In vitro susceptibility of selected isolates of Brucella abortus to penicillin

Brucella abortus isolates (n = 134) from 38 dairy herds in Vermont, Florida, and Canada exhibited 2 growth patterns on medium containing 5 U of penicillin/ml. The B abortus isolates from 7 herds in Vermont were always susceptible to the antibiotic at that concentration. Eleven of 12 isolates from 1 herd in Florida and 25 isolates (biotypes 1 and 4) from 30 Canadian herds grew at greater than or equal to 5 U of penicillin/ml, as determined using a gradient plate method. Penicillin susceptibility may be of value in identifying the source of infections in cattle.