Decreased vancomycin susceptibility in Staphylococcus aureus caused by IS256 tempering of WalKR expression

Chris R McEvoy, Brian Tsuji, Wei Gao, Torsten Seemann, Jessica Lee Porter, Kenneth D Doig, Dung Ngo, Benjamin Peter Howden, Timothy Paul Stinear

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36 Citations (Scopus)

Abstract

Vancomycin intermediate S. aureus (VISA) often arise by mutations in the essential two-component regulator walKR; however their impact on walKR function has not been definitively established. Here, we investigated ten MRSA recovered serially, after exposure of vancomycin-susceptible S. aureus JKD6009 to simulated human vancomycin dosing regimens (500mg - 4000mg every 12h) using a 10-day hollow fibre infection model. After continued exposure to the vancomycin regimens, two isolates displayed reduced susceptibility to both vancomycin and daptomycin, developing independent IS256 insertions in the walKR 5 -untranslated region (5 -UTR). Quantitative RT-PCR revealed a 50 reduction in walKR gene expression in the IS256 mutants compared to the VSSA parent. GFP reporter analysis, promoter mapping and site-directed mutagenesis confirmed these findings and showed that the IS256 insertions had replaced two SigA-like walKR promoters with weaker, hybrid promoters. Removal of IS256 reverted the phenotype to VSSA, showing that reduced expression of WalKR did induce the VISA phenotype. Analysis of selected WalKR-regulated autolysins revealed upregulation of ssaA but no change in expression of sak and sceD in both IS256 mutants. Whole genome sequencing of the two mutants revealed an additional IS256 insertion within agrC for one mutant and we confirmed that this mutation abolished agr function. These data provide the first substantial analysis of walKR promoter function and show that prolonged vancomycin exposure can result in VISA through an IS256 mediated reduction in walKR expression, however the mechanisms by which this occurs remain to be determined.
Original languageEnglish
Pages (from-to)3240 - 3249
Number of pages10
JournalAntimicrobial Agents and Chemotherapy
Volume57
Issue number7
DOIs
Publication statusPublished - 2013

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