Antimicrobial susceptibility and prevalence of resistance genes in Bacteroides fragilis isolated from blood culture bottles in two tertiary care hospitals in Japan.

The in vitro susceptibilities of Bacteroides fragilis to antimicrobial agents, especially to carbapenem, are a major concern in the treatment of patients with bloodstream infections. In this study, 50 isolates of B. fragilis were obtained from positive blood bottles from 2014 to 2019 in Saitama, Japan. Their susceptibility to ampicillin/sulbactam was reduced to 70.0% compared with a previous report, whereas they were still sufficiently susceptible to piperacillin/tazobactam (94.0%). Five cfiA-positive isolates (5/50, 10.0%) were identified that were resistant to doripenem and meropenem, and two of them carried an insertion sequence located upstream of the cfiA-coding region. In particular, imipenem should be considered as a first-line carbapenem for the empirical treatment of B. fragilis infection because only insertion sequence and cfiA double-positive strains showed resistance to imipenem. Thirty-six percent of the isolates had a reduced minimum inhibitory concentration for moxifloxacin. In addition, metronidazole should still be considered as an active agent for B. fragilis because all isolates were susceptible to this antibiotic and the prevalence of the nim gene was low in Japan.