Excreted antibiotics may be key to emergence of increasingly efficient antibiotic resistance in food-animal production

Authors: AVILLAN, JOHANNETSY - Washington State University; AHMADVAND, PARVANEH - Washington State University; Lu, Shao; HORTON, JENNIFER - Washington State University; LIU, JINXIN - Nanjing Agricultural University; LOFGREN, ERIC - Washington State University; DAVIS, MARGARET - Washington State University; KANG, CHULHEE - Washington State University; CALL, DOUGLAS - Washington State University

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2022
Publication Date: 7/14/2022
Citation: Avillan, J.J., Ahmadvand, P., Lu, S.Y., Horton, J., Liu, J., Lofgren, E., Davis, M.A., Kang, C., Call, D.R. 2022. Excreted antibiotics may be key to emergence of increasingly efficient antibiotic resistance in food-animal production. Applied and Environmental Microbiology. Article e00791-22. https://doi.org/10.1128/aem.00791-22.
DOI: https://doi.org/10.1128/aem.00791-22

Interpretive Summary: Antimicrobial resistance is an urgent and global public health threat. Genes that confer resistance to antibiotics are often quickly spread through bacterial populations resulting in the persistence of antimicrobial-resistant bacteria in the environment. The goal of this project was to understand factors that contribute to the rise of novel antimicrobial resistance genes in native bacterial populations. Antimicrobial genes isolated from animal bacterial pathogens were studied for their ability to confer resistance in mixed bacterial populations under different growth conditions. It was determined that exposure to high concentrations of antibiotic are needed to selectively favor emergence of novel genes in production environments. The necessary concentrations are most likely to arise in excreted urine, making the proximal animal environment an important component for both emergence and persistence of resistant organisms on farms. This research provides valuable information on the external environmental selection of antimicrobial resistance genes and the usage of antibiotics within animal husbandry for antibiotic stewardship.

Technical Abstract: At a time when antimicrobial resistance is seemingly ubiquitous worldwide, understanding the mechanisms responsible for successful emergence of new resistance genes may provide insights into the persistence and pathways of dissemination for antimicrobial resistant organisms in general. For example, E. coli strains harboring a Class A beta-lactamase encoding gene (blaCTX-M-15) appear to be displacing strains that harbor a Class C beta-lactamase gene (blaCMY-2) in Washington State dairy cattle. We cloned these genes with native promoters into low-copy-number plasmids that were transformed into isogenic strains of E. coli, and growth curves were generated for the two commonly administered antibiotics (ampicillin and ceftiofur). Both strains met the definition of resistance for ampicillin (=32 µg/ml) and ceftiofur (=16 µg/ml). Growth of the CMY-2-producing strain was compromised at 1 mg/ml ampicillin whereas the CTX-M-15-producing strain was not inhibited in the presence of 3 mg/ml ampicillin and for most concentrations of ceftiofur except for mixed outcomes when exposed to ceftiofur metabolites. Consequently, in the absence of competing genes, E. coli harboring either gene would experience a selective advantage if exposed to these antibiotics. Successful emergence of CTX-M-15-producing strains where CMY-2-producing strains are already established, however, requires high concentrations of antibiotics that can only be found in the urine of treated animals (e.g., known to achieve >2 mg/ml for ampicillin). This ex vivo selection pressure may be important for the emergence of new and more efficient antibiotic resistance genes, and likely for persistence of antimicrobial resistant bacteria in food-animal populations.