[Show abstract][Hide abstract] ABSTRACT: Using isogenic clinical bloodstream Staphylococcus aureus strains from a patient with relapsing endocarditis, we investigated the transcriptional profiles of the mprF and dlt genes in the context of cell-surface charge and daptomycin nonsusceptibility. As in prior studies, a point mutation within mprF was observed in the daptomycin-nonsusceptible strain. However, neither the transcriptional profile of mprF nor the membrane phospholipid analyses were compatible with the anticipated mprF gain-in-function phenotype. In contrast, we demonstrated enhanced dlt expression coincident with increased positive surface charge and reduced daptomycin binding.
The Journal of Infectious Diseases 12/2009; 200(12):1916-20. · 5.85 Impact Factor
[Show abstract][Hide abstract] 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.
Antimicrobial Agents and Chemotherapy 04/2009; 53(6):2312-8. · 4.57 Impact Factor