Background: The concurrence of mcr  and carbapenemase genes among Enterobacteriaceae  has been a great clinical concern. In our study, we aimed to investigate the prevalence of mcr-positive carbapenem-resistant Enterobacteriaceae  (CRE) in fresh vegetables and shed light on the possibility of transmission of mcr -positive CRE via fresh vegetables.
Methods: In this study, 712 fresh vegetable samples from 10 provinces in China were collected between May 2017 and Dec 2018 and were screened for mcr  and carbapenemase genes. Antibiotic susceptibilities for isolates co-harboring carbapenemase genes and mcr  were determined by an agar dilution or a broth microdilution method. Pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST) analysis were also performed. Transferability of the carbapenemase/mcr -bearing plasmids was determined by conjugation, replicon typing and S1-PFGE-Southern blotting. The sequences of these plasmids were analyzed by using whole-genome sequencing with Illumina Hiseq platform.
Results: Two E. coli  isolates concomitantly carrying mcr-1  and bla _NDM-5/9 from leaf rape and spinach, respectively, were found and both isolates showed multidrug resistance. Notably, mcr-1 -positive 690 harboring bla _NDM-5 and 701 carrying bla _NDM-9 belonged to ST156 and ST2847, respectively, similar to the prevalent MLST types of E. coli  co-carrying mcr-1  and bla _NDM from avian in our previous study. mcr-1  was on ∼33-kb IncX4 plasmid or ∼60-kb IncI2 plasmid, while bla _NDM-5/9 was on ∼46-kb IncX3 plasmid or ∼120-kb untypable plasmid. The plasmids were highly similar to those from animals and clinical patients reported in various countries.
Conclusion: E. coli  isolates concomitantly carrying mcr-1  and bla _NDM-5/9 in fresh vegetables may serve as a direct source of pathogens in humans, and such discovery in fresh vegetables emphasizes the importance of prompt surveillance and intervention in limiting the spread of E. coli  co-carrying bla _NDM and mcr-1 . To our knowledge, this is the first report of Enterobacteriaceae  co-carrying bla _NDM and mcr-1  in fresh vegetables.
Keywords: carbapenem resistance, colistin resistance, coexistence, plasmids, Enterobacteriaceae