Prevalence of the carbapenem-heteroresistant phenotype among ESBL-producing Escherichia coli and Klebsiella pneumoniae clinical isolates

Objectives: Carbapenem-heteroresistant (cHR) Enterobacteriaceae strains have been reported worldwide; however, the prevalence among clinical ESBL-producing Enterobacteriaceae isolates obtained from patients with repeated hospital admissions remains largely unknown. Methods: Heteroresistance was screened by disc diffusion and confirmed by a modified population analysis profiling (PAP) method against ertapenem, imipenem, meropenem and ceftolozane/tazobactam. MIC testing was performed by broth microdilution against carbapenems and a panel of agents with potential activity against ESBL-producing strains. Results: One hundred and seventy-three ESBL-producing meropenem-susceptible Escherichia coli and Klebsiella pneumoniae isolates were selected for testing. A total of 519 bacteria/carbapenem combinations were screened by disc diffusion; 84 combinations were identified as cHR. Modified PAP confirmed 70 bacteria/carbapenem combinations as heteroresistant; most (63%, 44/70) confirmed cHR colonies grew within the ertapenem zone of inhibition, followed by imipenem (30%, 21/70), then meropenem (7%, 5/70). In total, one-third of the unique patient isolates (32%, 55/173) were identified as being heteroresistant to at least one carbapenem; of those patients, 16% (9/55) had a carbapenem-non-susceptible isolate on subsequent visits. Only two cHR isolates screened positive for ceftolozane/tazobactam heteroresistance (1%, 2/173), of which one was confirmed heteroresistant by modified PAP. cHR isolates were more likely to be collected from a non-urinary source (e.g. respiratory) compared with non-cHR isolates (31% versus 19%, P = 0.02). MIC distributions of all tested antibiotic agents did not differ between non-cHR and cHR isolates. Conclusions: Our findings raise concerns for the continued use of carbapenems as first-line therapy for ESBL infections and for the potential selection for strains with full carbapenem resistance.