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 124.2). 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 vaginal flora and by 1000:1 in the colon. Factors predisposing to anaerobic infection include disruption of normal mucosal or cutaneous barriers by disease, surgery, or trauma; tissue injury (which reduces oxidation–reduction potential, favoring growth of anaerobes); impaired blood supply; obstruction of a hollow viscus; and foreign body. Other important factors include the numbers of organisms that get into deeper tissues (the inoculum size), various virulence factors (toxins, enzymes, and other substances) produced by anaerobes, and whether the host’s defense system is intact.

Types of infection involving anaerobes

In terms of overall frequency, there are four major sites of anaerobic infection: pleuropulmonary, intra-abdominal, female genital tract, and skin and soft tissue with or without involvement of underlying bone. Other infections that primarily involve anaerobic bacteria but are seen less commonly include brain abscess and bite-wound infections. Virtually all types of infection occurring in humans may involve anaerobic bacteria, and no organ or tissue of the body is immune to infection with these organisms. Table 124.3 lists infections commonly involving anaerobic bacteria. Abscess formation and tissue destruction are common characteristics of anaerobic infection. Synergy between various anaerobes or between anaerobes and aerobes is often important in mixed anaerobic infections.

Some anaerobic infections are unique (e.g., lung abscess, actinomycosis) and are readily suspected clinically. Major clues to anaerobic infection are listed in Table 124.4. Only the foul or putrid odor of a lesion or its discharge is specific; the other clues nonetheless may be highly suggestive. The Gram stain is useful because many anaerobes are unique morphologically. Information as to the relative numbers of various organisms may be extremely useful in directing empiric therapy.

Relatively recently, a number of serious infections involving various clostridia have been documented. Included are a more serious form of C. difficile-associated diarrheal disease or colitis involved in a number of hospital-acquired outbreaks, endometritis and toxic shock syndrome due to Clostridium sordellii following abortions, and serious soft-tissue infections (including necrotizing fasciitis and anaerobic myonecrosis) due to C. sordellii and Clostridium novyi in “skin-popping” drug addicts.

Collection and transport of specimens

Proper collection and transport of specimens is crucial for recovery of anaerobes in the laboratory. Because anaerobes are normal flora, the clinician should be certain not to contaminate the specimens with such flora; this may be difficult at times. A good example of the problem is the patient with suspected aspiration pneumonia. Expectorated sputum is unsuitable because of the large numbers of anaerobes and other organisms present in saliva as indigenous flora; it is necessary to bypass the normal flora. If an empyema is present, thoracentesis provides a good specimen and is indicated therapeutically. In the absence of pleural fluid, bronchoalveolar lavage or use of a plugged double-lumen catheter with a protected bronchial brush, with quantitative culture, should be used.

Proper transport requires placing the specimen under anaerobic conditions in a nonnutritive holding medium (in an oxygen-free glass tube or vial) for the trip to the laboratory.

Therapy

The two key approaches to treatment are surgery and antimicrobial therapy. Debridement and drainage usually are essential. Failure to carry out prompt and thorough surgical therapy may lead to lack of response to appropriate antimicrobial agents. Some abscesses are amenable to percutaneous drainage under guidance of ultrasound or computed tomography.

Hyperbaric oxygen (HBO) may have value in selected circumstances, such as gas gangrene, to help demarcate the infection; for example, it may indicate where amputation should be done in the case of an extremity infection. There has never been clear-cut clinical evidence of significant benefit from HBO; however, surgical therapy should never be delayed to administer HBO.

Initial antimicrobial therapy is necessarily empiric; it takes some time to get definitive information on the infecting flora because it is usually complex. Rational empiric therapy is based on the clinician’s assessment of the nature of the infectious process, knowledge of the usual infecting flora in such infections (Tables 124.5–124.9), and patterns of resistance of anaerobic bacteria to antimicrobial drugs in the particular hospital. Also, the clinician must take into account how the usual flora may have been modified by pathophysiology or disease and by prior antimicrobial therapy. Careful analysis of the Gram stain of the specimen may also suggest the need to modify the empiric approach. In certain situations, the pharmacologic properties of the drugs and whether they are bactericidal or not are important considerations. In central nervous system infections, for example, the drug must cross the blood–brain barrier well. In such infections and in endocarditis, bactericidal activity is important. A good clinician will be in close contact with the microbiology laboratory, particularly in the case of a very sick patient. Ideally, such contact begins before the specimen is submitted and is maintained until full culture results are available. The microbiologist may take advantage of information from the clinician to use special selective or other media in setting up the culture and can often look at cultures more often than is done with routine cultures, using an anaerobic chamber or other device to examine the culture without exposing it to oxygen. Preliminary culture information may dictate modification of the initial empiric antimicrobial regimen. The use of molecular techniques may lead to much more rapid identification than with conventional procedures.

Table 124.5 Usual flora in anaerobic pleuropulmonary infections^a

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Anaerobes Anaerobic gram-positive cocci Pigmented Prevotella (P. denticola, P. melaninogenica, P. intermedia, P. nigrescens, P. loescheii) Nonpigmented Prevotella (P. oris, P. buccae, P. oralis) Fusobacterium nucleatum Bacteroides fragilis group Non-spore-forming gram-positive rods (Actinomyces, Eubacterium, Lactobacillus)

Viridans streptococci

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^a In hospital-acquired infections (e.g., aspiration pneumonia), various nosocomial pathogens, such as Staphylococcus aureus

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