Gram negative anaerobe susceptibility testing in clinical isolates using Sensititre and Etest methods

Gram negative anaerobic organisms are important pathogens in a range of clinical infections, and susceptibility testing is not commonly performed in the microbiology laboratory. We performed anaerobic susceptibility testing on 70 clinically relevant Gram negative anaerobes isolated from routine cultures in a busy diagnostic laboratory which were identified by MALDI-TOF mass spectrometry (MALDI-TOF MS). The susceptibility testing was performed by two methods: Sensititre trays (ThermoFisher Scientific) against 15 different antibiotics, and Etests (bioMerieux) against five clinically relevant antibiotics (metronidazole, piperacillin-tazobactam, amoxicillin-clavulanate, meropenem and clindamycin). We found that all isolates were susceptible to metronidazole, and overall susceptibility to commonly used antibiotics such as piperacillin-tazobactam and amoxicillin-clavulanate was high (93-100% and 89-100%, respectively). Two isolates of Bacteroides fragilis were resistant to both broad spectrum beta-lactams and carbapenems. Comparing the two methods, using Sensititre broth microdilution as gold standard, there was high categorical agreement (92-100%). Antibiograms provide useful information for clinicians when choosing antimicrobials for infections caused by anaerobic organisms. This study has shown that in our area, use of metronidazole as a broad spectrum anti-anaerobic agent remains appropriate. 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 Etest method was considered superior to Sensititre trays given that the higher inoculum used may allow demonstration of heteroresistance, anaerobiasis can be maintained during setup, lower failure rates, and the ability to select only the antibiotics required.