transmitted to man and domestic animals. Pathogenic for ox, man, monkey, goat, sheep, pig, cat, parrot, cockatoo, and other birds. Experimentally highly pathogenic for rabbit and guinea pig. More highly pathogenic for animals than human type.

Source of Infection – Tuberculous cows may eliminate organisms in feces, urine, and milk. Milk from infected udders may contain enormous numbers of the organisms.

Mode of Transmission – Infection spread to healthy cows through milk, urine, and feces of diseased cows. Children, particularly those under five years of age, may become infected by drinking contaminated milk from diseased cows.

Susceptibility and Immunity – Susceptibility to infection is general; higher in children under three years of age, lowest from three to twelve years. Natural immunity to disease generally negative.

Prevention and Control – Education of public to danger of tuberculosis, mode of spread, and methods of control. Isolation and treatment of infected individuals. Pasteurization of milk and milk products. Slaughtering of tuberculous cattle. Tuberculous patients should be prohibited from handling foods for public consumption.

Disinfection of discharges and articles soiled with such discharges. Disinfection of rooms previously occupied by tuberculous patients. Vaccination of uninfected individuals with BCG may be useful. This is a bovine strain rendered avirulent by cultivating for many generations on a bile-glycerol-potato medium.

This change appears to be permanent, the loss of virulence not being restored when the organisms are again transferred to the usual culture media. Cure based largely on rest, good wholesome food, fresh air, sunshine, and freedom from worry. Surgical, chemotherapeutic, and antibiotic therapy are all supplementary and cannot be substituted for the basic rest treatment.

Combination of streptomycin and para-aminosalicylic acid (PAS) is commonly employed in adults for 6 months to a year or more. Isoniazid either alone or in combination with PAS or streptomycin, or both, is also useful.

Neisseria:

Genus includes a number of important Gram-negative cocci. Cells occur in pairs with adjacent sides flattened. Growth on nonenriched media may be poor. Aerobic or facultatively anaerobic. Parasites of animals so far as known.

Neisseria Gonorrhoeae:

Cells spherical, measuring 0.6 to 1 μ, occurring singly and in pairs, the sides flattened where they are in contact and are usually described as coffee-bean-shaped.

Disease Produced –  N. gonorrhoeae is the causative agent of gonorrhea in man. The organism attacks chiefly the human urethra in both sexes, producing an acute catarrhal condition. There is a marked tendency for the infection to spread, producing in the male epididymitis, prostatitis, cystitis, and other inflammatory conditions. In the female, the entire genitourinary tract may be involved, including the cervix.

The organism may also invade the blood stream and be carried to various parts of the body. The organism shows a predilection for the synovial membranes of the joints, producing gonorrheal rheumatism, and for the heart valves, causing endocarditis. Organisms may persist for many years and are probably never completely eliminated.

Ophthalmia Neonatorum:

This is an inflammation of the conjunctiva in the new born which is a consequence of maternal infection. Neisseria gonorrhoeae, the most common cause of blindness. Approximately 10 percent of all cases of blindness are due to gonorrheal infections. This is easily prevented by instillation of one or two drops of 2 percent silver nitrate in the eyes of the newborn.

Diagnosis – Direct smears are prepared from the urethral or conjunctival discharges and stained by the Gram method. The smears are examined for the presence of Gram-negative intracellular diplococci slightly flattened at their adjacent surfaces.

The organisms may be cultivated by streaking some of the urethral pus over the surface of a blood agar plate and incubating the plate in an atmosphere containing 10 per cent of carbon dioxide. This may be approximated by placing the plate in a screw-cap jar with a lighted candle and replacing the cover.

Oxidase Test – The gonococcus produces an oxidase which causes a 1 per cent solution of dimethyl-paraphenylenediamine to turn first pink, then rose, magenta, and finally black. The test is made by flooding a 24-hr. agar plate culture with the reagent and noting the development of the above series of colours.

Source of Infection – Purulent venereal discharges; blood and pus from infections of the conjunctiva and joints.

Mode of Transmission – Infection in almost every case is transmitted by direct contact from person to person (sexual intercourse); rarely by direct contact with articles freshly soiled with discharges of such persons.

Incubation Period – From 1 to 14 days, usually 3 to 5 days.

Susceptibility and Immunity – Susceptibility is general. An attack of gonorrhea produces very little, if any, immunity.

Prevention and Control – Adequate diagnostic facilities for identification of the gonococcus. Public facilities for prompt and adequate treatment of infected persons. Education of public on mode of
transmission and how to avoid infection. Control or elimination of houses of prostitution. Personal hygiene before or immediately after promiscuous sexual intercourse.

Disinfection of discharges from lesions and articles soiled with such discharges.

Specific treatment consists in administration of procaine penicillin in one intramuscular injection.

Pasteurella:

The pasteurellae are small, ellipsoidal to elongated rods which show bipolar staining by special methods. Gram-negative. Facultatively anaerobic. May require low oxidation-reduction potential on primary isolation. Parasitic on man, other animals, and birds.

Pasteurella Pestis – Rods, 1 by 2 μ, occurring singly. Characteristic bladder, safety-pin, and ring involution forms. Non-motile. Polar staining. Gram-negative.

Disease Produced – The causative organism of plague in man, rats, ground squirrels, and other rodents. Infectious for mice, guinea pigs, and rabbits. Human plague may be of three types – (i) bubonic, (ii) pneumonic, and (iii) septicemic.

Bubonic plague is a severe and highly fatal disease, characterized by high fever, weakness, buboes, sometimes pustules and subcutaneous hemorrhages. The bacteria are carried through the blood and lymph vessels to the lymph glands in the groin, armpits, neck, etc.

The bacteria multiply, produce pus, and cause an enlargement of the glands. The glands may ulcerate and discharge their contents. The enlarged glands are referred to as buboes, and the infection is known as bubonic plague.

Pneumonic plague gives the picture of a virulent septic pneumonia. The lungs become engorged, and hemorrhages appear under the pleura. Bacteria are found in large numbers in the peribronchial lymph spaces and in the adjoining alveoli.

The disease may be mild, or it may take an acute septicemic form which generally produces rapid death.

Diagnosis – Blood cultures prove positive in about 30 per cent of cases. Direct smears can be prepared from open buboes, or such material can be inoculated into culture media. If the material is contaminated, it can be purified by inoculating a guinea pig and isolating the plague bacillus from the heart’s blood. In the pneumonic type, the organisms are usually present in large numbers in sputum and may be recognized by direct smear.

Source of Infection – Organisms found in buboes, blood, pleural effusion, spleen, and liver of infected persons and rodents. In addition, organisms present in the sputum in cases of pneumonic plague. The infection usually reaches man through contact with diseased rats.

Mode of Transmission – Transmitted from rat to rat and from rat to man by infected rat fleas, the most important of which are Xenopsylla cheopis and Ceratophyllus fasciatus. The flea becomes infected by feeding on a diseased rat. The flea next feeds on a person and at the same time deposits feces and possibly some regurgitated blood. The bacilli are then rubbed into the skin by scratching.

Incubation Period – Usually 3 to 6 days.

Susceptibility and Immunity – Susceptibility to disease is general. Immunity after recovery is temporary and relative. Active immunization with a plague bacterin may confer protection for some months. Repeated stimulating injections are necessary.

Prevalence – Rarely occurs in North America. Sporadic cases occur where persons are exposed to infected wild rodents. Disease distributed in various parts of the world.

Prevention and Control – Rat extermination, ratproofing, and other necessary measures. Ratproofing of ships.

Isolation of Infected Individuals – Disinfection of sputum and articles soiled with sputum in pneumonic type. Terminal disinfection of walls, floors, and furniture of rooms occupied by patients with plague.

Streptomycin and the tetracyclines are highly effective for all forms of plague when used early. Recurrence of fever during streptomycin therapy may indicate pneumonia caused by Gram-positive cocci. Under such conditions penicillin should be used with streptomycin. Sometimes sulfadiazine used if antibiotics not available.

Pasteurella Tularensis:

Equal numbers of cocci and rods, 0.2 by 0.2 to 0.7 p, occurring singly. Extremely pleomorphic. Non-motile. May show bipolar staining. Gram-negative. Aerobic. Filterable through Berkefeld filters.

Disease Produced – Cause of tularemia in man. Onset of disease sudden with chills and fever. Patient usually prostrated and confined to bed. Fever may last for 3 to 4 weeks, followed by slow convalescence. Lymph glands may become swollen and tender, and suppurate in 50 per cent of infections. Organism infectious for rabbits, guinea pigs, rats, muskrats, water rats, gray mice, ground squirrels, beavers, and lemmings.

Diagnosis –  Disease diagnosed by  (i) animal inoculation, (ii) isolation of organism from lesions or discharges, and (iii) agglutination test with serum from patient.

Source of Infection – Originally isolated from California ground squirrels and later from more than 30 other forms of wild life in the United States and elsewhere. Also from rabbits, hares, deerfly (Chrysops discalis), wood tick (Dermacentor andersoni and D. variabilis), and possibly other biting insects. Hunters, cooks, and butchers may contract the disease during rabbit-hunting season.

Mode of Transmission – Transmitted by bites of infected flies and ticks and by inoculation through handling of diseased animals, especially in skinning rabbits during the hunting season. Ingestion of insufficiently cooked meat from a diseased animal. Also from drinking contaminated water.

Incubation Period – From 1 to 10 days, usually about 3 days.

Susceptibility and Immunity – All ages susceptible. Recovery from an attack followed by permanent immunity. An immune person exposed to the disease may develop a local infection through a break in the skin, but this does not cause any constitutional disturbance. Disease most prevalent during the hunting season.

Prevention and Control – Avoidance of bites from, or handling of, flies and ticks when working in infected areas during bloodsucking season. Use of rubber gloves in the handling and dressing of rabbits or in performing autopsies on animals likely to be infected. Meat from wild rabbits and other susceptible animals should be thoroughly cooked before eating. Water from infected areas should not be used unless first boiled or disinfected.

Disinfection of discharges from ulcer, lymph glands, or conjunctiva of infected individuals.

Streptomycin, the tetracyclines, and chloramphenicol effective when continued for 4 to 5 days after temperature becomes normal.