Bacteroides and Fusobacterium
Bacteroides account for as much as 30% of all faecal isolates and form an important part of the normal flora of the gut. Bacteroides are bile resistant and the principle pathogenic species is B. fragilis. Members of the anaerobic genus fusobacterium are also important in both the normal flora and infection. Six species are common inhabitants of the oral cavity, GI tract, and the genital tract.
Anaerobic infections are usually opportunistic in damaged areas where there is reduced oxygen supply. Anaerobic infections are characteristically polymicrobial, with aerobic and anaerobic components acting in concert. Certain members of the normal flora e.g. B. fragilis may predominate. The following are sites where anaerobic infections may occur:
1. Lung - anaerobic bacteria are implicated in 90% of aspiration pneumonia and in lung abscess.
2. Brain - 85% of brain abscesses yield anaerobes
3. Head and neck - anaerobic bacteria resident in the upper airways, in particular B. melaninogenicus and F. Nucleatum may move into normally sterile structures of the head and neck to cause deep infection.
4. Abdomen - intraperitoneal sepsis such as peritonitis and abscess following perforation of the bowel usually involve several species of anaerobes together with facultative organisms.
5. Genital tract - anaerobes are usually involved in pelvic abscesses.
6. Skin and soft tissues - anaerobic infection of the skin and soft tissues is commonly secondary to trauma, surgery, or restricted circulation.
Typically, bacteroides and fusobacterium infection involve abscess formation and tissue destruction. A foul-smelling discharge is pathognomonic although its absence does not exclude a diagnosis.
Several anaerobic culture systems are available. Normal blood agar plates can be used. Species are identified by gram stain, resistance to bile, antimicrobial susceptibility, and a battery of biochemical tests.
The selection of antimicrobials depends on whether the infection is above or below the diaphragm. Infections of the head, neck, lung, pleura, and the brain are usually associated with anaerobes of the oral flora. The bacteria involved i.e. fusobacterium, peptostreptococci, and B. melaninogenicus are ordinarily susceptible to penicillin and for many years, this was considered to be the drug of choice, with clindamycin and chloramphenicol being 2nd line drugs. In recent years, there had been a steady rise in the incidence of resistance and some of the newer cephalosporins are being used as front-line.
For infections below the diaphragm e.g. abdominal, pelvic, and soft tissue infections, the predominant pathogen is B. fragilis. Not only is this species the most virulent of the anaerobes, many strains have become resistant to tetracycline, penicillins, and many cephalosporins. The emergence of clindamycin resistance has also been documented and there had been scattered reports of resistance to metronidazole and chloramphenicol. In infections below the waist, a successful regimen must also cover the common accompanying aerobic and facultatively aerobic species.