Chromogenic cephalosporin spot test to detect beta-lactamase in clinically significant bacteria

Abstract

A chromogenic cephalosporin assay for .beta.-lactamase in which an impregnated filter paper is used is reported. Nitrocefin (Glaxo) is dissolved in buffered dimethyl sulfoxide, and 0.5 ml is used to impregnate a filter paper in a petri dish. An isolated bacterial colony is applied to the paper with a loop, and pink reaction within 15 min indicates .beta.-lactamase production. Results of this test on clincial isolates were correlated with standardized penicillin and ampicillin susceptibility tests. A correlation of 100% was observed with Staphylococcus aureus (428 resistant and 88 sensitive strains) and Haemophilus influenzae (161 sensitive and 15 resistant strains). Of 45 isolates of Bacteriodes fragilis, 1 was falsely negative for .beta.-lactamase and 1 was falsely positive; the remainder were all positive and were penicillin resistant. Of 27 strains of B. melaninogenicus, 14 were .beta.-lactamase positive and 12 of these were penicillin resistant.