Daptomycin resistance and tolerance due to loss-of-function in Staphylococcus aureus dsp1 and asp23 [Mechanisms of Resistance]

Lipopeptide daptomycin is a last line cell membrane-targeting antibiotic to treat multidrug-resistant Staphylococcus aureus. Alarmingly, daptomycin-resistant S. aureus isolates have emerged. The mechanisms underlying daptomycin resistance are diverse, share similarities with resistance to cationic antimicrobial peptides and other lipopeptides, but remain to be fully elucidated. We selected mutants with increased resistance to daptomycin from a library of transposon insertions in ST8 S. aureus HG003. Insertions in conferring increased daptomycin resistance were localized to two genes, one coding for a hypothetical lipoprotein (SAOUHSC_00362, Dsp1), and the other for an alkaline shock protein (SAOUHSC_02441, Asp23). Markerless loss of function mutants were then generated for comparison. All transposon mutants and knockout strains exhibited increased daptomycin resistance compared to wild type and complemented strains. Null and transposon insertion mutants also exhibited increased resistance to cationic antimicrobial peptides. Interestingly, dsp1 also showed increased resistance to vancomycin, a cell wall targeting drug with a different mode of action. Null mutations in both dsp1 and asp23 displayed increased tolerance as reflected by reduced killing to both daptomycin and vancomycin, as well as an increased tolerance to surfactant (Triton X-100). Neither mutant exhibited increased resistance to lysostaphin, a cell wall targeting endopeptidase. These findings identified two genes core to the S. aureus species, that make previously uncharacterized contributions to antimicrobial resistance and tolerance in S. aureus.

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