| Summary: | Background: Colistin is one of the last-line antimicrobial agents against life-threatening infections. The distribution of the colistin resistance gene <i>mcr-1</i> has been reported worldwide. However, most studies have focused on the distribution of <i>mcr-1</i>-positive bacteria in humans, animals, food, and sewage; few have focused on their distribution in natural environments. Method: We conducted a large spatial survey of <i>mcr-1</i>-positive <i>Escherichia coli</i> at 119 sites in 48 rivers, covering the entire island of Taiwan. We investigated the relationship between the livestock or poultry density in the surveyed riverine area and the number of <i>mcr-1</i>-positive <i>E. coli</i> in the river water. We then sequenced and characterized the isolated <i>mcr-1</i>-positive plasmids. Results: Seven <i>mcr-1</i> positive <i>E. coli</i> were isolated from 5.9% of the sampling sites. The <i>mcr-1</i>-positive sites correlated with high chicken and pig stocking densities but not human population density or other river parameters. Four of the <i>mcr-1</i>-positive <i>E. coli</i> strains harbored epidemic IncX4 plasmids, and three of them exhibited identical sequences with a size of 33,309 bp. One of the plasmids contained identical 33,309 bp sequences but carried an additional 5711-bp transposon (Tn3 family). To our knowledge, this is the first demonstration that <i>mcr-1</i>-carrying IncX4 plasmids can contain an insertion of such transposons. All <i>mcr-1</i>-positive isolates belonged to phylogenetic group A and harbored few known virulence genes. Conclusion: This study showed a positive relationship between the number of <i>mcr-1</i>-positive sites and high livestock and poultry density. The sequencing analyses indicated that the epidemic plasmid in the <i>mcr-1</i> isolates circulates not only in humans, animals, and food but also in the associated environments or natural habitats in Taiwan, suggesting that the surveillance of antibiotics-resistance genes for livestock or poultry farm quality control should include their associated environments.
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