Investigation of in vitro antibiotic susceptibilities of escherichia coli, klebsiella pneumoniae, acinetobacter baumannii and pseudomonas aeruginosa isolated from urine cultures of inpatients and determination of the mechanisms of resistance to fosfomycin.

The most common pathogens in urinary tract infections are E. coli, P. aeruginosa, K. pneumoniae. and A. baumannii. Therapy which is often empirical requires knowledge on local antibiotic resistance data. Therefore, it is necessary to determine the rate of resistance to antibiotics periodically in each hospital. Awareness of the resistance mechanisms against antibiotics that are currently in use is needed to prevent the spread of resistance. Fosfomycin, which is a widely used antibiotic in urinary tract infections is a broad-spectrum, bactericidal agent which was produced in 1969 and was re-introduced recently because of its effectivity against many problem pathogens such as multidrug-resistant (MDR) Enterobacterales. However, emergence of resistance to this agent is being reported worldwide. Resistance to fosfomycin may be due to three mechanisms; decreased permeability, modification of the target protein MurA and enzymatic inactivation. The first two mechanisms are chromosomal and the third is plasmid mediated. There are reports on fosfomycin resistance in our country but the prevalence of resistance mechanisms is not known. In this study, antibiotic susceptibility profiles of consecutive urinary isolates of E. coli (n=235), K. pneumoniae (n=56), A. baumannii (n= 7) and P. aeruginosa (n = 2) collected between February 2018 and February 2019 from inpatients in Hacettepe University Hospitals Bacteriology Laboratory were evaluated. In isolates resistant to fosfomycin, (MIC > 32mg/L), presence of plasmid mediated genes fosA, fosA3 and fosC2 which cause enzymatic inactivation were investigated. In our study, agar dilution method which is the gold standard for fosfomycin was employed for fosfomycin and broth microdilution test for other antibiotics (amikacin, gentamicin, ceftazidime, ceftriaxone, meropenem, ciprofloxacin, tigecycline, trimethoprim/sulfamethoxazole, colistin and piperacillin/ tazobactam) was used. PCR method was employed to investigate plasmid mediated fosfomycin resistance genes and for the analysis of blaCTXM. For amikacin, gentamicin, ceftazidime, ceftriaxone, meropenem, piperacillin / tazobactam, ciprofiloxacin, tigecycline, trimethoprim-sulfamethoxazole and colistin rates of resistance were 3.3%, 18.7%, 26.0%, 34.0%, 4.3%, 24.0%, 45.3%, 4.0%, 44.7% and 7.0% and respectively. Twenty isolates (6.6%) were resistant to fosfomycin. Resistance was most frequently observed in K. pneumoniae (n=9). fosA genes were detected in two K. pneumoniae isolates and fosA and fosA3 genes occured together in one isolate. This isoate also carried blaCTX-M. fosC2 gene was not detected in any of the isolates. In our study, only three resistance genes were investigated and other genes which may be responsible for fosfomycin resistance should also be investigated. Presence of plasmid originated genes in our isolates suggest that resistance will increase with the widespread use of fosfomycin. Therefore, antibiotic stewardship principles should be followed and the development of resistance should be closely monitored.