[Fosfomycin susceptibility of urinary Escherichia coli isolates producing extended-spectrum beta-lactamase according to CLSI and EUCAST recommendations].

The increasing rate of antibiotic resistance in Escherichia coli, the most common pathogen of urinary tract infections (UTIs), leads to difficulties in choosing appropriate antibiotic treatment and achieving treatment success. The aim of this study was to investigate in vitro activity of fosfomycin, presented as a favorable choice for the treatment of UTIs caused especially by extended-spectrum beta-lactamase (ESBL)-producing strains. A total of 244 E.coli strains, of them 118 were ESBL positive and 126 were negative, isolated from urine samples of inpatients and outpatients between May 2011-May 2012, were included in the study. Antibiotic susceptibilities of the isolates were determined by disk diffusion method (DDM) and ESBL production was confirmed by double-disc diffusion method according to the CLSI (Clinical and Laboratory Standards Institute) recommendations. Minimum inhibitor concentration (MIC) values for fosfomycin were detected by E-test method. Fosfomycin zone diameters and MIC values of isolates were interpreted according to the breakpoints of both CLSI and EUCAST (European Committee on Antimicrobial Susceptibility Testing). Susceptibilities of ESBL positive and negative isolates to fosfomycin and other antibiotics, and the results of fosfomycin susceptibility tests obtained by different methods were compared. The correlation between fosfomycin zone diameters and MIC values was calculated. In the study, the resistance rates of ESBL-producing isolates to ciprofloxacin, trimethoprim-sulfamethoxazole, gentamicin and amikacin were detected as 67%, 51%, 51% and 19%, respectively, while those rates were as 9%, 21%, 4% and 11%, respectively in non-ESBL producers. The difference between the two groups were found statistically significant (p< 0.001). Fosfomycin resistance of ESBL-producing and non-producing isolates were 3% and 1%, respectively, indicating no significant difference between the two groups (p= 0.356). According to fosfomycin MIC breakpoints defined by CLSI, 98.3% of ESBL-producing isolates and 100% of non-producing isolates were found susceptible to fosfomycin. According to EUCAST recommendations 98.3% of ESBL-producing isolates and 99.2% of non-producing isolates were found susceptible to fosfomycin. There was no significant difference between ESBL-positive and -negative strains according to CLSI and EUCAST recommendations (p= 0.233 and p= 0.611, respectively). When the methods were compared with each other, there were significant differences between DDM and CLSI-MIC or EUCAST-MIC (p= 0.033 and p= 0.049, respectively) and between CLSI-MIC and EUCAST-MIC (p< 0.001). There was a weak reverse linear correlation between fosfomycin zone diameters and MIC values (r= -0.138, p= 0.032). It was concluded that fosfomycin which had a high activity against ESBL-producing isolates was an appropriate alternative antibiotic in the treatment of UTIs.