Bloodstream infections caused by Bacteroides and Phocaeicola: clinical relevance and antimicrobial susceptibility. Presentation of twenty-four cases.

Introduction: Anaerobic bloodstream infections caused by Gram-negative bacilli are rare, but the mortality rate remains high. Increasing resistance complicates treatment strategies. Aim: This study aimed to assess the clinical and microbiological characteristics of BSIs caused by Bacteroides spp. and related genera, and to evaluate the antibiotic susceptibility of isolated strains in adult patients hospitalized in a multidisciplinary Polish hospital over a 10-year period. Materials and methods: A retrospective, observational, single-center study included adult patients diagnosed with anaerobic BSIs between 2014 and 2023. Microbiological identification was performed using MALDI-TOF MS. Antimicrobial susceptibility was evaluated via gradient diffusion assay and interpreted according to EUCAST. Results: Anaerobic BSIs caused by Bacteroides spp. and Phocaeicola spp. were identified in 24 patients (mean age: 65.6 years). B. fragilis was the most common isolate (62.5%). Mortality among these hospitalized patients was 46%, with a significantly higher risk observed in elderly patients (median age of deceased: 83 years). All isolates were susceptible to metronidazole and carbapenems; 25% showed resistance to clindamycin. Discussion: The study confirmed high mortality, significant age-related risk, and variable antimicrobial susceptibility. Notably, Bacteroides-related BSIs often originated from gastrointestinal and urinary sources and were associated with chronic comorbidities, including renal failure. Conclusions: Empirical treatment regimens must consider increasing resistance among anaerobes. Timely microbiological diagnostics, including anaerobic cultures and susceptibility testing, are essential to guide therapy and improve patient outcomes. Significance: This study underscores the necessity for ongoing surveillance of anaerobic BSIs and highlights gaps in empirical treatment practices, advocating for broader clinical awareness and targeted antimicrobial stewardship.