EVALUATION OF A RAPID BETA-LACTAMASE TEST FOR DETECTING AMPICILLIN-RESISTANT STRAINS OF HEMOPHILUS-INFLUENZAE TYPE-B

Abstract

Chloramphenicol is presently the drug of choice in the initial treatment of serious infections due to H. influenzae type b. Rapid detection of ampicillin resistance in clinical isolates would facilitate early discontinuation of chloramphenicol therapy in patients infected with ampicillin-sensitive bacteria. A total of 160 strains of H. influenzae type b were tested with a 1 h acidimetric microassay for .beta.-lactamase activity. All ampicillin-resistant strains rapidly hydrolyzed the .beta.-lactam ring of penicillin. When isolates were encoded and tested without knowledge of their MICs [minimum inhibitory concentration], the 40 ampicillin-resistant strains (MIC .gtoreq. 2 .mu.g/ml) were readily distinguished from 120 sensitive strains. Rapid .beta.-lactamase assay is therefore a reliable detector of ampicillin resistance in H. influenzae type b.