AUTHOR=Khine Nwai Oo , Lugsomya Kittitat , Niyomtham Waree , Pongpan Tawat , Hampson David J. , Prapasarakul Nuvee 

TITLE=Longitudinal Monitoring Reveals Persistence of Colistin-Resistant Escherichia coli on a Pig Farm Following Cessation of Colistin Use

JOURNAL=Frontiers in Veterinary Science

VOLUME=9

YEAR=2022

URL=https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2022.845746

DOI=10.3389/fvets.2022.845746

ISSN=2297-1769

ABSTRACT=<p>Colistin-resistant bacteria harboring plasmid-mediated <italic>mcr</italic> genes are of concern as they may be a cause of serious nosocomial infections. It is hypothesized that cessation of colistin use as a feed additive for pigs will reduce the occurrence and distribution of <italic>mcr</italic> genes in farms. The aim of this study was to investigate this hypothesis by longitudinal monitoring and characterizing of <italic>mcr</italic> positive <italic>Escherichia coli</italic> (MCRPE) isolates after colistin was withdrawn on a central Thailand pig farm that previously had a high frequency of MCRPE. Colistin use ceased at the beginning of 2017, and subsequently 170 samples were collected from farrowing sows and suckling piglets (<italic>n</italic> = 70), wastewater (<italic>n</italic> = 50) and farm workers (<italic>n</italic> = 50) over a 3.5-year period. Bacteria were identified by MALDI-TOF mass spectrometry and minimal inhibitory concentrations were determined by broth microdilution. The antibiogram of <italic>mcr</italic> positive <italic>E. coli</italic> isolates was determined using the Vitek2 automated susceptibility machine, and multiplex and simplex PCRs were performed for <italic>mcr</italic>-1–8 genes. MCRPE containing either <italic>mcr</italic>-1 or <italic>mcr</italic>-3 were isolated from pigs throughout the investigation period, but with a declining trend, whereas MCRPE isolates were recovered from humans only in 2017. MCRPE were still being recovered from wastewater in 2020. Most MCRPE isolates possessed the virulence genes <italic>Stap, Stb</italic>, or <italic>Stx2e</italic>, reflecting pathogenic potential in pigs, and showed high rates of resistance to ampicillin, gentamicin and tetracycline. Pulsed-field gel electrophoresis and multi-locus sequence typing showed that diverse MCRPE clones were distributed on the farm. The study identified a decline of pathogenic MCRPE following withdrawal of colistin, with pigs being the primary source, followed by wastewater. However, short-term therapeutic usage of other antibiotics could enhance the re-occurrence of <italic>mcr-</italic>carrying bacteria. Factors including the environment, management, and gene adaptations that allow maintenance of colistin resistance require further investigation, and longer-term studies are needed.</p>