Plasmid-encoded fosfomycin resistance in bacteria isolated from the urinary tract in a multicentre survey.

Sixty out of 219 fosfomycin-resistant bacteria selected from more than 7400 urinary pathogens in an epidemiological multicentre survey performed in Italy were screened for plasmid genes fosA and fosB conferring fosfomycin resistance. Only five strains, three enterobacteria and two staphylococci, carried plasmids harbouring, respectively, fosA and fosB genes. Fosfomycin resistance in the other isolates was caused by an alteration of the chromosomally encoded GlpT transport system. One strain, Morganella morganii 279, incorporated alpha-glycerolphosphate and its mechanism of fosfomycin resistance needs to be further investigated. Our study showed that PCR amplification is the most accurate, simple and rapid method for epidemiological studies of plasmid-encoded fosfomycin resistance, and that fosfomycin resistance conferred by plasmid genes (both fosA and fosB) accounts for only a low percentage of the fosfomycin-resistant strains.