Staphylococcus saprophyticus beta-lactamase production and disk diffusion susceptibility testing for three beta-lactam antimicrobial agents

Abstract

beta-Lactamase production and MIC determinations for penicillin, methicillin, and cephalothin were assessed for 67 strains of Staphylococcus saprophyticus and correlated with results of disk diffusion susceptibility testing. Fifty-five (82%) of the 67 strains produced beta-lactamase, and 40 (77%) of these beta-lactamase-producing strains were susceptible (zone size, greater than 29 mm) by disk diffusion techniques. Although the range of zone sizes for beta-lactamase producers was broad (26 to 36 mm), all 38 strains with a zone size of less than 31 mm by disk diffusion testing were beta-lactamase producers compared with 17 (59%) of 29 with larger zone sizes (P = 0.0000008). The median penicillin MIC for 12 S. saprophyticus strains was 0.25 micrograms/ml and was not related to beta-lactamase production. Although the methicillin MICs for 15 strains were in the susceptible range (4.0 micrograms/ml), interpretation of disk diffusion testing for oxacillin varied greatly among laboratories using identically prepared media and standardized techniques. Criteria presently used to define susceptibility of Staphylococcus aureus to penicillin and oxacillin by disk diffusion are inappropriate for S. saprophyticus. The clinical significance of the beta-lactamase produced by these strains needs further evaluation.