Anaerobic Lung Infections

Laura E. Crotty Alexander

Anaerobic bacteria aspirated into the lung can cause pneumonia, lung abscess, and empyema. Anaerobic lung infections typically smolder for weeks to months before a patient presents for medical care. Symptoms tend not to be as profound or specific as in other bacterial infections and include putrid sputum and breath, weight loss, and fever. Infection is caused by aspiration of multiple organisms normally residing in gingival crevices. Because anaerobes are not highly virulent organisms, they may require up to three or more concomitant bacterial species to establish infection in the lung via bacterial synergy. The concomitant species may be aerobic or anaerobic. Many times the concomitant bacteria will grow in culture, while the anaerobic bacteria do not. For this reason, the selective nature of diagnostic cultures may mask the true contribution of anaerobes to the infectious process. The top three anaerobic species that cause lung infections are Bacteroides melaninogenicus, Fusobacterium nucleatum, and Peptostreptococcus.

Poor dentition and gingival disease in a patient with pulmonary symptoms should raise the suspicion for anaerobic infection, since these conditions are associated with higher amounts of bacteria in the oropharynx. Anaerobic bacteria infect the lung after large quantities are aspirated. For this reason, anaerobic lung infection is associated with conditions that lead to aspiration such as alcohol abuse, seizure, stroke or other neurologic disorders, dysphagia, and esophageal motility disorders. In addition, patients with nosocomial sinusitis have a high incidence of anaerobes as the etiologic organism. Sinusitis is commonly associated with pneumonia due to the same organism, presumably because of aspiration of the organisms from the nasopharyngeal space. Anaerobic infections also can occur in the setting of obstructing lung lesions, such as tumors or aspirated objects.

Patients with anaerobic lung infections tend to be less acutely ill than those with other community-acquired pneumonias. In one review of all ICU admissions over one year, not a single case was due to anaerobic lung infections or their sequelae. The onset of symptoms tends to be more insidious than in pneumonia due to more virulent bacteria. Patients with anaerobic empyema and lung abscess typically present after 14 to 15 days of symptoms. Twenty percent of patients who have aspirated anaerobic organisms will develop lung necrosis and abscesses in the following weeks. Some patients will develop empyema in 4 to 6 weeks. Because of the chronic nature of anaerobic infections, symptoms such as weight loss, anemia, and night sweats are common. However, in the hours following aspiration itself, patients may have symptoms of chemical pneumonitis, which symptomatically and radiographically should clear within 24 to 48 hours.

Radiographic findings of anaerobic lung infection include necrotic lesions, cavitations, and abscesses and complicated pleural effusions. Because aspiration is the primary process leading to infection, dependent lung segments in the recumbent position are most commonly involved, especially the superior segments of the lower lobes and the posterior segments of the upper lobes.

Diagnosis is almost entirely clinical but the presence of putrid sputum or empyema fluid is diagnostic because anaerobes are the only organisms to produce the short-chain volatile fatty acids that are associated with a distinctive odor and sour and foul taste. There have been no significant advancements in diagnosis in over 20 years. Methods that may be useful in making a diagosis include transtracheal lung sampling and protected brush specimens via bronchoscopy. However, neither method has become commonplace. Samples need to be rapidly processed under anaerobic conditions; any contamination with more robust bacterial species (Staphylococcus) can lead to overgrowth that masks the presence of anaerobes.

New methods might include polymerase chain reaction (PCR) detection of bacterial genomic markers in sputum and bronchoalveolar lavage (BAL) samples. However, because the bacteria that cause anaerobic infections are found normally in the oral cavity, contamination of expectorated specimens is common. One method of removing oral contaminations is to run tap water over sputum samples to remove any bacteria not embedded in the mucous and biofilm. Recovery of anaerobes from pleural fluid is the most convincing finding. However, even in the absence of culture data, any pleural fluid that smells putrid is strong evidence of anaerobic infection.

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