Cell Wall Thickening Is Not a Universal Accompaniment of the Daptomycin Nonsusceptibility Phenotype in Staphylococcus aureus: Evidence for Multiple Resistance Mechanisms

The mechanism(s) of daptomycin (DAP) resistance (DAP(r)) is incompletely defined. Thickened cell walls (CWs) acting as either a mechanical barrier or an affinity trap for DAP have been purported to be a major contributor to the DAP(r) phenotype. To this end, we studied an isogenic set of methicillin-resistant Staphylococcus aureus (MRSA) isolates (pulsotype USA 300) from the bloodstream of a DAP-treated patient with endocarditis in which serial strains exhibited increasing DAP(r). Of interest, the DAP(r) isolate differed from its parental strain in several parameters, including acquisition of a point mutation within the putative synthase domain of the mprF gene in association with enhanced mprF expression, increased synthesis of lysyl-phosphotidylglycerol, an enhanced positive envelope charge, and reduced DAP surface binding. Transmission electron microscopy (TEM) revealed no significant increases in CW thickness in the two DAP(r) isolates (MRSA 11/21 and REF2145) compared with that in the DAP-susceptible (DAP(s)) parental strain, MRSA 11/11. The rates of Triton X-100-induced autolysis were also identical for the strain set. Furthermore, among six additional clinically isolated DAP(s)/DAP(r) S. aureus strain pairs, only three DAP(r) isolates exhibited CWs significantly thicker than those of the respective DAP(s) parent. These data confirm that CW thickening is neither universal to DAP(r) S. aureus nor sufficient to yield the DAP(r) phenotype among S. aureus strains.