Meropenem heteroresistance in clinical isolates of OXA-48-producing Klebsiella pneumoniae.

Abstract

OXA-48-producing Klebsiellapneumoniae isolates often show growth of colonies within inhibition zones in carbapenem diffusion assays. The nature of these colonies was investigated in a series of clinical isolates of OXA-48-producing K. pneumoniae obtained in the context of a hospital outbreak, and they were found to be persistent colonies that reproduced again the same phenotype when they were collected and tested in diffusion assays again. The frequency of mutations conferring resistance to meropenem (8 μg/mL) was determined for the same isolates. The average mutation frequency was 5.47·10-6 (range: 2.59·10-8-5.87·10-5), and the analysis of several resistant mutants showed that all of them had mutations in the ompK36 porin gene. Heteroresistance was investigated using population analysis profiling. The profiles were compatible with mutation frequency assays, and all the colonies analyzed were resistant mutants. In OXA-48-producing K. pneumoniae, the growth of persisters seems to be specific of diffusion assays.