A prospective study of 54 cases of pulmonary infection following aspiration was performed. Specimens utilized for bacteriologic study were either transtracheal aspirates, empyema fluid or blood. Appropriate anaerobic bacteriologie methods were employed. Anaerobic bacteria were recovered in 50 patients (93 per cent) and were the only pathogens in 25 (46 per cent). The predominant species were Bacteroides melanino-genicus, Fusobacterium nucleatum and anaerobic or microaerophilic gram-positive cocci. Bacteroides fragilis, which is resistant to many commonly used antibiotics, was recovered in nine patients (17 per cent). Aerobic and facultative bacteria were present in 29 patients (54 per cent), but anaerobes were present concurrently in all but 4. Enteric gram-negative bacilli and pseudomonads were particularly common in patients whose disease developed in the hospital. Eleven patients with mixed aerobic and anaerobic infections were treated successfully with antibiotics which were active only against the anaerobic isolates, thereby further implicating the pathogenic role of these microorganisms. The results indicate that anaerobes play a key role in most cases of infection following aspiration.
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