Analysis of cell membrane characteristics of in vitro-selected daptomycin-resistant strains of methicillin-resistant Staphylococcus aureus

Abstract

Our previous studies of clinical daptomycin-resistant (Dap(r)) Staphylococcus aureus strains suggested that resistance is linked to the perturbations of several key cell membrane (CM) characteristics, including the CM order (fluidity), phospholipid content and asymmetry, and relative surface charge. In the present study, we examined the CM profiles of a well-known methicillin-resistant Staphylococcus aureus (MRSA) strain (MW2) after in vitro selection for DAP resistance by a 20-day serial passage in sublethal concentrations of DAP. Compared to levels for the parental strain, Dap(r) strains exhibited (i) decreased CM fluidity, (ii) the increased synthesis of total lysyl-phosphatidylglycerol (LPG), (iii) the increased flipping of LPG to the CM outer bilayer, and (iv) the increased expression of mprF, the gene responsible for the latter two phenotypes. In addition, we found that the expression of the dlt operon, which also increases positive surface charge, was enhanced in the Dap(r) mutants. These phenotypic and genotypic changes correlated with reduced DAP surface binding, mirroring observations made in clinical Dap(r) isolates. In this strain, serial exposure to DAP induced an increase in vancomycin MICs into the vancomycin-intermediate S. aureus (VISA) range (4 microg/ml) in parallel with increasing DAP MICs. Also, this Dap(r) strain exhibited significantly thicker cell walls than the parental strain, potentially correlating with the coevolution of the VISA phenotype and implicating cell wall structure and/or function in the Dap(r) phenotype. Importantly, despite the overexpression of mprF and dlt, the relative net positive surface charge was decreased in the Dap(r) mutants, suggesting that other factors contribute to the surface charge alterations and that a simple charge repulsion mechanism could not entirely explain the Dap(r) phenotype in these strains.

FIG. 1.

Population analyses of study strains upon exposure to a range of vancomycin concentrations (conc.). These data represent the means (±SD) for three separate assays.

FIG. 2.

Net surface charge for the strain set based on fluorescein-isothiocyanate labeled PLL (PLL*) binding. The higher the intensity in fluorescent units (FUs), the more negative the relative surface charge.

FIG. 3.

Transmission EM of the parental strain (CB1118) and Dap^r mutant (CB2205). The thickness of the CB1118 (20.4 ± 2.7 nm) and CB2205 (31.3 ± 2.7 nm) CWs is indicated.

FIG. 4.

(A and B) Northern blot analyses for mprF and dlt operon expression. Total cellular RNA samples were isolated at 2, 6, and 12 h postinoculation from S. aureus cells cultured in TSB.