Identification and antibiotic susceptibility testing of Bacteroides fragilis directly from positive blood culture broths

This study aimed to develop a rapid and cost-effective method for the rapid identification (ID) and antimicrobial susceptibility testing (AST) of Bacteroides fragilis directly from positive blood cultures (BC) broths. Spiked BC bottles were used to compare the EUCAST standardized disk diffusion method for anaerobic bacteria (standard AST, sAST) with two alternative approaches: pellet-based AST (pAST) and direct disk diffusion (dDD). Rapid ID by MALDI-TOF MS from bacterial pellets correctly identified 93.5 % of isolates at the genus level and 85.9 % at the species level. pAST demonstrated excellent categorical agreement (CA) with sAST, achieving an overall CA of 98.5 %, with concordance rates exceeding 96 % for all tested antibiotics (meropenem, piperacillin-tazobactam, clindamycin, and metronidazole). In contrast, dDD showed poor performance, with an overall agreement of 69.5 %, especially low for piperacillin-tazobactam (48.8 %). These findings suggest that pAST offers a reliable and significantly faster alternative to conventional methods for anaerobic pathogens, whereas dDD remains insufficiently accurate for clinical use. Further studies with larger sample sizes are warranted to validate these results and support routine implementation.