Clinical Emergence of Bacteroides- Associated Anaerobic Bacteremia Due to Drug-Resistant Non- Bacteroides fragilis Species: A Multicentric Retrospective Observational Study

Abstract Background Bacteroides species are significant anaerobes that cause intra-abdominal infections. Recent data highlight increasing resistance, specifically to clindamycin (CLDM). However, the precise epidemiology of Bacteroides fragilis and non-fragilis remains unclear. Methods We retrospectively analyzed 528 obligate anaerobes that were isolated from blood cultures at 6 major Japanese hospitals between 2012 and 2022, with 102 B. fragilis and 72 non-B. fragilis isolates. Clinical characteristics and antimicrobial resistance were assessed. Drug resistance rates over time were analyzed using the Spearman rank correlation test. Results Compared with the B. fragilis group, non-B. fragilis group demonstrated higher resistance to CLDM (P = .006) and carbapenem (P = .005). Excluding deaths from underlying diseases, the 30-day mortality rate was higher in the non-B. fragilis group than in the B. fragilis group (P = .025). Risk factors in this group included carbapenem resistance (adjusted odds ratio = 7.05, P = .017). Notably, carbapenem (P = .014 vs P = .643) and tazobactam/piperacillin resistance rates (P = .012 vs P = .899) increased over time in the non-B. fragilis group, rather than the B. fragilis group. Conclusions Non-B. fragilis shows increasing resistance to key antibiotics and is linked to higher 30-day mortality compared with B. fragilis.