Abstract
The emergence of the plasmid-mediated colistin resistance gene mcr-1 is threatening the last-line role of colistin in human medicine. With mcr-1-positive Escherichia coli (E. coli) isolated from food animal being frequently reported in China, the prevalence of mcr-1 in food animal has attracted public attention. In the present study, a total of 105 colistin-resistant E. coli strains were isolated from 200 fecal samples collected from six swine farms in northeastern China. mcr-PCR revealed that the prevalence of mcr-1 in colistin-resistant E. coli was 53.33% (56/105). mcr-1-positive E. coli showed extensive antimicrobial resistance profiles with the presence of additional resistance genes, increased expression of multidrug efflux pump-associated genes, and increased biofilm formation ability. MLST differentiated all the mcr-1-positive E. coli into 25 sequence types (STs) and five unknown ST, and the most common ST was ST10 (n = 11). By phylogenetic group classification, the distribution of all mcr-1-positive E. coli belonging to groups A, B1, B2, and D was 46.43, 35.71, 5.36, and 5.36%, respectively. Conjugation experiment demonstrated that most of the mcr-1 were transferable at frequencies of 2.68 × 10–6–3.73 × 10–3 among 30 representative mcr-1-positive E. coli. The plasmid replicon types IncI2 (n = 9), IncX4 (n = 5), IncHI2 (n = 3), IncN (n = 3), and IncP (n = 1) were detected in the transconjugants. The results of growth assay, competition experiment, and plasmid stability testing showed that acquisition of mcr-1-harboring plasmids could reduce the fitness of bacterial hosts, but mcr-1 remained stable in the recipient strain. Due to the potential possibility of these mcr-1-positive E. coli being transmitted to humans through the food chain or through horizontal transmission, therefore, it is necessary to continuously monitor the prevalence and dissemination of mcr-1 in food animal, particularly in swine.
Highlights
The discovery and use of antibiotics in human medicine was regarded as one of the vast medical advancements over the past decades, and antibiotics play an important role in food-animal agriculture (Worthington and Melander, 2013)
We aimed to investigate the prevalence and characteristics of mcr-1 in swine farms in northeastern China by determining (1) the carriage rate of mcr-1 in colistin-resistant E. coli isolated from swine fecal samples; (2) the antimicrobial resistance profiles of mcr-1-positive E. coli isolates; (3) the presence of additional resistance genes, the relative expression levels of multidrug efflux pump-associated genes, and biofilm formation ability in mcr-1-positive E. coli isolates; (4) the genetic relationship of the mcr-1-positive E. coli isolates by multilocus sequence typing (MLST) and phylogenetic group; and (5) the transferability, conjugation frequency, fitness cost, and plasmid stability of mcr-1
The susceptibility of 56 mcr-1-positive E. coli isolates to other antimicrobials was determined
Summary
The discovery and use of antibiotics in human medicine was regarded as one of the vast medical advancements over the past decades, and antibiotics play an important role in food-animal agriculture (Worthington and Melander, 2013). Escherichia coli is one of the major pathogens in the swine industry, which is associated with gastrointestinal diseases and systemic infections, including diarrhea, edema disease, septicemia, polyserositis, mastitis, and urinary tract infections (Fairbrother et al, 2005). These diseases can lead to morbidity, mortality, and delayed growth, which are responsible for considerable economic losses and restrict the development of the swine industry. To maintain health and productivity, antibiotics are widely administered to treat E. coli infections in farms to swine via oral, either in feed or in water (Fairbrother et al, 2005). Among a variety of antibiotics used in swine farms, polypeptides and aminoglycosides are most frequently administrated (Sabine et al, 2017)
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