Comparative evaluation of disc diffusion, Liofilchem™ MTS strips, Etest® Biomérieux with broth microdilution for eravacylin susceptibility testing

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INTRODUCTION: Eravacycline is a novel glycylcycline with broad activity against Gram-negative bacteria, including carbapenem-resistant Enterobacterales. This study aimed to evaluate the performance of gradient diffusion strips and disc diffusion methods for eravacycline susceptibility testing in comparison with a reference broth microdilution method. MATERIALS AND METHODS: A total of 303 carbapenem-non-susceptible Enterobacterales clinical isolates collected at the French National Reference Center were included. Eravacycline susceptibility was determined using broth microdilution (reference method), two gradient diffusion strips (BioMérieux Etest® and Liofilchem) and two different discs (MAST and i2a). Performances were assessed according to ISO 20776-2 guidelines using essential agreement (EA), bias, categorical agreement (CA) and error rates. RESULTS: Eravacycline exhibited lower MICs than tigecycline across all species, with higher susceptibility observed in E. coli. Neither gradient strips nor disc diffusion methods met ISO performance criteria when considering the full collection. However, when only E. coli isolates were considered, Liofilchem gradient strips and both disc diffusion methods achieved acceptable performance, whereas BioMérieux Etest® showed acceptable EA despite high bias. CONCLUSION: While eravacycline shows strong in vitro activity against carbapenem-resistant Enterobacterales, current gradient strip and disc diffusion methods demonstrate limited reliability on Enterobacterales, although good performances were achieved for E. coli. These findings highlight the need for cautious interpretation and further optimization of routine susceptibility testing methods.

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