High incidence of multidrug-resistant Escherichia coli coharboring mcr-1 and blaCTX-M-15 recovered from pigs

Purpose: The coexistence of mobile colistin (COL)-resistant gene mcr-1 with extended-spectrum beta-lactamase (ESBL) gene in Escherichia coli has become a serious threat globally. The aim of this study was to investigate the increasing resistance to COL and in particular its coexistence with ESBL-producing E. coli recovered from pig farms in China. Materials and methods: E. coli were isolated from 14 pig farms in Jiangsu China. Susceptibility testing was identified by micro-dilution method. PCR assay and nucleotide sequencing were used to detect COL-resistant genes, mcr-1 to -5, as well as ESBL genes, bla(CTX-M), bla(SHV) and bla(TEM). Conjugation experiment, plasmid replicon typing of the multidrug resistance (MDR), S1-PFGE and DNA southern hybridization were performed to study the transferability of these genes. Results: Overall, 275 E. coli isolates were recovered from a total of 432 cloacal and nasal swabs. More than 90% of the isolates were MDR, of which 70.18% were resistant to COL. Of these 275 isolates, mcr-1 was identified as the most predominant gene carried by 71.63% (197/275) of isolates, 39.59% (78/197) of the isolates were harboring both mcr-1 and ESBL genes (bla(CTX-M), bla(SHV) and bla(TEM)). ESBL genotyping showed that bla(CTX-M) was the most predominant ESBL (68.49%) followed by bla(SHV) (16.4%) and bla(TEM) (15%). Sequencing revealed that the most common variants of blaCTX-M identified were, bla(CTX-M-15) (69%), bla(CTX-M-55) (29%) and bla(CTX-M-1) (1.8%). IncHI2, IncFIB, IncFIC, IncN and IncX4 were found to be the most common Inc-types found both in donors and in transconjugants and were associated with the transfer of the mcr-1 and ESBL encoding genes. Six strains carried a total of five different plasmids: approximately 97-, 130-, 160-, 227-and 242-kb plasmids. Conclusion: The coexistence of the mcr-1- and bla(CTX-M-15)-carrying isolates displaying high MDR, recovered from E. coli of pig origin, is a major concern for both humans and veterinary medicine.