Anaerobic infections are common and some are serious, with a high mortality rate. They are easily overlooked because special precautions are needed for specimen collection and transport to do good bacteriologic studies and because some clinical laboratories fail to grow many or most anaerobes (a number of laboratories do not even do anaerobic cultures).

Treatment of anaerobic infections may be difficult. Failure to treat for anaerobes in mixed infections may lead to poor or no response. Many antibacterial agents have poor activity against many or most anaerobes, particularly aminoglycosides, the older quinolones, trimethoprim–sulfamethoxazole, and monobactams. Resistance of anaerobes to antimicrobials is increasing.

The most important anaerobes clinically are various genera of gram-negative rods. Bacteroides, especially the Bacteroides fragilis group, made up of several species (including B. fragilis), is particularly important. The other principal gram-negative genera are Prevotella, Porphyromonas, Fusobacterium, Bilophila, and Sutterella. Among the gram-positive anaerobes are cocci (formerly in Peptostreptococcus, now in several genera) spore-forming (Clostridium), and non-spore-forming bacilli (especially Actinomyces and Propionibacterium) (Table 122.1).

SOURCE OF ANAEROBIC INFECTION

Virtually the only source of anaerobes causing infection is the indigenous flora of mucosal surfaces and, to a much lesser extent, the skin (Table 122.2). The major exception is Clostridium difficile, the principal cause of antimicrobial agent–associated colitis, which has caused nosocomial infections. Anaerobes outnumber aerobes by 10:1 in the oral and vagina flora and by 1000:1 in the colon.