Evaluation of a rapid beta-lactamase test for detecting ampicillin-resistant strains of Hemophilus influenzae type b.

Chloramphenicol is presently the drug of choice in the initial treatment of serious infections due to Hemophilus 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 one-hour acidimetric microassay for beta-lactamase activity. All ampicillin-resistant strains rapidly hydrolysed the beta-lactam ring of penicillin. When isolates were encoded and tested without knowledge of their MICs, the 40 ampicillin-resistant strains (MIC greater than or equal to 2 mug/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.