In addition to human and veterinary medicine,
antibiotics are extensively used in agricultural settings, such as for treatment of
infections, growth enhancement, and prophylaxis in food animals, leading to selection of
drug and multidrug-resistant bacteria. To help circumvent the problem of bacterial antibiotic resistance, it is first necessary to understand the scope of the problem. However, it is not fully understood how widespread
antibiotic-resistant bacteria are in agricultural settings. The lack of such surveillance data is especially evident in dairy farm environments, such as soil. It is also unknown to what extent various physiological modulators, such as
salicylate, a component of
aspirin and known model modulator of multiple antibiotic resistance (mar) genes, influence bacterial multi-drug resistance. We isolated and identified enteric soil bacteria from local dairy farms within Roosevelt County, NM, determined the resistance profiles to
antibiotics associated with mar, such as
chloramphenicol,
nalidixic acid,
penicillin G, and
tetracycline. We then purified and characterized plasmid
DNA and detected mar phenotypic activity. The minimal inhibitory concentrations (MIC) of
antibiotics for the isolates ranged from 6 to >50 microg/mL for
chloramphenicol, 2 to 8 microg/mL for
nalidixic acid, 25 to >300 microg/mL for
penicillin G, and 1 to >80 microg/mL for
tetracycline. On the other hand, many of the isolates had significantly enhanced MIC for the same
antibiotics in the presence of 5 mM
salicylate. Plasmid
DNA extracted from 12 randomly chosen isolates ranged in size from 6 to 12.5 kb and, in several cases, conferred resistance to
chloramphenicol and
penicillin G. It is concluded that enteric bacteria from dairy farm topsoil are multidrug resistant and harbor antibiotic-resistance plasmids. A role for dairy topsoil in
zoonoses is suggested, implicating this environment as a reservoir for development of bacterial resistance against clinically relevant
antibiotics.