Strain-Specific Adaptations of to Serial In Vitro Passage in Daptomycin (DAP): Genotypic and Phenotypic Characteristics.

Viridans group streptococci (VGS), especially the subgroup, are pivotal pathogens in a variety of invasive endovascular infections, including "toxic shock" in neutropenic cancer patients and infective endocarditis (IE). Previously, we showed that the serial in vitro passage of strains in sublethal daptomycin (DAP) resulted in rapid, high-level and stable DAP-resistance (DAP-R), which is accompanied by distinct changes in several genotypic and phenotypic signatures: (1) the disappearance of two key membrane phospholipids, phosphatidylglycerol (PG) and cardiolipin (CL); (2) increased membrane fluidity; (3) increased positive surface charge; (4) single nucleotide polymorphisms (SNPs) in two loci involved in CL biosynthesis (); and (5) DAP hyperaccumulation. The current study examined these same metrics following in vitro serial DAP passages of a separate well-characterized bloodstream isolate (SF100). Although some metrics seen in prior DAP post-passage strains were recapitulated with SF100 (e.g., SNPs, enhanced membrane fluidity), we observed the following major differences (comparing the parental versus post-passage variant): (1) no change in PG content; (2) reduced, but not absent, CL, with enhancement in phosphatidic acid (PA) content; (3) an unusual pattern of CL localization; (4) significantly decreased positive surface charge; (5) no difference in DAP accumulation; and (6) no SNPs. Thus, strains are not "pre-programmed" phenotypically and/or genotypically to adapt in an identical manner during the evolution of the DAP-R.