Characterization of FosL1, a Plasmid-Encoded Fosfomycin Resistance Protein Identified in Escherichia coli.

Fosfomycin is gaining renewed interest for treating urinary tract infections. Monitoring fosfomycin resistance is therefore important in order to detect the emergence of novel resistance mechanisms. Here, we used the Rapid Fosfomycin NP test to screen a collection of extended-spectrum-β-lactamase-producing Escherichia coli isolates from Switzerland and found a fosfomycin-resistant isolate in which a novel plasmid-mediated fosfomycin resistance gene, named fosL1, was identified. The FosL1 protein is a putative glutathione S-transferase enzyme conferring high-level resistance to fosfomycin and sharing between 57% to 63% amino acid identity with other FosA-like family members. Genetic analyses showed that the fosL1 gene was embedded in a mobile insertion cassette and had likely been acquired by transposition through a Tn7-related mechanism. In silico analysis over GenBank databases identified the FosL1-encoding gene in addition to another variant (fosL1 and fosL2, respectively) in two Salmonella enterica isolates from the United States. Our study further highlights the necessity of monitoring fosfomycin resistance in Enterobacteriaceae to identify the emergence of novel mechanisms of resistance.