Tetracycline- and macrolide-resistant Enterococcus species isolated from a mink farm in the United States

Authors: Agga, Getahun; Silva, Philip - Phil; MARTIN, RANDAL - Utah State University

Submitted to: Microbial Drug Resistance
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2022
Publication Date: 5/16/2022
Citation: Agga, G.E., Silva, P.J., Martin, R.S. 2022. Tetracycline- and macrolide-resistant Enterococcus species isolated from a mink farm in the United States. Microbial Drug Resistance. 28(6):734-743. https://doi.org/10.1089/mdr.2021.0438.
DOI: https://doi.org/10.1089/mdr.2021.0438

Interpretive Summary: Enterococci are gram positive bacteria inhabiting the gastrointestinal tract of humans and animals. Often, they are not associated with diseases in humans. However, as opportunistic pathogens they cause nosocomial infections in hospitalized people. Systemic infections usually require antibiotic treatment. Development of antimicrobial resistance in enterococci threatens the effectiveness of available treatments. Concentrated animal production such as mink farming can lead to a widespread occurrence of antimicrobial resistant bacteria including enterococci. People can be exposed to resistant strains through direct contact with infected animals, or through environmental exposure from water contamination. Environmental spread of antimicrobial resistant bacteria can occur through runoffs from farms, or when untreated animal manure is land applied as a source of organic fertilizer for crop production. Enterococci are used by the U.S. National Antimicrobial Resistance Monitoring System (NARMS), as indicator organism to monitor antimicrobial resistance of gram-positive bacteria. Status of antimicrobial resistance in mink farms has not been widely reported. We report the occurrence of antibiotic resistant enterococci on a U.S. mink farm. Resistant strains were detected from the feces and animal feed. Most of the isolates were multidrug resistant being resistant to more than three different classes of antimicrobials commonly used in humans. The two predominant antimicrobial resistant Enterococcus species identified from mink are commonly associated with human infections. This study is the first to report on the occurrence of antibiotic resistant enterococcus species of public health importance from a mink farm in the U.S. The results highlight the need for antimicrobial resistance monitoring in non-food animal production facilities such as mink farming.

Technical Abstract: Since Enterococcus species are a normal flora of animals and humans, they are commonly used as indicator organisms for the monitoring of antimicrobial resistance (AMR) in gram positive bacteria. They are also opportunistic pathogens causing diseases in humans and animals with life threatening infections caused by antibiotic resistant strains. Animals are reservoirs for pathogenic and resistant strains that can disseminate into the environment from production facilities. We investigated the abundance and prevalence of erythromycin (ERYr)- and tetracycline (TETr)-resistant enterococci in mink feces and feed. Fecal (n=42) and feed (n=8) samples obtained from a mink farm were cultured on erythromycin and tetracycline supplemented media for the enumeration and detection of ERYr- and TETr- enterococci, respectively. One isolate per sample was speciated and characterized for resistance genes and co-resistance to other antibiotics. ERYr- and TETr-enterococci were detected from 100% of feces (mean concentrations = 6 and 7 logs, respectively) and feed samples (mean concentrations=5 and 4 logs). Enterococcus faecium was predominant among generic (100%), fecal (79%)- and feed (62.5%)-ERYr isolates, and feed TETr isolates (87.5%). E. faecalis and E. faecium were detected at equal proportions among fecal TETr isolates. All ERYr isolates, and 90% of the TETr isolates were multidrug resistant (resistant to =3 antimicrobial classes). Among the ERYr isolates, while 83% of E. faecalis were positive for erm(B), 58% of E. faecium isolates were positive for msr(C). Among the ERYr isolates, tet(M) was detected from 89% of E. faecalis and 97% of E. faecium fecal- and all feed-isolates. Conversely, however, erm(B) was detected in 20% of E. faecalis and 40% of E. faecium TETr fecal isolates. Our study provides a baseline for future efforts to reduce AMR and improve antimicrobial stewardship in commercial mink production facilities.