Mutagenesis of the CTX-M-type ESBL-is MIC-guided treatment according to the new EUCAST recommendations a safe approach?

Objectives: Spurred by the latest EUCAST and CLSI recommendation to adjust antibiotic therapy on the basis of MICs instead of resistance mechanisms, we aimed to investigate the ability of CTX-M-1 and CTX-M-14 to achieve ceftazidime resistance under selective conditions. Methods: We exposed Escherichia coli transconjugants bearing natural plasmids that express CTX-M-1, CTX-M-14 or CTX-M-15 to various selective culture conditions and tracked their growth and mutational frequencies. For selected mutants we analysed the sequences of the bla(CTX-M) genes, determined the altered MICs of cefotaxime, cefepime, ceftazidime and meropenem, and measured the efflux properties and the changes in transcriptional levels of blaCTX-M genes. Results: The CTX-M-1-and CTX-M-14-bearing clones switched from ceftazidime-susceptible to ceftazidime-resistant phenotypes under selective conditions within 24 h. However, no mutations within the bla(CTX-M) genes were found, and the efflux was unlikely to be involved in the increased ceftazidime MICs. In CTX-M-1-bearing clones blaCTX-M-1 expression was 19-fold increased under ceftazidime-selective conditions but there was a high variance within the clones. Reasons for increased ceftazidime MICs of CTX-M-bearing clones remain unclear but might be the increased enzymatic activity or other intrachromosomal mutations. Conclusions: It can be speculated that different strategies to survive under selective conditions can be adopted by E. coli, thereby establishing an optimal mechanism with the lowest energy demand for each transconjugant. Based on our in vitro findings, we cannot fully recommend the use of ceftazidime, particularly in critically ill patients with infections due to ESBL producers, regardless of susceptibility to ceftazidime.