Characterization of fosA5 , a new plasmid-mediated fosfomycin resistance gene in Escherichia coli

A clinical strain of extended-spectrum β-lactamase-producing Escherichia coli E265, with a fosfomycin MIC of 512 μg ml(-1), was isolated from an inpatient with hospital-acquired pneumonia. This strain was negative for known fos genes, had no mutation in the target enzyme by polymerase chain reaction amplification and had functional transport systems for fosfomycin uptake. Fosfomycin resistance could be transferred from strain E265 to E. coli J53 azide(R) by conjugation. The DNA fragment containing fosfomycin resistance determinants was cloned into E. coli TOP10. The minimal inhibitory concentrations of fosfomycin for the transconjugant and transformant were 512 and 1024 μg ml(-1). By sequencing, a plasmid-mediated fosA subtype, designated fosA5, was found and characterized. The fosA5 gene was 420 bp in length and encoded a 139-amino-acid protein that shared 69 to 80% identity with FosA, FosA2, FosA3 and FosA4, and 31, 14 and 25% identity with FosB, FosC and FosX, respectively. The analysis of genetic environment of fosA5 suggested that a strain such as Klebsiella pneumoniae CG4 might be the origin of plasmid-mediated fosA5, with IS10 playing an important role in its mobilization.This study aimed to clone and characterize a plasmid-mediated fosA subtype gene, fosA5, in a clinical strain of ESBL-producing Escherichia coli, which confers fosfomycin resistance. Detection of the fosA5 gene clarified the mechanism of fosfomycin resistance in a strain that was negative for known fosfomycin resistance genes. Monitoring and surveillance will be important to follow the changes in fosfomycin resistance and prevent further dissemination of fos genes.