Chapter Capsule

A. Endosporeformers: Highly resistant spores are produced.

1. The aerobic genus Bacillus anthracis causes anthrax, a zoonosis of herbivorous animals; the bacterium inhabits the soil, where it is picked up by grazing animals; humans are infected by contact with animals or their other products; the cutaneous form is manifest by eschar, a black ulcer at the site of entrance; the pulmonic form, acquired by inhaling spores, is highly fatal. Anthrax is controlled by antibiotics and animal vaccination.

2. The anaerobic genus Clostridium is common in soil and spread throughout most habitats, even the human body; wound and tissue diseases are due to introduction of the bacterium into damaged tissues; food intoxications arise when pathogenic clostridia grow in food; the basis for disease is powerful exotoxins that act upon specific target tissues.

a. Cl. perfringens causes gas gangrene, a soft-tissue and muscle infection (myonecrosis) in areas of damage caused by injuries or diabetes; spores germinate in anaerobic tissues and produce gas bubbles; may invade nearby healthy tissue; toxin necrotizes muscle and other tissue; treatment is by careful tissue debridement and cleansing, hyperbaric oxygen, and antibiotics.

b. Cl. difficile causes antibiotic-associated colitis, an acute diarrheal disease associated with disruptions in the GI tract flora due to antimicrobic therapy.

c. Cl. tetani is the cause of tetanus or lockjaw, a disease associated with dirty puncture wounds or lesions; spores in anaerobic areas of the wound grow and produce a toxin called tetanospasmin, which travels to target cells in the CNS; causes severe uncontrollable spasms of large muscles; treatment is by antitoxin; control is through vaccination with tetanus toxoid.

d. Clostridial food poisoning occurs when Cl. perfringens is eaten with inadequately cooked meats and fish, producing toxin in the intestine; marked by severe diarrhea, vomiting; not usually fatal.

e. Botulism, caused by Cl. botulinum, is associated with improperly home-canned foods; spores withstand food processing, grow in stored food, and produce botulin. Botulism is a true food intoxication; ingested toxin enters circulation and acts on myoneural junctions to block muscle contraction, leading to flaccid paralysis; treated by antitoxin; control by proper canning, heating of foods prior to eating.

f. Infant and wound botulism are unusual types of infectious botulism contracted like tetanus.

B. Non-spore-forming Rods: Straight, regular rods and irregular rods.

1. Straight, nonpleomorphic rods stain evenly. Genera that are regular in morphology include Listeria monocytogenes and Erysipelothrix rhusiopathiae.

a. L. monocytogenes are widely distributed resistant bacteria that cause listeriosis; primary habitat appears to be water, soil, and the intestines; most cases are food infections associated with contaminated dairy products and meats; disease is mild, self-limited in young adults, but may be severe and complicated in fetuses, neonates, the elderly, or the immunocompromised; treated with penicillin; prevention requires proper pasteurization of milk and dairy products.

b. E. rhusiopathiae is carried by animals (swine) and widely dispersed into the environment; cause of erysipeloid, a disease common among occupations that handle animals or animal products; symptom is inflamed red sores on fingers at portal of entry.

2. Irregular, non-spore-forming rods include Corynebacterium and Propionibacterium.

a. C. diphtheriae is a highly pleomorphic aerobic rod with metachromatic granules and palisades arrangement; agent of diphtheria, a disease whose reservoir is healthy human carriers; spread by droplets; occurs first as a localized infection of the throat with tenacious pseudomembrane that may cause suffocation; later symptoms are due to a classic toxemia; toxin spreads in blood to targets; may damage heart and nervous system; treated by antitoxin, penicillin (erythromycin); controlled by vaccination (DTaP).

b. Propionibacterium acnes is an anaerobic rod found in the skin of humans; is regularly associated with acne vulgaris lesions; acts on skin oils to create an inflammatory condition that erupts into pimples.

3. Acid-Fast Bacilli (AFB): Contain large amounts of mycolic acids and waxes; are strict aerobes; slender filamentous rods; widely distributed; tend to be resistant to environmental conditions.

a. Mycobacterium tuberculosis causes tuberculosis (TB), a common lung infection; tubercle bacillus is spread by droplets in close quarters; susceptible persons have weakened immunities and poor nutrition and living conditions. The disease is divided into (1) primary pulmonary infection, often marked by formation of granulomas in the lungs called tubercles; (2) secondary TB, a severe lung complication that occurs in people with reactivated primary disease; and (3) disseminated extrapulmonary infections due to blood-borne TB bacilli carried to bone, kidneys, lymph nodes, or brain; TB diagnosed by tuberculin testing, chest X rays, acid-fast stain for bacilli in sputum, and laboratory cultivation and identification; managed by combined drug therapy, BCG vaccine in some countries.

b. M. leprae causes leprosy, a chronic disease that begins in skin and mucous membranes and progresses into nerves; prevalent in endemic regions throughout the world; spread through direct inoculation from leprotics; two forms are (1) tuberculoid, a superficial infection without skin disfigurement which damages nerves and causes loss of pain perception, and (2) lepromatous, a deeply nodular infection that causes severe disfigurement of the face and extremities. In both forms, mutilation of parts may occur due to loss of pain receptors; treatment is by long-term combined therapy.

c. NTM (non-tuberculous mycobacteria) are common environmental species that may be involved in disease. For example, M. avium complex (MAC) causes a disseminated disease of AIDS patients, and M. marinum causes swimming pool granuloma.

A. Actinomycosis: A. israelii occurs in the human oral cavity, tonsils, intestine; oral disease or surgery may result in cervicofacial infection, also abdominal, thoracic, uterine complications.

B. Nocardiosis: N. brasiliensis causes pulmonary disease similar to TB, with abscesses and nodules that may spread to chest wall; one agent of mycetoma.