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

Abstract

,B-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 14-lactamase, and 40 (77%) of these ,B-lactamase-producing strains were susceptible (zone size, >29 mm) by disk diffusion techniques. Although the range of zone sizes for ,-lactamase producers was broad (26 to 36 mm), all 38 strains with a zone size of <31 mm by disk diffusion testing were 3-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 ,g/ml and was not related to (-lactamase production. Although the methicillin MICs for 15 strains were in the susceptible range (4.0 ,Ig/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 l-lactamase produced by these strains needs further evaluation.Staphylococcus saprophyticus, a coagulase-negative staphylococcus, is a common cause of acute urinary infections in young women (3,6,15). Unlike most other coagulase-negative staphylococci, S. saprophyticus strains are reported to be uniformly susceptible to most antibiotics active against gram-positive organisms (3,9,15). Yet, in a recent study of urinary infections with this organism among college women (6), we found that the majority of strains were not susceptible to penicillin and methicillin by disk diffusion (DD) techniques, but were susceptible to cephalothin. To explain this observation, we assessed the prevalence of P-lactamase production among strains of S. saprophyticus, determined the MICs of penicillin, methicillin, and cephalothin for these strains, and correlated these results with susceptibility testing by DD. MATERIALS AND METHODS Sixty-seven S. saprophyticus strains isolated from ambulatory patients with acute urinary infections were evaluated.

Evaluation of four qualitative methods for detection of beta-lactamase production inStaphylococcus andMicrococcus species

“…Determination of/3-Iactamase production from the zone edge appearance resulted in a large number of false positive results, contrary to findings of other investigators (7) (7,13). It is evident that file sharp zone edge can be formed due to/3-1actamase production as well by other, as yet undetermined factors.…”

In vitro susceptibilities of four species of coagulase-negative staphylococci

“…saprophyticus have yielded inconsistent and methoddependent results (15,20,27,44), these organisms had a unimodal distribution of penicillin G MICs (5,15,33,39). Based on our studies, we concluded that all strains either were ,-lactamase negative or produced ineffective enzyme and were susceptible to penicillin G; application of S. aureus MIC criteria was not appropriate to S. saprophyticus.…”

Oxacillin Resistance and Antimicrobial Susceptibility Profile of Staphylococcussaprophyticus and Other Staphylococci Isolated from Patients with Urinary Tract Infection

“…According to Hovelius and Mardh [60], chromogenic cephalosporin (nitrocefin) does not detect β-lactamase production in S. saprophyticus . In contrast, Latham et al [61] identified β-lactamase production by the chromogenic cephalosporin method in 50 (75%) of 67 S. saprophyticus isolates. McDougal and Thornsberry [62] evaluated β-lactamase production in 66 S. aureus strains using the nitrocefin disk test and all strains tested positive.…”