Association between obligatory anaerobes and empyema caused by Streptococcus anginosusgroup bacteria

Co-infections of Streptococcus anginosusgroup (SAG) bacteria and obligate anaerobes are observed in patients with empyema; however, their epidemiology and pathology remain unknown. A retrospective study was performed with 44 patients who underwent pleural effusion microbiota evaluation between January 2006 and March 2018, using the clone library method for detecting empyema caused by SAG bacteria. Based on culture analysis of pleural effusion, 12 patients were diagnosed with empyema caused by SAG bacteria. Obligate anaerobe phylotypes were detected in eight patients (66.7%) using the clone library method, whereas anaerobic culture analysis detected anaerobes in only two patients (16.7%). No significant difference was observed between the clinical features of SAG-mediated empyema with and without anaerobes using the clone library method, except for chest computed tomographic data. Co-infection of SAG bacteria and obligate anaerobes may be underestimated if conventional culture methods are used. SAG-mediated empyema with and without anaerobes may present different radiological features; therefore, further studies are required.