FIG 2.

Analysis of vancomycin-resistant subpopulation profiles (population analysis [PA]). (A) Profiles of Mu3, 6R-P, and Mu50 observed after 48, 72, 96, 120, and 144 h of incubation at 37°C. (B) 6R-P and its phenotypic revertant strains L1, L2, and L3 as observed after 96 h of incubation at 37°C. (C) ΔIP and H14 strains and their derivatives introduced with the rpoB(R512P) mutation. About 10⁷ CFU of the overnight culture of each strain were inoculated on BHI agar plates containing various concentrations of vancomycin. The numbers of colonies formed after incubation of 96 h at 37°C were counted and plotted in a semilogarithmic graph. Note that the shape of the PA curve of H14 is reminiscent of hVISA; however, its resistant subpopulation, as judged by the growth on the plate containing 4 mg/liter vancomycin, is much smaller than that of Mu3, which is >1 × 10⁻⁶. We call such an S. aureus strain with reduced susceptibility to vancomycin pre-hVISA.