Phenotypic and Genotypic Characterization of Daptomycin-Resistant Methicillin-Resistant Staphylococcus Aureus Strains: Relative Roles of mprF and DLT Operons

PLoS One. 2014 Sep 16;9(9):e107426. doi: 10.1371/journal.pone.0107426. eCollection 2014.

Abstract

Development of in vivo daptomycin resistance (DAP-R) among Staphylococcus aureus clinical isolates, in association with clinical treatment failures, has become a major therapeutic problem. This issue is especially relevant to methicillin-resistant S. aureus (MRSA) strains in the context of invasive endovascular infections. In the current study, we used three well-characterized and clinically-derived DAP-susceptible (DAP-S) vs. resistant (DAP-R) MRSA strain-pairs to elucidate potential genotypic mechanisms of the DAP-R phenotype. In comparison to the DAP-S parental strains, DAP-R isolates demonstrated (i) altered expression of two key determinants of net positive surface charge, either during exponential or stationary growth phases (i.e., dysregulation of dltA and mprF), (ii) a significant increase in the D-alanylated wall teichoic acid (WTA) content in DAP-R strains, reflecting DltA gain-in-function; (iii) heightened elaboration of lysinylated-phosphatidylglyderol (L-PG) in DAP-R strains, reflecting MprF gain-in-function; (iv) increased cell membrane (CM) fluidity, and (v) significantly reduced susceptibility to prototypic cationic host defense peptides of platelet and leukocyte origins. In the tested DAP-R strains, genes conferring positive surface charge were dysregulated, and their functionality altered. However, there were no correlations between relative surface positive charge or cell wall thickness and the observed DAP-R phenotype. Thus, charge repulsion mechanisms via altered surface charge may not be sufficient to explain the DAP-R outcome. Instead, changes in the compositional or biophysical order of the DAP CM target of such DAP-R strains (i.e., increased fluidity) may be essential to this phenotype. Taken together, DAP-R in S. aureus appears to involve multi-factorial and strain-specific adaptive mechanisms.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Anti-Bacterial Agents / pharmacology*
  • Antimicrobial Cationic Peptides / pharmacology
  • Cell Wall / metabolism
  • Daptomycin / pharmacology*
  • Drug Resistance, Bacterial*
  • Fatty Acids / metabolism
  • Gene Expression Regulation, Bacterial
  • Genotype*
  • Humans
  • Methicillin-Resistant Staphylococcus aureus / drug effects*
  • Methicillin-Resistant Staphylococcus aureus / genetics*
  • Methicillin-Resistant Staphylococcus aureus / metabolism
  • Microbial Sensitivity Tests*
  • Operon
  • Phospholipids / metabolism
  • Teichoic Acids / metabolism

Substances

  • Anti-Bacterial Agents
  • Antimicrobial Cationic Peptides
  • Fatty Acids
  • Phospholipids
  • Teichoic Acids
  • Daptomycin