Once a parC mutation is fixed, a profound change occurs in the behavior of the strain with respect to selection of subsequent resistance mutations. The slide shows the effect of moxifloxacin concentration on selection of mutants with wild-type cells (open squaqres) and with a parC mutant (solid squares). The latter displays a much higher MPC and a broad, high plateau. Since the MIC is not raised very much in the parC mutant, such a strain might be susceptible to moxifloxacin treatment. However, we would expect moxifloxacin-resistant gyrA mutants to arise readily. Thus a first-step mutation is likely to accelerate the development of resistance. This phenomenon is likely to be important because first-step parC mutants are already widespread but not considered to be levofloxacin-resistant by standard approaches (7).