Investigation of Fosfomycin Susceptibility in Extended Spectrum Beta-Lactamase Producing Enteric Bacteria by Disk Diffusion, Automated System and Agar Dilütion Kit

Abstract

Due to the increase of bacteria that develop resistance to classical antibiotics in recent years, fosfomycin has become one of the antibiotics gaining importance again. Fosfomycin exerts its bactericidal effect on many bacteria by binding to a cytoplasmic enzyme, MurA, and inhibiting the enzyme enopurivil transferase, which is involved in peptidoglycan synthesis. Bacteria producing ESBL are common pathogens in nosocomial infections and resistance is frequently encountered in their treatment. Fosfomycin is considered as an alternative drug in these infections. With its reintroduction, it is important to establish reliable and reproducible susceptibility testing methods for fosfomycin. In this study, it was aimed to evaluate the utility of disk diffusion, agar dilution kit and automated MicroScan methods (Beckman Coulter, Brea, CA, ABD) for fosfomycin susceptibility of ESBL-producing enteric bacteria. The study included 120 ESBL positive isolates of enteric bacteria (89 Escherichia coli, 31 Klebsiella pneumoniae) cultivated from the samples sent to the Microbiology Laboratory of the Mersin University Medical Faculty of Medicine Hospital between May 2019 and November 2019. Fosfomycin susceptibilities of the isolates included in the study were tested by disk diffusion, automated MicroScan and the commercial agar dilution kit. In the study, 37 (30.8%) of 120 isolates were determined as resistant to fosfomycin by disk diffusion method and six (5%) of the isolates were found to be resistant to fosfomycin with the MicroScan automatized system. With the agar dilution kit, 11 (9.1%) of 120 isolates were found to be resistant as the minimum inhibitory concentration (MIC) values were > 32 µg/ml. Fosfomycin resistance was significantly higher in K.pneumoniae isolates than E.coli isolates by agar dilution method (p= 0.006). Ertapenem resistance was detected in seven of 120 isolates. All seven isolates that were resistant to ertapenem were susceptible to fosfomycin by agar dilution method. All three isolates, which were found resistant to colistin, were susceptible to fosfomycin by agar dilution method. In the study, the categorical agreement of the disk diffusion and MicroScan methods in E.coli isolates with the agar dilution was high and the error rates were low. For K.pneumoniae, categorical concordance was calculated as 22.5% for disk diffusion test and 74.1% for MicroScan, while major and minor error rates were found to be quite high. As a result; fosfomycin is thought to be an alternative drug option in the treatment of infections caused by especially carbapenem and colistin resistant bacteria, with its high activity on ESBL-producing enteric bacteria. It has been determined that there is a great agreement between the agar dilution, which is the reference method for the determination of fosfomycin resistance for E.coli, and the MicroScan and disk diffusion methods, and that the disk diffusion method which provides ease of use in terms of cost and labor compared to the agar dilution method, can be used as a reliable method. However, it has been demonstrated that the agar dilution method should be used in K.pneumoniae isolates. Nevertheless; It was concluded that if the automatized system is used in laboratories where agar dilution cannot be performed, the results should be interpreted carefully, since the error rates are very high.