The purpose of this project was to compare antibiotic susceptibilities of Staphylococcus aureus isolates from 22 certified organic and 16 conventional (antibiotic using) dairy herds located throughout New York State and Vermont. Isolates from the organic and conventional herds, when compared on a qualitative basis of sensitive, intermediate and resistant susceptibilities, showed statistically significant differences for ampicillin, penicillin and tetracycline. More isolates from the conventional herds were resistant to these three antibiotics than isolates from the organic herds. In addition, when compared on the basis of quantitative zone diameters, an even larger difference between isolates from organic herds and those from conventional herds was noted. Isolates from organic herds were more susceptible (in the sense of larger growth inhibition zones in millimeters) than the isolates from conventional herds to most of the antibiotics that we tested.
