Improved detection of beta-lactamase activity in isolates of Staphylococcus saprophyticus with the use of a modified Cefinase disk procedure.

The Cefinase disk (BBL Microbiology Systems, Cockeysville, MD) technique has been reported to be relatively insensitive for detection of beta-lactamase production by Staphylococcus saprophyticus. The effects of time (up to 24 hours) and temperature (25 degrees C vs. 35 degrees C) of test incubation as well as enzyme induction on Cefinase detection of beta-lactamase activity in clinical isolates of S. saprophyticus was investigated. Results were compared to beta-lactamase detection with the use of conventional nitrocefin and cloverleaf methods and with penicillin minimal inhibitory concentrations (MICs). Of 85 S. saprophyticus isolates, 84 (98.8%) had penicillin MIC end points of greater than or equal to 0.25 microgram/mL. Beta-lactamase was detected by at least one method in 73 (85.9%) of the isolates: 63 (74.1%) by nitrocefin, 55 (64.7%) by cloverleaf, and 63 (74.1%) by Cefinase. Of Cefinase-positive isolates, 56 (88.9%) and 63 (100%) were detected at 25 degrees C and 35 degrees C, respectively, and none required enzyme induction. At 35 degrees C, 4 (6.3%) and 19 (30.2%) of the Cefinase-positive isolates were detected at 1 hour and 24 hours, respectively. Incubation of Cefinase tests at 35 degrees C for 24 hours provides a satisfactory alternative to the conventional nitrocefin assay for detection of beta-lactamase activity in S. saprophyticus.