Genetic Environment and Stability ofcfrin Methicillin-Resistant Staphylococcus aureus CM05

Abstract

ABSTRACTThe Cfr methyltransferase confers resistance to many 50S ribosomal subunit-targeted antibiotics, including linezolid (LZD), via methylation of the 23S rRNA base A2503 in the peptidyl transferase center. Methicillin-resistantStaphylococcus aureusstrain CM05 is the first clinical isolate documented to carrycfr. Whilecfris typically plasmid borne, in CM05 it is located on the chromosome and is coexpressed withermBas part of themlroperon. Here we evaluated the chromosomal locus, association with mobile genetic elements, and stability of thecfrinsertion region in CM05. Thecfr-containingmlroperon is located within a 15.5-kb plasmid-like insertion into 23S rRNA allele 4. The region surrounding thecfrgene has a high degree of sequence similarity to the broad-host-range toxin/antitoxin multidrug resistance plasmid pSM19035, including a secondermBgene downstream of themlrlocus andistAS-istBS. Analysis of several individual CM05 colonies revealed two distinct populations for which LZD MICs were either 8 or 2 μg/ml. In the LZDscolonies (designated CM05Δ), a recombination event involving the twoermBgenes had occurred, resulting in the deletion ofcfrand the 3′ flanking region (cfr-istAS-istBS-ermB). The fitness advantage of CM05Δ over CM05 (though not likely due to thecfrdeletion itself) results in the predominance of CM05Δ in the absence of selective pressure. Minicircles resulting from theermBrecombination event and the novel association ofcfrwith the pSM19035 plasmid system support the potential for the continued dissemination ofcfr.