Up-regulation of resident chromosomal fosB gene expression: a novel mechanism of acquired fosfomycin resistance in MRSA

Objectives This study investigated fosfomycin susceptibility and mechanisms of resistance in a collection of 99 Staphylococcus aureus isolated from cases of hospital-acquired pneumonia, previously collected from a multicentre survey carried out in Italy. Methods Fosfomycin susceptibility was tested by reference agar dilution. Bioinformatic and gene expression analysis, mutant selection experiments and WGS were executed to characterize fosfomycin resistance mechanisms. Results Fosfomycin resistance rates were 0% (0 of 35) among MSSA and 22% (14 of 64) among MRSA, with no evidence of clonal expansion. Resistance mechanisms were putatively identified in 8 of the 14 resistant strains, including: (i) chromosomal mutations causing loss of function of the UhpT transporter; (ii) overexpression of the gene encoding the Tet38 efflux pump; and (iii) overexpression of a fosB gene encoding a fosfomycin-inactivating enzyme, which was found to be resident in the chromosome of several S. aureus lineages but not always associated with fosfomycin resistance. The latter mechanism, which had not been previously described and was confirmed by results of in vitro mutant selection experiments, was associated in two cases with transposition of an IS1182 element upstream of the chromosomal fosB gene, apparently providing an additional promoter. Conclusions This study showed that some S. aureus clonal lineages carry a resident chromosomal fosB gene and can evolve to fosfomycin resistance by overexpression of this gene.