Evaluation of S1 chromogenic cephalosporin beta-lactamase disk assay tested against gram-positive anaerobes, coagulase-negative staphylococci, Prevotella spp. and Enterococcus spp.

The efficacy of three rapid colorimetric disk assays to detect beta-lactamase production in 60 clinical isolates was evaluated. Two chromogenic cephalosporin substrates (S1 and nitrocefin) and an acidimetric test were in complete agreement when tested against Enterococcus spp. (20 strains, not Enterococcus faecalis), Prevotella spp. (10 strains) and Gram-positive anaerobic cocci (10 strains). However, the acidimetric test produced documented false-negative results in detecting the beta-lactamases from coagulase-negative staphylococci (two of 20 strains tested). The time required to produce a positive result for the discordant Staphylococcus epidermidis isolate favored S1 compared with nitrocefin. These studies indicate that the acidimetric test was less sensitive than the chromogenic cephalosporin substrates and that nitrocefin and S1 could be used to screen for beta-lactamase production in these tested species.