Objective: This retrospective study was conducted to determine the prevalence and molecular epidemiology characteristics of carbapenem-resistant Escherichia coli  (CRE).
Methods: A total of 593 Escherichia coli  (E. coli ) isolates were recovered from pigs and urban river from 2009 to 2014 in Heilongjiang Province of China. Forty CRE including 22 strains isolated from fecal samples of pigs and 18 strains isolated from water samples were selected. PCR detection of resistance determinants, multi-locus sequence typing (MLST), pulsed-field gel electrophoresis (PFGE), and phylogenetic groups were performed to characterize CRE isolates. Conjugation experiments, plasmid stability testing, PCR-based replicon typing (PBRT), and PCR mapping were conducted to analyze bla _NDM-carrying plasmids. In vitro time–growth studies and competition experiments were carried out to assess the fitness impact of NDM carriage.
Results: Five NDM-1-positive E. coli  isolates were identified from water samples. Genetic environment analysis revealed that a cluster of genes (ISAba125-bla _NDM-1-ble _MBL-ΔtrpF ) was detected in all of the NDM-1-positive isolates. Conjugation assays showed that bla _NDM-1 could be successfully transferred to E. coli  J53 from 5 donor strains at frequencies of 4.6×10⁻⁵ to 2.6×10⁻². The plasmids from all transconjugants belonged to different plasmid replicon types including IncA/C (n=2), IncFII (n=1) and IncX3 (n=2). In vitro time–growth studies revealed that bla _NDM-1 did not have a significant impact on cell proliferation. Meanwhile, competition experiments showed that the acquisition of bla _NDM-1 can place an energy burden on the bacterial host and incur fitness cost. However, plasmid stability testing showed that bla _NDM-1-carrying plasmid remained stable in the hosts after seven passages without antimicrobial selection.
Conclusion: The study revealed the early molecular epidemiology and dissemination characteristics of CRE. In addition, the overall antimicrobial resistance in E. coli  recovered from water samples is higher than the strains isolated from fecal samples of pigs. Furthermore, we isolated and identified five NDM-1-producing E. coli strains from water samples.
Keywords: carbapenem-resistant, Escherichia coli , NDM-1, fitness cost