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 × 10−1997776442 2 232222
MI85.00 × 10−110101010108107776665544 2
NJ57.91 × 10−1777777555543 3 3333
PC43.09 × 10−58885332 2 222222222
IL52.96 × 10−5777777767555532 2 2
AMC1109475.33 × 10−288777776655444332 2
99/3759-V53.71 × 10−266555555554445432 2
99/3700-W51.42 × 10−26655443 2 222222222
LIM241.39 × 10−255444432 2 22222222
9814147.23 × 10−7544444434454443 3 3
2816031.48 × 10−6554445444444432 2 2
BR183.15 × 10−2997655332 2 2223322
BR251.26 × 10−59998652 2 222222222
BR382.76 × 10−299987455552453 3 33
BR473.16 × 10−56653 3 333332323323
BR5-159.17 × 10−699754432 2 22222222
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.