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 ||Download (.pdf) TABLE 17–1 Gram-Positive Rods of Medical Importance
|Growth ||Anaerobic Growth ||Spore Formation ||Exotoxins Important in Pathogenesis |
|Bacillus ||– ||+ ||+ |
|Clostridium ||+ ||+ ||+ |
|Corynebacterium ||– ||– ||+ |
|Listeria ||– ||– ||– |
|Gardnerella ||– ||– ||– |
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.
SPORE-FORMING GRAM-POSITIVE RODS
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 ||Download (.pdf) TABLE 17–2 Important Features of Pathogenesis by Bacillus Species
|Organism ||Disease ||Transmission/Predisposing Factor ||Action of Toxin ||Prevention |
|B. anthracis ||Anthrax || |
Cutaneous anthrax: spores in soil enter wound
Pulmonary anthrax: spores are inhaled into lung
|Exotoxin has three components: protective antigen binds to cells; edema factor is an adenylate cyclase; lethal factor is a protease that inhibits cell growth resulting in cell death (necrosis) ||Vaccine contains protective antigen as the immunogen |
|B. cereus ||Food poisoning ||Spores germinate in reheated rice, then bacteria produce exotoxins, which are ingested || |
Two exotoxins (enterotoxins):
Similar to cholera toxin, it increases cyclic AMP
Similar to staphylococcal enterotoxin, it is a superantigen
|No vaccine |
B. 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.
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 the area of edema surrounding the eschar, which is caused ...