Multidrug Resistance and Virulence Factors of Escherichia coli Harboring Plasmid-Mediated Colistin Resistance: mcr-1 and mcr-3 Genes in Contracted Pig Farms in Thailand

Abstract

The presence of the plasmid-mediated colistin resistance encoding <i>mcr</i> gene family in the <i>Enterobacteriaceae</i> is one of the crucial global concerns. The use of colistin in livestock rearing is believed to be the cause of <i>mcr</i> gene spreading and is of impact to public health. The objective of this research was to detect the frequency and virulent genes of <i>mcr-</i>positive <i>Escherichia coli</i> (MCRPE) in fecal samples from healthy pigs in a contract farming system across Thailand. A total of 696 pooled samples were derived from 80 farms, located in 49 provinces across six regions of Thailand. The colistin-resistant <i>E. coli</i> were identified by MALDI-TOF mass spectrometry and antimicrobial susceptibility testing by broth microdilution. The antibiogram was determined using an automated susceptibility machine, and the genetic characteristics were investigated for <i>mcr-1-5</i> genes, phylogenetic group, replicon types, and virulent genes. In total, 31 of 696 samples were positive, with <i>E. coli</i> containing <i>mcr-1</i> or combination of <i>mcr-1</i> and <i>mcr-3</i> with incidence of 4.45 and 0.43%. Phylogenetic groups A and B1 and the IncF and IncFIB replicon types were predominantly found in the MCRPE located in the central area, with multidrug-resistant traits against 3-14 types of antimicrobials. Additionally, 19 of 31 isolates identified as enterotoxigenic <i>E. coli</i> were with the <i>stap</i> and <i>stb</i> (enterotoxin-encoding genes). In conclusion, a low carriage rate of <i>mcr-</i>positive <i>E. coli</i> was detected in the large-scale farming of healthy pigs. The association between multidrug-resistant MCRPE and their pathogenic potential should be of concern.