TABLE 2.

Gradual loss of vancomycin resistance among of VRSA strains during passage in drug-free BHI medium

StrainMIC (mg/liter)bPEcMIC (mg/liter) after the following number of passages (days)a:
01251015202530354045505560657084
Mu5062.12 × 10−19977764422232222
MI85.00 × 10−1101010101081077766655442
NJ57.91 × 10−177777755554333333
PC43.09 × 10−588853322222222222
IL52.96 × 10−577777776755553222
AMC1109475.33 × 10−2887777766554443322
99/3759-V53.71 × 10−2665555555544454322
99/3700-W51.42 × 10−266554432222222222
LIM241.39 × 10−255444432222222222
9814147.23 × 10−754444443445444333
2816031.48 × 10−655444544444443222
BR183.15 × 10−299765533222223322
BR251.26 × 10−599986522222222222
BR382.76 × 10−299987455552453333
BR473.16 × 10−566533333332323323
BR5-159.17 × 10−699754432222222222
Mu321.81 × 10−522222222222222222
  • a Bacteria were subjected to daily passage in drug-free BHI medium, and vancomycin MICs were determined every 5 days. The MICs of cultures that were regarded as vancomycin-susceptible substrains are in boldface. These cultures were used for further studies.

  • b Vancomycin MICs for strains when they were received by JCSC.

  • c PE, plating efficiency. The frequency of subpopulations resistant to 4 mg of vancomycin/liter was calculated by dividing the number of colonies grown on the vancomycin-containing BHI agar (4 mg/liter) by the estimated number of cells in the inoculum.