Strain-Specific Adaptations ofStreptococcus mitis-oralisto Serial In Vitro Passage in Daptomycin (DAP): Genotypic and Phenotypic Characteristics

Viridans group streptococci (VGS), especially theStreptococcus mitis-oralissubgroup, 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 ofS. mitis-oralisstrains 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 (pgsA; cdsA); and (5) DAP hyperaccumulation. The current study examined these same metrics following in vitro serial DAP passages of a separate well-characterizedS. mitis-oralisbloodstream isolate (SF100). Although some metrics seen in prior DAP post-passage strains were recapitulated with SF100 (e.g.,pgsASNPs, 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) nocdsASNPs. Thus,S. mitis-oralisstrains are not "pre-programmed" phenotypically and/or genotypically to adapt in an identical manner during the evolution of the DAP-R.