Chapter 35 The Genus Bacteroides

Gram-negative anaerobic, non–spore-forming bacteria of veterinary importance are in the genera Bacteroides (the subject of this chapter) and Fusobacterium, Prevotella, Porphyromonas, and Dichelobacter (covered in subsequent chapters) (Table 35-1). Many are found in the environment in large numbers, and residence in the intestinal tract is common. The Eh in necrotic and suppurative lesions may approach −240 mV, which favors the growth of strict anaerobes. Concurrent infection with facultative organisms contributes to the lowered Eh, and these mixed infections are common. Given their common occurrence in the intestine and the environment, isolation of these organisms does not necessarily imply involvement in the genesis of an infection.

TABLE 35-1 Bacteroides Species of Veterinary Significance

DISEASE AND EPIDEMIOLOGY

Members of the genus Bacteroides represent nearly 50% of isolates of anaerobic bacteria in some veterinary hospital situations. The most common conditions from which these organisms are recovered include soft-tissue abscesses, cellulitis, periodontal abscesses, lung and liver abscesses, peritonitis, pyometritis, osteomyelitis, postoperative wound infections, and mastitis. Bacteroides spp. have little host or tissue predilection; bile-resistant, nonpigmented members of Bacteroides fragilis are often encountered in the respiratory tract but less so in abscesses (Figure 35-1). Bacteroides fragilis has also been isolated from aborted bovine fetuses with bronchopneumonic lesions. Multiple species have been isolated from uteri of dairy cows with retained fetal membranes and postparturient endometritis. Bacteroides asaccharolyticus has been isolated from lesions of osteomyelitis in small animals and horses, and Bacteroides levii is associated with bovine mastitis. Cats with empyema have been a source of Bacteroides salivosus (see Table 35-1). Bacteroides thetaiotaomicron is a member of the normal flora, but occasionally causes infections in immunocompromised patients (Figure 35-2); Bacteroides ovatus and Bacteroides vulgatus cause osteomyelitis and soft tissue infections (Figure 35-3).

FIGURE 35-1 Bacteroides fragilis grown in Schaedler’s broth (Gram stain).

(Courtesy Public Health Image Library, PHIL #2996, Centers for Disease Control and Prevention, Atlanta, 1974, Don Stalons.)

FIGURE 35-2 Bacteroides thetaiotaomicron, with morphology reminiscent of Greek letters (thioglycollate broth, 48-hour incubation).

(Courtesy Public Health Image Library, PHIL #2956, Centers for Disease Control and Prevention, Atlanta, 1972, V.R. Dowell.)

FIGURE 35-3 Bacteroides ovatus (thioglycollate broth, 48-hour incubation).

(Courtesy Public Health Image Library, PHIL #2962, Centers for Disease Control and Prevention, Atlanta, 1972, V.R. Dowell.)

Some strains of B. fragilis are associated with watery nonhemorrhagic diarrhea in livestock and young children. Piglets become anorexic and dehydrated, and lesions include swelling and vacuolation of enterocytes, with eventual exfoliation. Attachment to and invasion of enterocytes have not been demonstrated. Crypt hyperplasia is extensive in the colon and less so in the ileum. Experimental inoculation of rabbits with strains from pigs or humans produces watery diarrhea and dehydration, with lesions similar to those in piglets. Clinical signs and microscopic lesions suggest a mechanism based on net secretion of fluid into the small intestine and decreased absorption by the large intestine.