Molecular characterization of non-carbapenemase-producing Escherichia coli clinical isolates with reduced carbapenem susceptibility from a Thai university hospital

Abstract

Twelve non-replicate carbapenemase-negative ertapenem-non-susceptible (CNENS) Escherichia coli isolates from a Thai university hospital between 2010 and 2014 were characterized and compared with two carbapenemase-producing E. coli isolates from the same hospital. Eight unique PFGE patterns were obtained. All produced CTX-M-15 β-lactamase and two isolates co-expressed either CMY-2 cephalosporinase or increased efflux pump. Amino acid sequence analysis revealed that OmpF defect (seven isolates) due to mutations generating premature proteins or an IS1 insertion was more prevalent than in OmpC (no truncated proteins found). Seven out of 10 isolates possessing OmpC variants with any OmpF defect were low-level ertapenem-resistant (MICs of 1-4 µg/mL) and imipenem- and meropenem-susceptible (0.125-0.5 µg/mL). Two isolates with ompC PCR-negative and OmpF defect showed higher carbapenem MICs (8-32, 1-8 and 1-4 µg/mL for ertapenem, imipenem and meropenem, respectively) with the highest MICs by the additional efflux pump activity. Both carbapenemase producers possessing CTX-M-15 and porin background identical to the CNENS isolates exhibited ertapenem, imipenem and meropenem MICs of 128-256, 8 and 2-32 µg/mL, respectively. These findings suggest porin defect combined with CTX-M-15 production as the major mechanism responsible for reduced carbapenem susceptibility among our CNENS isolates, which have potential to become high-level carbapenem-resistant by additional carbapenemase or efflux pump activities.