Date Full Report Received


Date Abstract Report Received



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Antibiotic-resistant bacterial infections are a growing health crisis, which has prompted calls for decreased antibiotic use in human medicine and animal agriculture. With increased restrictions on antibiotics in animal agriculture, alternatives products are utilized. Heavy metals, including zinc and copper, can have antimicrobial properties at high enough concentrations and are used as one alternative product. However, the impacts of dietary zinc or copper on antibiotic resistance genes (ARGs) abundance and transmission are not completely understood. The proposed research project identified and compared the ARG carriage and dissemination potential in one of the groups of swine gut bacteria most relevant to public health, the Gram-negative bacteria (mostly Enterobacteriaceae; this group includes Escherichia coli), in groups of nursery-age pigs that were fed either antibiotics or dietary heavy metals (zinc and copper).
When the presence of ARGs in swine gut bacteria was examined, no statistically significant differences among treatment groups was observed. However, limitations in study design may be responsible for this result, as the sequencing data suggested that the Enterobacteriaceae sequenced may be an incomplete representation of the Enterobacteriaceae in the gut due to how the organisms were cultured. Nevertheless, Enterobacteriaceae present in swine carried an average of 9.2 ARGs. These ARGs encoded resistances to a wide range of antibiotics, which may have clinical implications. Thus, despite the dietary additive, a number of ARGs were detected in Enterobacteriaceae cultured from pig fecal samples. Bacterial plasmids, as opposed to the chromosome, encode ARGs with other mobile genetic elements. In the Enterobacteriaceae cultured from pigs, certain ARGs were associated with specific types of plasmids. One of these plasmid types, IncI1, is a narrow host-range plasmid of Escherichia coli and Salmonella enterica species. Thus, carriage of ARGs on IncI1 can readily transfer to organisms associated with disease in humans or animals.

The relative abundance of ARGS in feces tested were not impacted by the addition of therapeutic zinc oxide and copper chloride or chlortetracycline when compared to ARG abundance in feces of pigs on unamended diet. The qPCR targets examined included genes which encoded resistance to different classes of antibiotics, the beta-lactams (blaCMY-2 and blaTEM), tetracyclines (tet32, tetA, and tetW), aminoglycosides (aph2′-id) as well as the IncF plasmid replicon. Again, ARGS were detected in fecal DNA of all the pigs in the trial, and there was a wide range in the abundance of ARGs within a treatment group.
Collectively, swine gut bacteria encoded a number of different ARG in Enterobacteriaceae, and the abundance of the ARGs in swine fecal bacteria varies widely between pigs. Methodology used in the study limited the interpretation of the impact of dietary heavy metals on ARG mobility, and a larger number of samples may be required to fully appreciate the impact of heavy metals on the abundance of ARGs given the range in abundance in pigs in the same treatment group.

Crystal Loving, PhD
Research Microbiologist
USDA-ARS-National Animal Disease Center