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Anaerobes form the predominant bacterial components of the normal human skin and mucous membranes. They are responsible either alone or in combination with aerobes for a wide variety of infections ranging from superficial skin infections to intra-abdominal and intracranial infections.


Anaerobic organisms are widely distributed in nature. They are present in the soil as well as the skin, mucous membranes, and gastrointestinal tracts of animals and humans. Only a few of these organisms have been identified as responsible for disease in humans (Table 244-1).


Infection with these organisms usually results secondary to disruption in the normal skin or mucous membrane barriers of the host, resulting in entry of the bacteria into deeper tissues and leading to, at times, potentially severe infections from an individual’s own endogenous flora. Although some are strict anaerobes, others may be facultative anaerobes, able to survive in conditions with or without oxygen. The presence of devitalized tissues, low oxygen tension, and low pH serve to greatly contribute to the pathogenesis of anaerobic infections. Other conditions that may play a role include host defense mechanisms, virulence factors (bacterial adherence factors), production of toxins (eg, Clostridium species), and the presence of other bacteria in polymicrobial infections.

Due to their fastidious nature, as well as inconsistent use of adequate methods for isolation and identification, anaerobic bacteria are not easily isolated, which makes their exact frequency difficult to ascertain. Although anaerobes have been reported to account for 1% to 20% of episodes of bacteremia in adults, anaerobic organisms have rarely been isolated from blood cultures in pediatric patients. This may partly be explained by higher prevalence of chronic or debilitating conditions in adults, like malignant neoplasms, secondary immunodeficiencies, diabetes, obstetric and gynecologic surgery, and the presence of decubitus ulcers.

In recent years, increased use of advanced technology such as matrix-assisted laser desorption/ionization–time of flight mass spectrometry (MALDI-TOF MS) has greatly increased the ability to identify, speciate, and type anaerobes. Determination of in vitro antimicrobial susceptibility or resistance is also possible. This will serve to enhance early recognition as well as institution of appropriate and timely therapy in patients with severe infections.


Commonly encountered diseases caused by anaerobic bacteria in children are listed in Table 244-2. The principal sites of infection are deep soft tissue around the mouth and oropharynx, peritonitis and peritoneal abscesses following appendicitis or bowel rupture, and brain and lung abscesses. In females, beyond menarche, anaerobic bacteria ...

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