Anaerobic organism susceptibility testing is not commonly performed in the microbiology laboratory and the antibiotic susceptibility patterns of local isolates are often not known. We performed anaerobic susceptibility testing on 70 prospective clinically relevant Gram negative anaerobes isolated from routine cultures in a busy diagnostic laboratory which were identified by Maldi-Tof. The susceptibility testing was performed by two methods – Sensititre trays (Thermo Fisher Scientific) against 15 different antibiotics; and E-tests (Biomerieux) against five clinically relevant antibiotics – metronidazole, piperacillin-tazobactam, amoxicillin-clavulanate, meropenem and clindamycin.
We found that the all of the isolates were susceptible to metronidazole, and overall susceptibility to commonly used antibiotics such as piperacillin-tazobactam and amoxicillin-clavulanate were high (96% and 95% respectively). Two isolates of Bacteroides fragilis were resistant to both broad spectrum beta-lactamases and carbapenems.
The E-test method was easier to perform than Sensititre trays; both demonstrated trailing endpoints classical to anaerobe susceptibility testing. Comparing the two methods, using Sensititre broth microdilution as gold standard, found 32 (14%) minor errors, 12 (4%) major errors and 3 (1%) very major errors.
Anaerobic organism antibiograms provide useful information for clinicians when choosing antimicrobials for infections with anaerobic organisms. This study has shown that in our area, use of metronidazole as a broad spectrum anti-anaerobic agent remains advisable. Anaerobic susceptibility testing is also important to perform in individual clinical isolates, especially from sterile sites or in pure culture. The emergence of broad spectrum beta lactam and carbapenem resistance in clinical isolates of Bacteroides fragilis is of concern and will require further monitoring. The method chosen to perform susceptibility testing depends on the needs and expertise of the laboratory.