Prevalence of fosfomycin resistance and gene mutations in clinical isolates of methicillin-resistantStaphylococcus aureus

Background Fosfomycin exhibits excellent in vitro activity against multidrug-resistant pathogens, including methicillin-resistantStaphylococcus aureus(MRSA). Increasing fosfomycin resistance among clinical MRSA isolates was reported previously, but little is known about the relative abundance of Fosfomycin resistance genes in MRSA isolates circulating in Taiwan. Methods All MRSA isolates, collected in 2002 and 2012 by the Taiwan Surveillance of Antimicrobial Resistance (TSAR) program, were used in this study. Susceptibility to various antimicrobial agents, including fosfomycin, was determined by broth microdilution. Genetic determinants of fosfomycin resistance, includingfosBcarriage andmurA,glpTanduhpTmutations, were investigated using PCR and sequencing of amplicons. Staphylococcal protein A (spa) typing was also performed to determine the genetic relatedness of MRSA isolates. Results A total of 969 MRSA strains, 495 in the year 2002 and 474 in the year 2012, were analyzed. The overall in vitro susceptibility was 8.2% to erythromycin, 18.0% to clindamycin, 29.0% to tetracycline, 44.6% to ciprofloxacin, 57.5% to trimethoprim/sulfamethoxazole, 86.9% to rifampicin, 92.9% to fosfomycin and 100% to linezolid and vancomycin. A significant increase in the fosfomycin resistance rate was observed from 3.4% in 2002 to 11.0% in 2012. Of 68 fosfomycin-resistant MRSA isolates, several genetic backgrounds probably contributing to fosfomycin resistance were identified. Twelve isolates harbored thefosBgene, and various mutations inmurA,uhpT, andglpTgenes were noted in 11, 59, and 66 isolates, respectively. The most prevalent gene mutations were found in the combination of uhpT and glpT genes (58 isolates). The vast majority of the fosfomycin-resistant MRSA isolates belonged tospatype t002. Conclusions An increased fosfomycin resistance rate of MRSA isolates was observed in our present study, mostly due to mutations in theglpTanduhpTgenes. Clonal spread probably contributed to the increased fosfomycin resistance.