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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. Bacillus species are aerobes, and the Clostridium species are anaerobes (see also Chapter 21).

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. B. cereus causes food poisoning and occasionally eye or other localized infections.

The genus Clostridium is extremely heterogeneous, and more than 200 species have been described. The list of pathogenic organisms, as well as novel species isolated from human feces whose pathogenic potential remains undetermined, continues to grow. Clostridia cause several important toxin-mediated diseases, including tetanus (Clostridium tetani), botulism (C. botulinum), gas gangrene (C. perfringens), and antibiotic-associated diarrhea and pseudomembranous colitis (C. difficile). Other clostridia are also found in mixed anaerobic infections in humans (see Chapter 21).


The genus Bacillus includes large aerobic, Gram-positive rods occurring in chains. The members of this genus are closely related but differ both phenotypically and in terms of pathogenesis. Pathogenic species possess virulence plasmids. Most members of this genus are saprophytic organisms prevalent in soil, water, and air, and on vegetation (eg, B. subtilis). Some are insect pathogens, such as B. thuringiensis. B. cereus can grow in foods and cause food poisoning by producing either an enterotoxin (diarrhea) or an emetic toxin (vomiting). B. cereus 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 and Identification

A. Typical Organisms

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 bacilli.

B. 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 B. cereus and the saprophytic bacilli. Gelatin is liquefied, and growth in gelatin stabs resembles an inverted fir tree.

C. Growth Characteristics

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 ...

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