Evaluation of in vitro methods for testing tigecycline combinations against carbapenemase-producing Klebsiella pneumoniae isolates

Objectives: Treatment of infections caused by carbapenemase-producing Klebsiella pneumoniae (CPKP) frequently involves combination therapy with various antimicrobial agents in the hope of achieving synergistic effects. Routine laboratory antimicrobial synergy testing is a service that is currently unavailable owing to the laborious nature of the reference time-kill assay (TKA) as well as the widely used chequerboard method. In this study, we explored whether easier methods, based on the Etest technique, might offer a suitable alternative. Methods: In vitro interactions of tigecycline combination with colistin, gentamicin, fosfomycin or meropenem against 26 CPKP isolates were evaluated employing the TKA, chequerboard method and three Etest methodologies (the MIC/MIC ratio, the cross formation and the agar/Etest method). Rates of consequent synergy and concordance of the studied methods were determined. Results: All antimicrobial combinations demonstrated some degree of synergy against the CPKP isolates tested. No antagonism was observed for any of the combinations. All methods showed poor synergy concordance with the TKA, producing non-significant kappa (kappa) results. Etest methods (MIC/MIC ratio and agar/Etest) exhibited fair agreement (kappa = 0.29 and 0.38, respectively) with the chequerboard method. Conclusion: There is a poor correlation between synergy testing methods of tigecycline combinations, which may be associated with their different endpoints. To elucidate method comparability and reliability, their correlation with clinical outcomes appears important. (C) 2019 International Society for Antimicrobial Chemotherapy. Published by Elsevier Ltd.