Diseases
Members of the genus Bacteroides are the most common cause of serious anaerobic infections (e.g., sepsis, peritonitis, and abscesses). Bacteroides fragilis is the most frequent pathogen. Prevotella melaninogenica is also an important pathogen. It was formerly known as Bacteroides melaninogenicus, and both names are still encountered.
Important Properties
Bacteroides and Prevotella organisms are anaerobic, non–spore forming, gram-negative rods. Of the many species of Bacteroides, two are human pathogens: B. fragilis and Bacteroides corrodens.
Members of the B. fragilis group are the predominant organ isms in the human colon, numbering approximately 1011/g of feces and are found in the vagina of approximately 60% of women. P. melaninogenica and B. corrodens occur primarily in the mouth.
Pathogenesis & Epidemiology
Because Bacteroides and Prevotella species are part of the normal flora, infections are endogenous, usually arising from a break in a mucosal surface, and are not communicable. These organisms cause a variety of infections, such as local abscesses at the site of a mucosal break, metastatic abscesses by hematogenous spread to distant organs, or lung abscesses by aspiration of oral flora.
Predisposing factors such as surgery, trauma, and chronic disease play an important role in pathogenesis. Local tissue necrosis, impaired blood supply, and growth of facultative anaerobes at the site contribute to anaerobic infections. The facultative anaerobes, such as E. coli, utilize the oxygen, thereby reducing it to a level that allows the anaerobic Bacteroides and Prevotella strains to grow. As a result, many anaerobic infections contain a mixed facultative and anaerobic flora. This has important implications for therapy; both the facultative anaerobes and the anaerobes should be treated.
The polysaccharide capsule of B. fragilis is an important virulence factor. The host response to the capsule plays a major role in abscess formation. Note also that the endotoxin of B. fragilis contains a variant lipid A that is missing one of the fatty acids and consequently is 1000-fold less active than the typical endotoxin of bacteria such as Neisseria meningitidis.
Enzymes such as hyaluronidase, collagenase, and phospholipase are produced and contribute to tissue damage. Enterotoxin producing strain of B. fragilis can cause diarrhea in both children and adults.
Clinical Findings
The B. fragilis group of organisms is most frequently associated with intra-abdominal infections, either peritonitis or localized abscesses. Pelvic abscesses, necrotizing fasciitis, and bacteremia occur as well. Abscesses of the mouth, pharynx, brain, and lung are more commonly caused by P. melaninogenica, a member of the normal oral flora, but B. fragilis is found in about 25% of lung abscesses. In general, B. fragilis causes disease below the diaphragm, whereas P. melaninogenica causes disease above the diaphragm. Prevotella intermedia is an important cause of gingivitis, periodontitis, and dental abscess.
Laboratory Diagnosis Bacteroides species can be isolated anaerobically on blood agar plates containing kanamycin and vancomycin to inhibit unwanted organisms. They are identified by biochemical reac tions (e.g., sugar fermentations) and by production of certain organic acids (e.g., formic, acetic, and propionic acids), which are detected by gas chromatography. P. melaninogenica produces characteristic black colonies (Figure 1).
Fig1. Prevotella melaninogenica—black pigmented colonies. Arrow points to a black pigmented colony of P. melaninogenica. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
Treatment & Prevention
Members of the B. fragilis group are resistant to penicillins, first generation cephalosporins, and aminoglycosides, making them among the most antibiotic-resistant of the anaerobic bacteria. Penicillin resistance is the result of β-lactamase production. Metronidazole is the drug of choice, with cefoxitin, clindamycin, and chloramphenicol as alternatives. Aminoglycosides are frequently combined to treat the facultative gram-negative rods in mixed infections. The drug of choice for P. melaninogenica infections is either metronidazole or clindamycin. β-Lactamase-producing strains of P. melaninogenica have been isolated from patients. Surgical drainage of abscesses usually accompanies antibiotic therapy, but lung abscesses often heal without drainage.
Prevention of Bacteroides and Prevotella infections centers on perioperative administration of a cephalosporin, frequently cefoxitin, for abdominal or pelvic surgery. There is no vaccine.
