Characterization of Passage-Selected Vancomycin-Resistant Staphylococcus aureus Strains of Diverse Parental Backgrounds

Abstract

A series of 12 Staphylococcus aureus strains of various genetic backgrounds, methicillin resistance levels, and autolytic activities were subjected to selection for the glycopeptide-intermediate S. aureus (GISA) susceptibility phenotype on increasing concentrations of vancomycin. Six strains acquired the phenotype rapidly, two did so slowly, and four failed to do so. The vancomycin MICs for the GISA strains ranged from 4 to 16 g/ml, were stable to 20 nonselective passages, and expressed resistance homogeneously. Neither ease of acquisition of the GISA phenotype nor the MIC attained correlated with methicillin resistance hetero-versus homogeneity or autolytic deficiency or sufficiency. Oxacillin MICs were generally unchanged between parent and GISA strains, although the mec members of both isogenic methicillin-susceptible and methicillin-resistant pairs acquired the GISA phenotype more rapidly and to higher MICs than did their susceptible counterparts. Transmission electron microscopy revealed that the GISA strains appeared normal in the absence of vancomycin but had thickened and diffuse cell walls when grown with vancomycin at one-half the MIC. Common features among GISAs were reduced doubling times, decreased lysostaphin susceptibilities, and reduced whole-cell and zymographic autolytic activities in the absence of vancomycin. This, with surface hydrophobicity differences, indicated that even in the absence of vancomycin the GISA cell walls differed from those of the parents. Autolytic activities were further reduced by the inclusion of vancomycin in whole-cell and zymographic studies. The six least vancomycin-susceptible GISA strains exhibited an increased capacity to remove vancomycin from the medium versus their parent lines. This study suggests that while some elements of the GISA phenotype are strain specific, many are common to the phenotype although their expression is influenced by genetic background. GISA strains with similar glycopeptide MICs may express individual components of the phenotype to different extents.

Why are bacteria refractory to antimicrobials?

“…For instance, the intermediate vancomycin-resistance phenotype in S. aureus is due to an increased amount of non-amidated glutamine residues in the peptidoglycan. It has been suggested that the increased resistance in these strains is due to the "affinity trapping" of vancomycin in the thickened cell wall [17,18].…”

Alterations in phage-typing patterns in vancomycin-intermediate Staphylococcus aureus

“…5 ìg ml À1 for BB270 V5 ) and not the actual vancomycin MIC (Pfeltz et al, 2000). The MICs of the strains for vancomycin, teicoplanin and oxacillin and other characteristics of these strains were described by Pfeltz et al (2000).…”

“…BB270), and the number associated with the subscript V (such as V5) refers to the vancomycin concentration at which the strain was maintained directly following VISA phenotype selection (e.g. 5 ìg ml À1 for BB270 V5 ) and not the actual vancomycin MIC (Pfeltz et al, 2000). The MICs of the strains for vancomycin, teicoplanin and oxacillin and other characteristics of these strains were described by Pfeltz et al (2000).…”

“…The strains analysed represent various genetic backgrounds and methicillin resistance levels and their isogenic in vitro step-selected VISA derivatives (Pfeltz et al, 2000). The subscript V in a strain designation refers to a vancomycin-intermediate derivative (e.g.…”

“…Vancomycin-intermediate expression in S. aureus is mediated by chromosomal mutation(s) (Avison et al, 2002;Pfeltz et al, 2000;Sakoulas et al, 2002; and is not due to horizontal gene transfer of vancomycin-resistance determinants (van) from organisms such as the vancomycin-resistant enterococci. VISA strains harbour a chromosomal mutation(s) that leads to alterations in cellwall synthesis and structure, which are thought to impart resistance by sequestering glycopeptide molecules away from their target (for review, see .…”