01 Set First detection of a cfr-positive MRSA in Italy

Alberto Antonelli

Infections mediated by MRSA represent a worldwide threat, and linezolid is considered one of the last resort antibiotics for these infections. In recent years few cases of acquired linezolid resistance mediated by the cfr gene, were reported worldwide. The diffusion of this transferable gene is worrisome because it confers resistance to several anti-ribosomal drugs including oxazolidinones, phenicols, lincosamides, pleuromutilins, streptogramin A and certain 16 membered-ring macrolides. A linezolid resistant cfr-positive MRSA, named AOUC-0915, was isolated in September 2015 from the respiratory secretions of a cystic fibrosis patient, following an erroneous under-dosage linezolid therapy. In this study the complex resistome of AOUC-0915 was characterized. Whole-genome sequencing of Staphylococcus aureus AOUC-0915 revealed the presence of multiple resistance factors. In S. aureus AOUC-0915, cfr gene was inserted into Tn6349, an original chromosomally located composite transposon of 48.4-kb with a scaffold resembling a plasmid previously found in E. faecalis. Tn6349 carried also erm(B) and fexB resistance genes, and a previously uncharacterized putative efflux pump. The latter gene was cloned into an expression vector in Escherichia coli and S. aureus, in order to study its possible contribution to the multi-drug resistance phenotype of S. aureus AOUC-0915. Finally, an original transferable plasmid of 35 kb carrying tet(M) and erm(C) resistance genes was also characterized.