Pathogens of Animals: Bacillus anthracis and B. cereus are the predominant pathogens of medical importance. Paenibacillus alvei, B. megaterium, B. coagulans, Brevibacillus laterosporus, B. subtilis, B. sphaericus, B. circulans, Brevibacillus brevis, B. licheniformis, P. macerans, B. pumilus and B. thuringiensis have been occasionally isolated from human infections.

B. anthracis is the causative agent of anthrax, and B. cereus causes food poisoning. Nonanthrax Bacillus species can also cause a wide variety of other infections, and they are being recognized with increasing frequency as pathogens in humans.

Anthrax
Anthrax is primarily a disease of domesticated and wild animals, particularly herbivorous animals, such as cattle, sheep, horses, mules and goats. Humans become infected incidentally when brought into contact with diseased animals, which includes their flesh, bones, hides, hair and excrement. In the United States, the incidence of naturally-acquired anthrax is extremely rare (1-2 cases of cutaneous disease per year). Worldwide, the incidence is unknown, although B. anthracis is present in most of the world's soils.

The most common form of the disease in humans is cutaneous anthrax, which is usually acquired via injured skin or mucous membranes. A minor scratch or abrasion, usually on an exposed area of the face or neck or arms, is inoculated by spores from the soil or a contaminated animal or carcass. The spores germinate, vegetative cells multiply, and a characteristic gelatinous edema develops at the site. This develops into papule within 12-36 hours after infection. The papule changes rapidly to a vesicle, then to a pustule (malignant pustule), and finally into a necrotic ulcer, from which infection may disseminate, giving rise to septicemia. Lymphatic swelling also occurs within seven days. In severe cases, where the blood stream is eventually invaded, the disease is frequently fatal.

Another form of the disease, inhalation anthrax (woolsorters' disease), results most commonly from inhalation of spore-containing dust where animal hair or hides are being handled. The disease begins abruptly with high fever and chest pain. It progresses rapidly to a systemic hemorrhagic pathology and is often fatal if treatment cannot stop the invasive aspect of the infection.

Gastrointestinal anthrax is analogous to cutaneous anthrax but occurs on the intestinal mucosa. As in cutaneous anthrax, the organisms probably invade the mucosa through a preexisting lesion. The bacteria spread from the mucosal lesion to the lymphatic system. Intestinal anthrax results from the ingestion of poorly cooked meat from infected animals. Gastrointestinal anthrax is rare but may occur as explosive outbreaks associated with ingestion of infected animals.

The pathology of anthrax is mediated by two primary determinants of bacterial virulence: presence of an antiphagoytic capsule, which promotes bacterial invasion, and production of a powerful lethal toxin, the anthrax toxin.

For more information on anthrax, including use and detection of Bacillus anthracis as an agent of bioterrorism, please see the chapter on Bacillus anthracis and Anthrax.

Bacillus anthracis Gram stain. CDC.

Bacillus cereus food poisoning
Bacillus cereus causes two types of food-borne intoxications. One type is characterized by nausea and vomiting and abdominal cramps and has an incubation period of 1 to 6 hours. It resembles Staphylococcus aureus food poisoning in its symptoms and incubation period. This is the "short-incubation" or emetic form of the disease. The second type is manifested primarily by abdominal cramps and diarrhea with an incubation period of 8 to 16 hours. Diarrhea may be a small volume or profuse and watery. This type is referred to as the "long-incubation" or diarrheal form of the disease, and it resembles more food poisoning caused by Clostridium perfringens. In either type, the illness usually lasts less than 24 hours after onset.

The short-incubation form of disease is caused by a preformed heat-stable enterotoxin. The mechanism and site of action of this toxin are unknown. The long-incubation form of illness is mediated by a heat-labile enterotoxin, which apparently activates intestinal adenylate cyclase and causes intestinal fluid secretion.

This bacterium is dealt with separately in the medical section of the text at Bacillus cereus and Food Poisoning.

Colonies of Bacillus anthracis (right) and Bacillus cereus (left) on a plate of blood agar. CDC.