The genus bacillus consists of large gram-positive rods that form spores and grow best under aerobic conditions. Most species are saprobic and are found on vegetation and in soil, water, and air. The only species that is highly pathogenic for man is B. anthracis. B. cereus is responsible for outbreaks of food poisoning.
The anthrax bacillus is unusually large and was the first bacterium shown to cause disease. It is a large gram-positive non-motile spore-forming rod. It is readily cultured on ordinary nutrient media. Anthrax is primarily a disease of domesticated and wild animals. Humans become infected only incidentally when in contact with diseased animals, their hides, hair, or excreta. The anthrax toxin plays a major role in the pathogenesis of disease.
Malignant Pustule - the most common form of anthrax in humans is the cutaneous variety known as malignant pustule. The primary lesion usually develops at the site of a minor scratch or abrasion on an exposed area of the face, neck, or upper extremities into which spores had been inoculated. The spores germinate, and after an incubation period of 2 to 5 days, a papule develops, later becoming a vesicle. Eventually the vesicle breaks down and is replaced by a black eschar. In severe cases of anthrax, the blood stream is invaded and systemic disease develops.
Intestinal anthrax – results from ingestion of poorly cooked meats from infected animals. The clinical picture varies from a severe gastroenteritis to an acute abdomen with abdominal pain, fever, vomiting, bloody diarrhoea, intestinal obstruction and shock). The mortality rate is extremely high.
Diagnosis, treatment and prevention
Anthrax bacilli may be seen in stained smears of the exudate from skin lesions in the vesicular stage. Penicillin is the drug of choice; tetracyline is an alternative. Anthrax still causes heavy loss of livestock in the Middle East, Africa, and Asia. Animal anthrax can be controlled by immunisation with attenuated vaccines.
B. cereus is a common aerobic sporing organism. Its spores are often found in cereals and other foods, some of which will survive cooking and germinate into bacilli, which under warm storage conditions in cooked foods, will multiply and produce toxin. A wide variety of foods has been associated with outbreaks, particularly cornflour sauce in Norway and fried rice in Britain and other countries. As with C. perfringens, long moist storage of warmed cooked food.
There is no serological typing scheme for B. Cereus: certain serotypes occur more commonly in outbreaks than others and there are two different patterns of symptoms. The outbreaks arising from rice typically have a short incubation period of 1-3 hours with sudden onset of acute vomiting and some diarrhoea: the symptoms resembling those of staphylococcal enterotoxin food poisoning. The other pattern resembles C. perfringens food poisoning with an incubation period of 9-18 hours and diarrhoea as the chief symptom. The toxin responsible for the vomiting syndrome, which is probably preformed when the organism grows in food, is extremely heat resistant; it is not destroyed after 1 ˝ hours at 121oC. Other aerobic spore-bearing organisms such as B. Subtilis have also been described as agents of food poisoning with similar characteristics to those caused by B. Cereus.