Comparative in vitro activity of 11 beta-lactam antibiotics against 91 Staphylococcus intermedius strains isolated from staphylococcal dermatitis in dogs.

A study was made of the susceptibility of 91 strains of Staphylococcus intermedius, isolated from staphylococcal dermatitis of dogs, to 11 beta-lactam antimicrobial agents. The activity of these antimicrobial agents against all strains was tested at pH 7.2 and 8.5. Finally, a comparative study was made of the susceptibility of these micro-organisms, dividing them into six groups: beta-lactamase positive and negative, methicillin resistant and susceptible and oxacillin resistant and susceptible. Only three of the 11 antimicrobial agents studied exhibited satisfactory behaviour at the two pH levels studied: mezlocillin, cefazolin and cefalotine. There was a good correlation at pH 8.5 between methicillin-resistant strains and those resistant to penicillin G, ampicillin, amoxycillin, cefamandole and cefuroxime. This correlation was not observed, however, between oxacillin-resistant strains and strains resistant to the other antimicrobial agents studied.