Nature of bacitracin resistance in Staphylococcus aureus

Bacitracin is employed for topical treatment of staphylococcal infection, though information is lacking regarding the nature of resistance to this antibiotic in the staphylococci. Here we examined bacitracin resistance in a large collection (n = 1,470) of multidrug-resistant isolates of Staphylococcus aureus. Susceptibility testing of the entire collection revealed a broad range of bacitracin minimum inhibitory concentrations (MICs) (from 32 to >4,096 µg/mL) and allowed us to define a tentative epidemiological cut-off value (TECOFF) (apparent upper end of the wild-type distribution in terms of bacitracin susceptibility) of 256 µg/mL. On the basis of this TECOFF, 101 strains (6.8% of the total) for which bacitracin had an MIC of ≥512 µg/mL were considered resistant. Bacitracin resistance was found across multiple sequence types (STs), though over half the resistant strains belonged to the ST8:USA300 lineage. In nearly all bacitracin-resistant strains (99 of 101), whole genome sequence analysis identified operons similar to the bcrABD operon that confers bacitracin resistance in Enterococcus faecalis; 3 strains carried an operon [bcrAB(ISL3)D] closely related to the latter (~90% identity in the encoded resistance proteins), while 96 strains harbored a more distantly related operon (<50% identity in the encoded proteins) that we distinguished with the designation bcrEFH. Molecular cloning experiments confirmed the ability of both bcr operons to confer bacitracin resistance in S. aureus. Both bcrAB(ISL3)D and bcrEFH reside on IS6-flanked pseudo-compound transposons on multidrug resistance plasmids, highlighting their potential to spread and become more widely disseminated among staphylococci.