Clinical recognition of anaerobic infections
Knowledge that anaerobic bacteria are common in both community- and hospital-acquired infections has evolved during the 1970s mainly through improvements in laboratory culture procedures and a classification system for anaerobes. The availability of effective antimicrobials for treatment of these infections also has encouraged efforts to isolate these oxygen-sensitive bacteria. Certain features of anaerobic infections aid in their clinical recognition. The source of these infections usually is the host's endogenous flora which is located on the skin and on the mucus membrane surfaces of the upper respiratory tract and mouth and the gastrointestinal and genitourinary tracts. This endogenous anaerobic flora, often with aerobic species, can produce tissue infection when there is a diminution of host defenses, primarily the lowering of the oxygen tension or oxidation-reduction potential in tissue. This occurs in settings that produce tissue necrosis, impairment of blood supply, growth of aerobic bacteria, or heavy contamination of tissue with anaerobes. Thus, infections usually occur in areas contiguous to this endogenous flora in the respiratory tract, the abdomen, and female genital tract, but may also occur at distant sites by hematogenous spread. Since mixtures of anaerobic species or of anaerobes and aerobes are most common in these infections, they are termed 'mixed' or 'polymicrobic'. Typically, there is abscess formation and necrosis of tissue, and a foul odor and gas may be evident. These infections may be chronic with intermittent recrudescence of symptoms, but they can be acute. Septic thrombophlebitis may complicate the infection, particularly with infections of the female pelvis. A direct gram stain of clinical material may presumptively indicate the presence of anaerobes of various types and be useful in selection of antimicrobial therapy. Accurate bacteriologic diagnosis depends on rapid delivery of specimens to the laboratory with protection against oxygen exposure and drying and the use of supplemented media and proper anaerobic conditions for culture; otherwise, cultures may be negative for growth or only aerobes may be reported. Anaerobic gram negative rods such as Bacteroides fragilis. Bacteroides melaninogenicus and Bacteroides bivius are common from infection and often are resistant to many commonly used antibiotics. Anaerobic gram positive cocci, particularly species of Peptococcus and Peptostreptococcus are also frequent isolates from infection.
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