Comparative in vitro studies of a number of anti-mastitis antibiotics.

In vitro activities of penicillin, ampicillin, dihydrostreptomycin, nafcillin, cloxacillin and commercially available combinations of these antibiotics against a variety of Gram-positive mastitis-causing micro-organisms were examined. As a rule, penicillin proved to be more active against penicillin-sensitive strains of micro-organisms than ampicillin and nafcillin and far more so than cloxacillin. In combination with di-hydrostreptomycin, which in itself showed hardly any activiy, the activity of penicillin against streptococci was enhanced considerably. This confirms the synergistic effect of this combination. No clear synergism was noted in the activity against staphylococci. Against strains which were less sensitive to penicillin, there was not much difference in activity between penicillin, ampicillin, nafcillin and cloxacillin. Nafcillin was found to be much more active against streptococci than cloxacillin. The anti-staphylococcal activities of these two antibiotics however were identical. The activity of the combination of penicillin, dihydrostreptomycin and nafcillin against most micro-organisms was mainly due to the penicillin in the combination. Against Streptococcus agalactiae and the most penicillin-resistant staphylococci, nafcillin contributed clearly to the activity. The activity of the combination ampicillin + cloxacillin against Streptococcus agalactiae was mainly based on the activity of the ampicillin in the combination. Against penicillin-resistant staphylococci, the activity of the combination was mainly due to the activity of cloxacillin. Against the other micro-organisms, both compounds contributed to the activity of the combination.