Effect of procaine benzylpenicillin alone or in combination with dihydrostreptomycin on udder pathogens in vitro and in experimentally infected bovine udders.

Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of benzylpenicillin (PEN) and dihydrostreptomycinsulfate (DHS) were determined for freshly isolated bovine udder pathogens. Killing curves were determined to study synergism of PEN and DHS on Staphylococcus aureus and Streptococcus uberis. Healthy udder quarters of cows in their first lactation were inoculated with Str dysgalactiae or Str uberis strains. After development of acute mastitis, procaine PEN alone or combined with DHS were given IM, and the concentrations of PEN and DHS in plasma and normal and mastitic milk were determined. Twenty-four hours after 10 mg of procaine PEN/kg of body weight (IM), concentrations in mastitic milk equal to MIC or MBC were maintained only for Str dysgalactiae. Twenty-four hours after 20 mg of procaine PEN/kg (IM), MBC in mastitic milk was maintained for Str uberis and 50% of the S aureus. When concentrations equal to MIC or MBC Of PEN were combined with a sub-MIC amount of DHS, the in vitro synergistic killing of Str uberis was not observed in vivo. In agreement with in vitro killing effect at PEN concentrations equal to MBC, Str dysgalactiae was 100% eliminated in 5 cows when MBC of PEN in mastitic milk was maintained for 24 to 36 hours. On the contrary, only 2 of 6 cows inoculated with Str uberis were free of this pathogen, although MBC of PEN was maintained for 24 to 36 hours in mastitic milk.