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There are five medically important genera of gram-positive rods: Bacillus, Clostridium, Corynebacterium, Listeria, and Gardnerella. Bacillus and Clostridium form spores, whereas Corynebacterium, Listeria, and Gardnerella do not. Members of the genus Bacillus are aerobic, whereas those of the genus Clostridium are anaerobic (Table 17–1).

Table 17–1Gram-Positive Rods of Medical Importance

These gram-positive rods can also be distinguished based on their appearance on Gram stain. Bacillus and Clostridium species are longer and more deeply staining than Corynebacterium and Listeria species. Corynebacterium species are club-shaped (i.e., they are thinner on one end than the other). Corynebacterium and Listeria species characteristically appear as V- or L-shaped rods. Gardnerella vaginalis is a short gram-variable rod.

Additional information regarding the clinical aspects of infections caused by the organisms in this chapter is provided in Part IX entitled Infectious Diseases beginning on Chapter 70.



There are two medically important Bacillus species: Bacillus anthracis and Bacillus cereus. Important features of pathogenesis by these two Bacillus species are described in Table 17–2.

Table 17–2Important Features of Pathogenesis by Bacillus Species

1. Bacillus anthracis


Bacillus anthracis causes anthrax (Figure 17–1), which is common in animals but rare in humans. Human disease occurs in three main forms: cutaneous, pulmonary (inhalation), and gastrointestinal. In 2001, an outbreak of both inhalation and cutaneous anthrax occurred in the United States. The outbreak was caused by sending spores of the organism through the mail. There were 18 cases, causing five deaths in this outbreak.

Figure 17–1

Skin lesion of anthrax. Note the black eschar, a necrotic lesion covered by a crust, caused by lethal factor, an exotoxin produced by Bacillus anthracis. Note ...

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