Antimicrobial Resistance Gene Delivery in Animal Feeds

Avoparcin, a glycopeptide antimicrobial agent related to vancomycin, has been used extensively as a growth promoter in animal feeds for more than 2 decades, and evidence has shown that such use contributed to the development of vancomycin-resistant enterococci. A cluster that includes three genes, vanH, vanA, and vanX, is required for high-level resistance to glycopeptides. In the vancomycin producer Amycolatopsis orientalis C329.2, homologs of these genes are present, suggesting an origin for the cluster. We found substantial bacterial DNA contamination in animal-feed-grade avoparcin. Furthermore, nucleotide sequences related to the cluster vanHAX are present in this DNA, suggesting that the prolonged use of avoparcin in agriculture led to the uptake of glycopeptide resistance genes by animal commensal bacteria, which were subsequently transferred to humans.