[Microbiology of Aspiration Pneumonia]

The number of patients with pneumonia has been increasing as the population ages, and most fatal pneumonia cases are the elderly with aspiration pneumonia. Although aspiration pneumonia leads to poor short- and long-term prognosis, there have been no practical ways to diagnose it precisely. Persistent subclinical aspiration without any subjective symptoms is problematic in clinical practice in patients with aspiration pneumonia, and physicians can only use aspiration risks such as brain infarction to diagnose aspiration pneumonia. Anaerobes have been believed to be major causative pathogens in aspiration pneumonia, based on data from the 1970's. In relation to these data, Marik insisted that there is a possible overestimation of anaerobes because 1) the sampling of microbiologic specimens was in the late phase in the course of the illness, especially frequently after developing complications such as abscesses, necrotizing pneumonia, or empyema thoracis; 2) the organisms recovered by percutaneous transtracheal aspiration (PTA) sampling could have been contaminated by the aspiration of oropharyngeal flora during the PTA procedure or colonized in the trachea; and 3) many of the patients had chronic alcoholism or were under general anesthesia. In addition, 4) oral care was not common in the 1970s, and 5) the patients in these reports were relatively young. Molecular biological approaches using the 16S ribosomal RNA (rRNA) gene have recently been used, and have enabled us to detect more exact pathogens compared to conventional bacterial culture. Using the method with the detection of the 16S rRNA gene, we evaluated the bacterial phylotypes in bronchoalveolar lavage fluid in patients with aspiration pneumonia and found that oral streptococci were the most detected phylotypes (31.0%), while anaerobes were only 6.0%. Our results suggest that oral streptococci are important, and anaerobes may have been overestimated as causative pathogens in patients with aspiration pneumonia.