Bacillus

Introduction
The gram-positive spore-forming bacilli are the Bacillus and Clostridium species. These bacilli are ubiquitous, and because they form spores they can survive in the environment for many years. Bacillus species are aerobes, whereas clostridia are anaerobes.
Several species of them cause important disease in humans. Anthrax, a prototype disease in the history of microbiology, is caused by Bacillus anthracis. Anthrax remains an important disease of animals and occasionally of humans, and B anthracis is a major agent of bioterrorism and biologic warfare. Bacillus cereus causes food poisoning and occasionally eye or other localized infections. Clostridia cause several important toxin-mediated diseases: Clostridium tetani, tetanus; Clostridium botulinum, botulism; Clostridium perfringens, gas gangrene; and Clostridium difficile, pseudomembranous colitis. Other clostridia are also found in mixed anaerobic infections in humans .

Bacillus Species
The genus bacillus includes large aerobic, gram-positive rods occurring in chains. Most members of this genus are saprophytic organisms prevalent in soil, water, and air and on vegetation, such as Bacillus cereus and Bacillus subtilis. Some are insect pathogens. B cereus can grow in foods and produce an enterotoxin or an emetic toxin and cause food poisoning. Such organisms may occasionally produce disease in immunocompromised humans (eg, meningitis, endocarditis, endophthalmitis, conjunctivitis, or acute gastroenteritis). B anthracis, which causes anthrax, is the principal pathogen of the genus.
Morphology & Identification
The typical cells, measuring 1 x 3–4 mm, have square ends and are arranged in long chains; spores are located in the center of the nonmotile bacilli.
Culture
Colonies of B anthracis are round and have a "cut glass" appearance in transmitted light. Hemolysis is uncommon with B anthracis but common with the saprophytic bacilli. Gelatin is liquefied, and growth in gelatin stabs resembles an inverted fir tree.
Growth Characteristics
The saprophytic bacilli utilize simple sources of nitrogen and carbon for energy and growth. The spores are resistant to environmental changes, withstand dry heat and certain chemical disinfectants for moderate periods, and persist for years in dry earth. Animal products contaminated with anthrax spores (eg, hides, bristles, hair, wool, bone) can be sterilized by autoclaving.
Bacillus anthracis
Pathogenesis
Anthrax is primarily a disease of herbivores—goats, sheep, cattle, horses, & other animals (eg, rats) .Humans become infected incidentally by contact with infected animals or their products. In animals, the portal of entry is the mouth and the gastrointestinal tract. Spores from contaminated soil find easy access when ingested with spiny or irritating vegetation. In humans, the infection is usually acquired by the entry of spores through injured skin (cutaneous anthrax) or rarely the mucous membranes (gastrointestinal anthrax), or by inhalation of spores into the lung (inhalation anthrax).
The spores germinate in the tissue at the site of entry, and growth of the vegetative organisms results in formation of a gelatinous edema and congestion. Bacilli spread via lymphatics to the bloodstream, and they multiply freely in the blood and tissues shortly before and after the animal s death.
B anthracis that does not produce a capsule is not virulent and does not induce anthrax in test animals. The poly-D-glutamic acid capsule is antiphagocytic. The capsule gene is on a plasmid.