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Objectives:  Daptomycin (DAP) resistance in  Staphylococcus aureus  is uncommon but there are increasing reports of the emergence of resistance during DAP therapy. Most clinical DAP-resistant  S. aureus  isolates investigated carried mutations in the  mprF  gene. The aim of this study was to identify mutations between a clinical pair of methicillin-susceptible  S. aureus  (MSSA) isolates (DAP-susceptible and DAP-resistant). Additionally, the activity of genes previously associated with DAP resistance was assessed.   Materials and Methods:  Two MSSA isolates from patient with left-sided endocarditis were analyzed by whole genome sequencing (WGS) and reverse transcription-quantitative real-time PCR (RT-qPCR). The first isolate, DAP-susceptible, was obtained before initiation of treatment and the second isolate, DAP-resistant, was recovered after 4 weeks of DAP therapy.   Results:  Comparison of complete genomes of DAP-susceptible and its DAP-resistant variant identified two non-synonymous and one synonymous mutations. The non-synonymous mutations consisted of a S829L substitution in  mprF  and a T331I substitution in  vraS . The RT-qPCR experiments revealed an increased expression of  vraS, dltA, mprF , and  sceD  genes in DAP-resistant variant. Strikingly, the expression of  dltA  and  mprF  genes was significantly downregulated by DAP.   Conclusion:  The  mprF  and  vraS  genes were previously associated with DAP resistance, however, none of the mutations described in this study had been previously identified and linked to DAP resistance. Moreover, we provide a new insight into the DAP action on  S. aureus , in which the expression of key genes in DAP resistance is decreased by the antibiotic.

frontiers in microbiology

Daptomycin Resistant Staphylococcus aureus Clinical Strain With Novel Non-synonymous Mutations in the mprF and vraS Genes: A New Insight Into Daptomycin Resistance