Therapy of experimental staphylococcal mastitis in the mouse with cloxacillin and rifampicin, alone and in combination.

Cloxacillin effectively killed Staphylococcus aureus in vitro and in experimental acute mastitis in the mouse, but was ineffective in reducing bacterial counts in those with experimental chronic mastitis. Rifampicin was similarly bactericidal for staphylococci in vitro and in acute mastitis but it also significantly reduced numbers of viable bacteria in chronic mastitis. Exposure of multiplying staphylococci in vitro and in vivo to rifampicin alone led to the emergence of resistant bacteria, but this was prevented by the use of a combination of rifampicin with cloxacillin. This combination showed neither antagonism nor synergy in vitro, was more effective than either antibiotic alone in acute mastitis and killed S aureus in the chronic infection as effectively as rifampicin alone.