Species Distribution and Antimicrobial Susceptibility of Anaerobic Bacteria Recovered from Clinical Specimens in Warsaw University Hospitals.

Objectives

The aim of this study was to characterize the distribution of anaerobic bacteria isolated from clinical specimens across major specimen types and to determine the antimicrobial susceptibility patterns of the most frequently isolated species.

Methods

Clinical specimens collected in 2022 from three university hospitals in Warsaw were analyzed. Anaerobes were identified using MALDI-TOF MS, and antimicrobial susceptibility testing was performed according to EUCAST guidelines. A total of 1,425 isolates were included and classified as Gram-negative anaerobic bacilli (GNAB), Gram-positive anaerobic cocci (GPAC), Gram-positive anaerobic bacilli (GPAB), and spore-forming bacilli.

Results

Skin and soft tissue specimens yielded the largest number of isolates (n = 720), predominantly Finegoldia magna, Bacteroides fragilis, Anaerococcus vaginalis, and Prevotella bivia. Among intra-abdominal specimens (n = 366), the Bacteroides species constituted the majority of GNAB. Among head and neck specimens (n = 261), Prevotella spp. and Parvimonas micra were most common. Isolates recovered from blood cultures (n = 59) were dominated by Cutibacterium acnes, followed by Bacteroides spp. and Clostridium spp. Metronidazole and meropenem showed the highest overall activity (>95% susceptibility). β-lactam-β-lactamase inhibitor combinations remained broadly effective, with susceptibility rates exceeding 90% for most anaerobic species, although lower susceptibility was observed among certain Bacteroides species, particularly B. thetaiotaomicron (69.1%). Clindamycin susceptibility was 50% or lower among Bacteroides species and variable among Prevotella spp. and GPAC.

Conclusions

Anaerobic bacteria isolated from clinical specimens demonstrated considerable species diversity and characteristic specimen-specific distribution patterns. Bacteroides and Prevotella species, predominated in intra-abdominal specimens, whereas F. magna and other GPAC were most commonly recovered from skin and soft tissue infections. Species belonging to Actinomyces and related genera were frequently identified in specimens from head and neck infections. Although susceptibility rates exceeded 95% for carbapenems and metronidazole, species-dependent resistance-particularly to clindamycin and piperacillin-tazobactam-highlights the need for ongoing surveillance and precise species-level identification to support optimal empirical therapy.