Detection and occurrence of antimicrobially resistant enteric bacteria in groundwater on or near swine farms in eastern North Carolina

The use of antibiotics for growth promotion and disease treatment by the commercial swine industry has led to high proportions of multiply antibiotic-resistant enteric bacteria fecally shed by these animals and concerns about the spread of these enteric bacteria into the environment. A study was conducted to quantify the extent of release of antibiotic-resistant enteric bacteria from swine farms into groundwater. Four study sites with known groundwater flow paths were screened for enteric bacteria and coliphages. These sites include two swine farms with lagoons and land application of the swine waste, and two agricultural farms with only crops and no land application or animals (reference sites). There were statistically significantly higher E. coli concentrations at the two swine farm sites than at the reference sites. There were similar levels of enterococci found at the two swine farm sites and the second reference site, and these levels were higher than those at the other reference site. The enterococci from the swine farms were determined to be predominantly E. faecium, E. faecalis, and E. hirae , which are all associated with swine fecal matter. The enterococci from the reference site were determined to be predominantly E. mundtii and E. casseliflavus, which are considered primarily or exclusively non-fecal, environmental sources of enterococci, respectively. The bacterial isolates were tested for antibiotic resistance using a panel of 17 drugs that are typical of human and veterinary use. The majority of the groundwater E. coli isolates from the swine farms was resistant to 0 to 3 antimicrobials, with a few showing multiple resistances to up to 6 drugs. The groundwater enterococci isolates from the swine farms were predominantly resistant to 0 to 2 antimicrobials, although a few were multi-drug resistant with up to 6 different resistance traits. Two vancomycin-resistant enterococci (VRE) isolates were found in groundwater, one each from the two swine farm sites, and their species identities were confirmed by molecular characterization. Biochemical and molecular methods of speciation were compared, and the molecular characterization of both E. coli and enterococci species had a higher rate of accuracy than the biochemical method. This study demonstrated that antibiotic-resistant enteric bacteria are being found in groundwaters associated with swine farms that have the lagoon and land application system for waste management. The extent to which such contamination of groundwater with multiple antibiotic-resistant enteric bacteria poses risks to human health is uncertain and deserves further investigation.