Obtaining precise etiological information regarding causative bacteria is important for the proper use of antimicrobials in hospital-acquired pneumonia (HAP), which is associated with a high rate of mortality. The aim of this study was to comparatively investigate the bacterial diversity in bronchoalveolar lavage fluid (BALF) in Japanese patients with HAP by the clone library method using the 16S rRNA gene. This study included Japanese patients with HAP who were treated at our hospital and referring hospitals. BALF specimens were obtained from pneumonia lesions identified on chest radiographs and/or computed tomography. Sputum specimens were also evaluated in patients with sputum production. Sixty-eight patients were ultimately enrolled. BALF cultivation revealed bacterial positivity in 53 of 68 (77.9%) patients, and Staphylococcus aureus (30.9%) was the most frequently isolated, followed by Pseudomonas aeruginosa (16.2%), and Escherichia coli (10.3%). In contrast, the clone library analysis identified the presence of some bacterial phenotype in 65 of 68 (95.6%) patients, and streptococci (16.2%), Corynebacterium species (11.8%), anaerobes (10.3%) were frequently detected as the predominant phylotypes. Both methods tended to detect S. aureus, Klebsiella pneumoniae, and E. coli in patients with late-onset pneumonia. In addition, the cases that phylotypes of S. aureus and P. aeruginosa were found to account for > 5% of the bacterial flora of each case were 42.9% and 72.7%, respectively. These results indicate that attention should be paid to the roles of gram-positive bacilli such as streptococci, Corynebacterium species and anaerobes, in addition to Gram-negative bacilli, in the pathogenesis of HAP.
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