Re-incubation of positive anaerobic blood culture bottles for detecting additional anaerobes in polymicrobial bacteremia.

Anaerobes are important causes of bloodstream infections, but may be overlooked in routine blood culture workflows because of their slow growth and frequent involvement in polymicrobial infections. We hypothesized that in polymicrobial bacteremia, rapidly growing organisms may trigger early blood culture positivity before slow-growing anaerobes have sufficiently proliferated, potentially leading to underdetection during routine subculture. In this observational study, we evaluated whether re-incubation of positive anaerobic blood culture bottles improves the detection of anaerobic bacteria. All blood samples were cultured using the BacT/Alert Virtuo automated blood culture system. Adult patients with at least one positive anaerobic blood culture bottle were included. Positive anaerobic bottles were subcultured using the conventional method and then re-incubated for an additional 4 days before repeat subculture to detect additional anaerobes. Species were identified using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, supplemented by 16S rRNA gene sequencing. Of 310 patients with positive anaerobic blood cultures, 29 (9.4%) had an additional 36 bacterial isolates identified after re-incubation, of which only 3 were judged to be contaminants. Of 27 cases with two positive anaerobic bottles, additional organisms were detected in both bottles in 23 cases (85%), and concordant organisms were detected from the suspected infection focus in 9 cases (31%). Additional species identified following repeat incubation included