Heterogeneity of Genetic Pathways toward Daptomycin Nonsusceptibility in Staphylococcus aureus Determined by Adjunctive Antibiotics

Abstract

ABSTRACTDaptomycin is increasingly used in combination with other antibiotics to enhance antimicrobial efficacy and/or to mitigate the emergence of daptomycin nonsusceptibility (DNS). This study used a clinical methicillin-resistantStaphylococcus aureus(MRSA) strain in which DNS emerged upon therapy to examine the influence of antibiotic combinations on the development of mutations in specific genes (mprF,rpoBC,dltA,cls2, andyycFG) previously associated with DNS. Whole genomes of bacteria obtained following 28 days ofin vitroexposure to daptomycin with or without adjunctive clarithromycin, linezolid, oxacillin, or trimethoprim-sulfamethoxazole were sequenced, and the sequences were compared to that of the progenitor isolate. The addition of oxacillin to medium containing daptomycin prevented the emergence ofmprFmutation but did not preventrpoBCmutation (P< 0.01). These isolates maintained susceptibility to daptomycin during the combined exposure (median MIC, 1 mg/liter). Daptomycin plus clarithromycin or linezolid resulted in low-level (1.5 to 8 mg/liter) and high-level (12 to 96 mg/liter) DNS, respectively, and did not preventmprFmutation. However, these same combinations preventedrpoBCmutation. Daptomycin alone or combined with linezolid or trimethoprim-sulfamethoxazole resulted in high-level DNS and mutations inmprFplusrpoBC,cls2, andyycFG. Combining daptomycin with different antimicrobials alters the mutational space available for DNS development, thereby favoring the development of predictable collateral susceptibilities.