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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 (bioMérieux) 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 β-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 β-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., (Copyright © 2018 Royal College of Pathologists of Australasia. Published by Elsevier B.V. All rights reserved.)

Melioidosis is caused by the Gram-negative soil saprophyte, Burkholderia pseudomallei and is endemic in tropical and subtropical regions of southeast Asia and northern Australia. Cotrimoxazole has been traditionally used for the therapy of melioidosis despite results indicating resistance often produced in the disc diffusion test against B. pseudomallei. This inconsistency was addressed by comparing this method with the agar dilution, MicroScan and E-test methods. The results demonstrated that by disc diffusion, 41.3% of 80 B. pseudomallei clinical isolates tested were susceptible to cotrimoxazole, whereas the MicroScan, agar dilution and the E-test demonstrated 92.5, 90 and 97.5% of the isolates to be susceptible, respectively. These results indicate that an MIC based method is required to test the susceptibility of B. pseudomallei against cotrimoxazole.