Comparison of in vitro fosfomycin susceptibility testing methods with agar dilution for carbapenem resistant Klebsiella pneumoniae and Escherichiacoli.

Purpose: The emergence of Extensively drug resistant (XDR) pathogens like Carbapenem Resistant Klebsiella pneumoniae (CR Kpn) and Carbapenem Resistant Escherichia coli (CR Eco) has limited therapeutic options for treating them. Fosfomycin a broad-spectrum antibiotic, has emerged as a potential treatment option in combination with other agents. It is therefore important that accurate drug susceptibility testing (DST) results of fosfomycin should be available to all clinical microbiology laboratories. Agar dilution which is the recommended method for fosfomycin DST is not convenient to adopt in a routine set-up. This study aimed to determine the susceptibility pattern of CR Kpn and CR Eco to fosfomycin and to evaluate the discrepancies of the available manual MIC based alternative methods.

Methods: Agar dilution (AD), broth microdilution (BMD), E-test and Ezy MIC test were performed on 235 CR-Kpn and Eco isolates respectively.

Results: Of 177 CR Kpn, 31.63% (n ​= ​56/177) of the isolates were susceptible by AD. Categorical Agreement (CA) by BMD, E-test and Ezy MIC were lower than the acceptable limit while Very Major Errors (VMEs) and Major Errors (MEs) were beyond the acceptable limits. In the case of CR Eco, 96.55% (n ​= ​56/58) were susceptible by AD. CA of 100% (n ​= ​58/58) was shown by both BMD and Ezy MIC while 86.20% (n ​= ​50/58) was shown by E-test, with no VME observed for CR Eco. ME was only observed for E-test method.

Conclusion: The alternative methods were in poor agreement with AD method for CR Kpn and for CR Eco, BMD and Ezy MIC have shown reliable results.