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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. Whereas the Bacillus species are aerobes, the Clostridium species are anaerobes.

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Of the many species of Bacillus and related genera, most do not cause disease and are not well characterized in medical microbiology. There are a few species, however, that cause important diseases in humans. Anthrax, a classical disease in the history of microbiology, is caused by Bacillus anthracis. Anthrax remains an important disease of animals and occasionally of humans. Because of its potent toxins, B anthracis is a major potential agent of bioterrorism and biologic warfare. Bacillus cereus and Bacillus thuringiensis cause food poisoning and occasionally eye or other localized infections.

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The genus Clostridium is extremely heterogeneous, and more than 190 species have been described. The list of pathogenic organisms, as well as novel species isolated from human feces whose pathogenic potential remain undetermined, continues to grow. Clostridia cause several important toxin-mediated diseases, including 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 (see Chapter 21).

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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, such as B thuringiensis. This organism is also capable of causing disease in humans. B cereus can grow in foods and cause food poisoning by producing either an enterotoxin (diarrhea) or an emetic toxin (vomiting). Both B cereus and B thuringiensis 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.

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Morphology and Identification

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Typical Organisms

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The typical cells, measuring 1 × 3–4 μm, have square ends and are arranged in long chains; spores are located in the center of the nonmotile bacilli.

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Culture

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Colonies of B anthracis are round and have a "cut glass" appearance in transmitted light. Hemolysis is uncommon with B anthracis but common with B cereus and the saprophytic bacilli. Gelatin is liquefied, and growth in gelatin stabs resembles an inverted fir tree.

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Growth Characteristics

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The saprophytic bacilli use 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.

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Pathogenesis

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Anthrax is primarily a disease of herbivores—goats, ...

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