Fosfomycin is an orally bioavailable bactericidal agent which concentrates significantly in urine and is being increasingly prescribed for patients with Multidrug Resistant Enterobacteriaceae (MDRE) urinary isolates. According to the Clinical and Laboratory Standards Institute (CLSI), disc diffusion (DD) and agar dilution (AD) are the standards for fosfomycin susceptibility testing for urinary Escherichia coli and Enterococcus faecalis isolates, whereas the European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines suggest both AD and broth microdilu- tion (BD), but not DD. A prospective study was done in the Department of Microbiology in a tertiary care teaching hospital in Kolkata. MDRE urinary isolates that were resistant to oral and third-generation cephalosporins, either ciprofloxacin or levofloxacin and either of cotrimoxazole or nitrofurantoin were included in the study and tested further for fosfomycin. DD was carried out on Mueller-Hinton agar (MHA) supplemented with 25 µg/ml G6P with 50 and 200 µg discs. E-test was carried out on the same medium with drug concentrations ranging from 0.064 to 1024 µg/ml. Broth dilution was carried out on G6P supplemented Mueller Hinton broth with drug concentrations ranging from 0.25 to 1024 µg/ml. A total of 723 urine samples were obtained in a 2-month period, which yielded 79 Enterobacteriaceae, out of which 30 were MDRE. About 26 (86.67%) of these isolates were susceptible to fosfomycin by CLSI criteria and 25 (83.33%) by EUCAST criteria for minimum inhibitory concen- tration (MIC) by E-test and broth dilution. Only 4 (13.33%) were resistant to fosfomycin by DD. However, heteroresistant colonies were found among 21 (70%) isolates by E-test. None of these were picked up by DD or BD. The MIC of fosfomycin was between 0.25 and 512 µg/ml. These heteroresistant colonies when further subjected to E-test and BD showed resistance with MIC 512 µg/ml, depicting prevalence of heteroresist- ance. E-test can be used as a simple and effective screening method for identifying the fosfomycin heteroresistance and in turn changing the myth of the so high susceptibility rates among MDRE.
