Effect of beta-lactamase of Staphylococcus intermedius on disk agar diffusion susceptibility tests

Clinical strains of canine Staphylococcus intermedius (n = 120) were tested for susceptibility to beta-lactam antimicrobics (n = 6) by the standardized disk agar diffusion method and for beta-lactamase (BL) production. Significant differences between susceptibilities for BL producing (n = 68) and non-producing (n = 52) strains were found for penicillin G and ampicillin. Zone sizes of BL producing strains were significantly smaller than those of non-producers for penicillin G, ampicillin, cephalothin, carbenicillin and amoxicillin-clavulanic acid but not for methicillin. However, all strains were sensitive to cephalothin and amoxicillin-clavulanic acid; only one strain was resistant to methicillin; and one strain was intermediate in susceptibility to carbenicillin. Although 62 (52%) strains were sensitive to penicillin G and ampicillin based on established zone size interpretive criteria, 15 (24%) of these strains produced BL. Zone size measurements obtained with beta-lactam antimicrobics that are highly susceptible to inactivation by BL are not reliable and should be disregarded for canine S. intermedius which produce BL.