Comparison of disc diffusion and Sensititre® Broth Microdilution methods for antimicrobial susceptibility testing of Bacteroides fragilis group clinical isolates following the French guidelines

This study aims to evaluate the concordance between the disc diffusion test (DDT) and custom-designed Sensititre® Broth Microdilution plates and to estimate their respective sensitivity and specificity for antimicrobial susceptibility testing (AST) of Bacteroides fragilis group (BFG) clinical isolates.

AST was performed on 126 BFG isolates using both DDT and Sensititre® methods according to the 2024 CA-SFM guidelines. Cohen's kappa coefficients were used to assess concordance, while a Bayesian approach estimated sensitivity and specificity with 95 % confidence intervals (95 % CIs) for amoxicillin-clavulanate (AMC), piperacillin-tazobactam (PTZ), imipenem (IPM), clindamycin (CLI), metronidazole (MTR), and moxifloxacin (MXF).

Cohen's kappa coefficients with 95 % CIs for AMC, PTZ, IPM, CLI, MTR, and MXF were 0.92 (0.85-1.00), 0.70 (0.56-0.83), 0.96 (0.92-1.00), 0.98 (0.94-1.00), 0.96 (0.89-1.00), and 0.70 (0.58-0.82), respectively. DDT demonstrated sensitivity > 0.8 for all antibiotics, and specificity > 0.8 except for PTZ (0.51; 0.37-0.65) and CLI (0.71; 0.58-0.82). Sensititre® exhibited sensitivity > 0.9 for all antibiotics except for PTZ (0.86; 0.70-0.99), and specificity > 0.9 except for MXF (0.88; 0.75-0.99).

DDT and Sensititre® Broth Microdilution demonstrated strong concordance for most antibiotics. Predictive values estimated using Bayesian model showed that Sensititre® offered the highest overall sensitivity and specificity.