Evaluation of MICROANAUT-S Anaerobes MIC broth microdilution panels for antibiotic susceptibility testing of anaerobes

Accurate and routine-friendly methods for MIC determination of anaerobes are demanded. Here, we evaluate the performance of a commercially available microdilution panel in comparison to a gradient MIC Strip test.

N=163 anaerobes clinical isolates (60 species, 23 genera) were tested with the MICRONAUT-S Anaerobes MIC panel (MERLIN Diagnostika, Germany). The same bacterial suspension was used for the panel inoculum, and for testing by MIC Strips (Liofilchem, Italy). The panels were incubated at 37 °C in anaerobic conditions for ≥24 h, and read visually. In case of no bacterial growth for the growth control, incubation was prolonged to 48-72 h. Results were interpreted according to EUCAST guidelines. Comparison was performed in terms of essential agreement, category agreement and error rates.

Category agreement with MIC strips resulted overall 95% (from 95.6% to 100%). Essential agreement resulted 91.1% for piperacillin/tazobactam, and 95% for all the otherantibiotics. Overall, n=15 minor errors, n=2 major errors and n=5 very major errors were observed. For doxycycline, tigecycline and moxifloxacin (for which no breakpoints are available), MICRONAUT results diverged for ≤1 dilution fold from MIC Strips results. For most isolates (117/163) the panels were readable after overnight incubation, in 42 cases after 48 h, in 4 cases after 72 h.

The MICRONAUT-S Anaerobes MIC panels proved to be a reliable microdilution method for antibiotic susceptibility testing of anaerobes, providing results consistent with gradient methodology, with both fast and slow growing species. The ease-of-handling and -result interpretation makes this method suitable for routine implementation.