Antimicrobial susceptibility testing of anaerobic bacteria causing bacteremia: A 13-year (2010-2022) retrospective study in a tertiary hospital

Infections from anaerobic microorganisms result from breached mucosal barriers, posing a significant mortality risk. A retrospective study at Hospital Universitario La Paz (Madrid) from 2010 to 2022 analyzed 491 (6.17 %) anaerobic bacteremia cases out of 7956 significant bacteremia cases among 171,833 blood culture requests. Bacteroides fragilis was the most frequently isolated species (28.3 %), followed by Clostridium perfringens (13.6 %). B. fragilis showed good susceptibility to amoxicillin/ clavulanic acid (86 %), piperacillin/tazobactam (86 %), and metronidazole (87.7 %). In general, non-fragilis Bacteroides species showed low susceptibility to penicillin (7 %), amoxicillin (17.5 %), and clindamycin (64.9 %). Of our 13 non-perfringens Clostridium isolates, four exhibited resistance to penicillin and four showed resistance to clindamycin. Lactobacillus species were highly susceptible to antibiotics tested. Prevotella spp. showed low susceptibility to penicillin (20 %), amoxicillin (20 %), and clindamycin (40 %). The study contributes valuable data for monitoring and improving anaerobic bacteremia treatment.