Development of resistance in wild-type and hypermutable Pseudomonas aeruginosa strains exposed to clinical pharmacokinetic profiles of meropenem and ceftazidime simulated in vitro

Abstract

In this study we investigated the interplay of antibiotic pharmacokinetic profiles and the development of mutation-mediated resistance in wild-type and hypermutable Pseudomonas aeruginosa strains. We used in vitro models simulating profiles of the commonly used therapeutic drugs meropenem and ceftazidime, two agents with high levels of antipseudomonal activity said to have different potentials for stimulating resistance development. During ceftazidime treatment of the wild-type strain (PAO1), fully resistant mutants overproducing AmpC were selected rapidly and they completely replaced wild-type cells in the population. During treatment with meropenem, mutants of PAO1 were not selected as rapidly and showed only intermediate resistance due to the loss of OprD. These mutants also replaced the parent strain in the population. During the treatment of the mutator P. aeruginosa strain with meropenem, the slowly selected mutants did not accumulate several resistance mechanisms but only lost OprD and did not completely replace the parent strain in the population. Our results indicate that the commonly used dosing regimens for meropenem and ceftazidime cannot avoid the selection of mutants of wild-type and hypermutable P. aeruginosa strains. For the treatment outcome, including the prevention of resistance development, it would be beneficial for the antibiotic concentration to remain above the mutant prevention concentration for a longer period of time than it does in present regimens.

FIG. 1.

(A and B) CFU counts indicating the total populations of PAO1 cells and the populations of mutant cells during treatment with meropenem (three 1-g doses) (A) and during treatment with ceftazidime (three 2-g doses) (B). (C) CFU counts indicating the total population of 12-09-15 cells and the population of mutant cells during treatment with meropenem (three 1-g doses). Arrows indicate dosing times.

FIG. 2.

Results of the competitive growth assays with parent and mutant strains selected in in vitro model experiments with P. aeruginosa PAO1 and meropenem (PAO1_PAO1/MEM), P. aeruginosa PAO1 and ceftazidime ((PAO1_PAO1/CAZ), and P. aeruginosa 12-09-15 and meropenem (12-09-15_12-09-15/MEM). CFU counts indicating the total populations (A) and the proportion of the respective mutant (B) are shown.

FIG. 3.

The concentration-time curve (concentrations were taken from references and 20), the duration of the mutant selection window, and the amount of time that the drug concentration was below the MIC (T_<MIC) for the parent strain in each dosing interval for PAO1 and meropenem (MEM) (A), PAO1 and ceftazidime (CAZ) (B), and 12-09-15 and meropenem (C) are shown.