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Bacillus
ORGANISM:
- Genus: Bacillus
- Species: anthracis, cereus
GENERAL CONCEPTS:
- At least 48 species are known but only B. anthracis and B. cereus cause disease in humans.
- B. anthracis is responsible for the disease anthrax. This is a disease primarily of animals but humans can acquire via handling, inhaling or ingesting contaminated animal products.
- B. cereus is predominantly responsible for food poisoning in humans.
DISTINCTIVE PROPERTIES:
- Members of the genus Bacillus are Gram-positive, rod-shaped, spore-formers that require oxygen. However, this is a very diverse group of organisms and some species are actually Gram-negative or facultative.
- B. anthracis produces a single antigenic type of capsule and several exotoxins.
- B. cereus produces enterotoxins that causes food poisoning.
PATHOGENESIS:
- Anthrax infections result only if the bacteria produce a i) capsule and ii) exotoxins. The capsule allows the bacteria to survive phagocytosis. Three exotoxins (all of which are required for virulence) include:
- edema factor (adenylate cyclase)
- protective antigen factor
- lethal factor.
- Anthrax infections are classified by route of entry:
- Cutaneous anthrax; Bacillus spores enter the skin through a cut or animal bite and germinate. A small red lesion develops after 1-7 days, eventually producing local necrosis (the "black eschar"). Spread of the bacteria causes regional lymph tenderness which may be followed by a toxic septicemia and death. Only about 5% of cutaneous infections become septic.
- Inhalation anthrax; Bacillus spores are inhaled and ingested by aveolar macrophages. These cells carry the bacteria to the regional lymph nodes, causing necrotic hemorrhaging which leads to death.
- Gastrointestinal anthrax; ingestion of contaminated meat produces systemic symptoms which can lead to death. Mortality by gastrointestinal anthrax may be 50%.
- B. cereus food poisoning results from the ingestion of preformed enterotoxins, producing predominantly vomiting and diarrhea. The vomiting form is most often associated with ingestion of a heat stable toxin from contaminated rice, while the diarrheal form is most often associated with ingestion of a heat labile toxin from contaminated meat or vegetables.
HOST DEFENSES:
- Animals with high level phagocytic activity are more resistant. The capsule of B. anthracis is poorly immunogenic.
- Specific immunity can result due to the production of antitoxin Ab (if the affected animal/human survives).
EPIDEMIOLOGY:
- Bacillus species are worldwide soil saprophytes. The spores can survive 60 or more years under harsh environmental conditions.
- Most human anthrax infections occur in persons whose occupation brings them in contact with infected animals or their products. For example, woolsorters and hidehandlers are the most frequently affected.
- There are approximately 2000-5000 cases of human anthrax worldwide per year and 95% of these are of the cutaneous form.
- B. cereus outbreaks occur sporadically. There have been outbreaks associated with Chinese restaurants due to the way in which their fried rice was prepared.
DIAGNOSIS:
- Clinical: Cutaneous anthrax may be suspected upon observing the characteristic "black eschar" lesions. Inhalation and gastrointestinal anthrax are very difficult to diagnose based solely on clinical presentation.
B. cereus food poisoning may present in two different forms: the vomiting form occurs within 1-6 hours (average 2 hours) following ingestion while the diarrheal form occurs from 8-12 hours (average 9 hours) following ingestion.
- Laboratory: A Gram stain of lesion material or feces can indicate the presence of these Gram-positive bacteria. Immunofluorescence techniques are also available.
CONTROL:
- Sanitary: Decontamination of infected animal products, deep burial of animal carcasses and the use of protective clothing can reduce the incidence of anthrax. Proper food handling, preparation and storage are essential to preventing food poisoning.
- Immunological: An avirulent spore vaccine for animals and those at high risk is available against anthrax.
Chemotherapeutic: Penicillin, erythromycin or tetracycline are drugs of choice for anthrax.
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