Activity of penicillin and novobiocin against bovine mastitis pathogens.

: Penicillin (P), novobiocin (N), and both (P-N) were evaluated in vitro against 143 clinical isolates of bovine mastitis, including, Staphylococcus aureus, Streptococcus agalactiae, Streptococcus dysgalactiae, and Streptococcus uberis. Greater antistaphylococcal activities were demonstrated with N alone and P-N than with P alone. Streptococcus spp were more susceptible to P alone than to N alone, and the effectiveness of P-N corresponded with that of P alone. In vitro, P-N had a wider spectrum of antibacterial activity than did either P or N. Similar results were also obtained in vivo, using the mouse protection test. The P-N gave a greater protection rate in mice experimentally infected with S aureus and Str agalactiae than did P or N.