Table 3.

Effect of mexAB-oprM operon on LVX susceptibility of strains with multiple target mutations

gyrA orparC mutationMIC (μg/ml) for indicated strain with the following pump status:
None2 (PAM1032)0.25 (PAM1020)0.015 (PAM1154)
gyrA(Thr83→Ile)8 (PAM1573)2 (PAM1548)0.125 (PAM1665)
gyrA (Thr83→Ile) parC(Ser87→Leu)32 (PAM1582)4 (PAM1667)0.5 (PAM1600)
gyrA (Thr83→Ile) parC (Ser87→Leu)gyrA(Asp87→Tyr)128 (PAM1609)16 (PAM1669)2 (PAM1640)
  • a The strains were obtained by stepwise selection with increasing concentrations of LVX. Strain PAM1032 was selected from wild-type strain PAM1020. The order of the strains in the column corresponds to the order in which the strains were selected, so that for example, PAM1032 is a parent of PAM1573. All mutant selections were performed with LVX at 4× the MIC for the corresponding parent.

  • b WT, wild type. PAM1548 was obtained as a spontaneous LVX-resistant mutant selected from strain PAM1020. PAM1667 and PAM1669 were constructed by transduction of themexAB-oprM operon with the wild-type level of expression (nonalB mutation) from strain PAM1064 into PAM1582 and PAM1609, respectively, as described in Materials and Methods.

  • c Strains PAM1665, PAM1600, and PAM1640 were constructed by transduction of the Hg resistance from strain PAM1154 (PAM1020 oprM::ΩHg) into strains PAM1573, PAM1582, and PAM1609, respectively.