Nature of bacitracin resistance in <i>Staphylococcus aureus</i>

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 (<i>n</i> = 1,470) of multidrug-resistant isolates of <i>Staphylococcus aureus</i>. 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 <i>bcrABD</i> operon that confers bacitracin resistance in <i>Enterococcus faecalis</i>; 3 strains carried an operon [<i>bcrAB</i>(ISL3)<i>D</i>] 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 <i>bcrEFH</i>. Molecular cloning experiments confirmed the ability of both <i>bcr</i> operons to confer bacitracin resistance in <i>S. aureus</i>. Both <i>bcrAB</i>(ISL3)<i>D</i> and <i>bcrEFH</i> reside on IS<i>6</i>-flanked pseudo-compound transposons on multidrug resistance plasmids, highlighting their potential to spread and become more widely disseminated among staphylococci.