Causal Role of Single Nucleotide Polymorphisms within the mprF Gene of Staphylococcus aureus in Daptomycin Resistance

Single nucleotide polymorphisms (SNPs) within the mprF open reading frame (ORF) have been commonly observed in daptomycin-resistant (DAP(r)) Staphylococcus aureus strains. Such SNPs are usually associated with a gain-in-function phenotype, in terms of either increased synthesis or enhanced translocation (flipping) of lysyl-phosphatidylglycerol (L-PG). However, it is unclear if such mprF SNPs are causal in DAPr strains or are merely a biomarker for this phenotype. In this study, we used an isogenic set of S. aureus strains: (i) Newman, (ii) its isogenic Delta mprF mutant, and (iii) several in trans plasmid complementation constructs, expressing either a wild-type or point-mutated form of the mprF ORF cloned from two isogenic DAP-susceptible (DAP(s))-DAP(r) strain pairs (616-701 and MRSA11/11-REF2145). Complementation of the Delta mprF strain with singly point-mutated mprF genes (mprF(S295L) or mprF(T345A)) revealed that (i) individual and distinct point mutations within the mprF ORF can recapitulate phenotypes observed in donor strains (i.e., changes in DAP MICs, positive surface charge, and cell membrane phospholipid profiles) and (ii) these gain-in-function SNPs (i.e., enhanced L-PG synthesis) likely promote reduced DAP binding to S. aureus by a charge repulsion mechanism. Thus, for these two DAPr strains, the defined mprF SNPs appear to be causally related to this phenotype.