Determination of Fosfomycin Susceptibility in Escherichia coli Isolates by Various Methods

Introduction: Because of its broad-spectrum activity and its safety profile, fosfomycin is often used as a first-line antibiotic for treating uncomplicated urinary tract infections. It is also one of the treatment options in various infections such as central nervous system, bone and joints, lung, soft tissue infections and sepsis. Escherichia coli is frequently encountered as a causative agent in such infections and is responsible for more than 80% of community-acquired urinary tract infections. There is a need to develop a fast and easily applicable test to evaluate the efficacy of fosfomycin. The aim of this study is to determine the susceptibility of fosfomycin in Escherichia coil isolates obtained from clinical isolates by various methods and to evaluate these methods in terms of ease of use, sensitivity, and specificity. Materials and Methods: A total 175 E. coli isolates obtained from various cultures sent from clinics between November 2020 and April 2021 were included in the study. Agar dilution, disc diffusion method, in-tube macrodilution method with fosfomycin discs and rapid fosfomycin/E. coli NP test was used for determining fosfomycin susceptibility of isolates. Results: 169 (97%) of E. coil isolates were by agar dilution, 165 (94%) by disc diffusion test (DDT), 165 (94%) by macrodilution tube using fosfomycin discs, 167 (95%) fast fosfomycin/E. coil NP test was found to be susceptible to fosfomycin. With agar dilution, MIC50 = 1 mu g/mL, MIC90 = 32 mu g/mL were determined. Considering the agar dilution as a reference method; the sensitivity and specificity of the DDT, macrodilution tube method using fosfomycin discs, rapid fosfomycin/E. coli test were 100% and 98.8%, 100% and 97.6%, 100% and 97.6%, respectively. Conclusion: As fosfomycin use increases, the demand for susceptibility testing of this agent is expected to rise. Despite numerous reports of its activity, broader clinical use of fosfomycin is hindered by the difficulty in performing susceptibility testing. Therefore, it is imperative to implement the best method to deliver reliable results in a timely manner. Laboratories need rapid, easily applicable and evaluable antimicrobial susceptibility tests. In this sense, the gold-standard method of agar dilution is not a valid option for many clinical laboratories. Although DDT is a more applicable test routinely, rapid fosfomycin/E. coil NP test is promising both in terms of speed and ease of application