Prevalence and Genetic Analysis of Chromosomal mcr-3/7 in Aeromonas From U.S. Animal-Derived Samples

The prevalence of -positive bacteria in 5,169 domestic animal-derived samples collected by USDA Food Safety and Inspection Service between October 2018 and May 2019 was investigated. A procedure including enriched broth culture and real-time PCR targeting to were used for the screening. Fifteen positive isolates were identified, including one plasmid-borne -positive strain, EC2492 (reported elsewhere) and 14 -positive strains from poultry (1), catfish (2), and chicken rinse (11) samples, resulting in an overall prevalence of -positive bacteria 0.29% in all meat samples tested. Analysis of 16S rRNA and whole genome sequences revealed that all 14 strains belonged to . Data from phylogenetic analysis of seven housekeeping genes, including , and , indicated that nine strains belonged to and five strains belonged to . Antimicrobial tests showed that almost all -positive strains exhibited high resistance to colistin with MICs ≥ 128mg/L, except for one strain, which showed a borderline resistance with a MIC of 2 mg/L. A segment containing two adjacent and lik genes was found in two and one strains and a variety of IS-like elements were found in the flanking regions of this segment. A -related lipid A phosphoethanolamine transferase gene was present in all 14 strains, while an additional -related lipid A phosphoethanolamine transferase gene was found in 5 strains only. In addition to genes, other antimicrobial resistance genes, including , , , , , , , , and were observed in chromosomes of some strains. The relative high prevalence of chromosome-borne genes and the close proximity of various IS elements to these genes highlights the need for continued vigilance to reduce the mobility of these colistin-resistance genes among food animals.