Evaluation of the capillary beta-lactamase test and antimicrobial susceptibility of Haemophilus influenzae.

The capillary beta-lactamase test for the detection of Haemophilus influenzae resistance to ampicillin was evaluated against 132 strains of H. influenzae recently isolated from clinical materials and four reference strains. Nineteen strains, including two of serotype b, were beta-lactamase-positive. The minimal inhibitory concentrations (MIC) of ampicillin for the 117 beta-lactamase-negative strains ranged from less than or equal to 0.125 to 2 microgram/ml (only one strain had a MIC of 2 microgram/ml). The range of MIC's of ampicillin was 4 to 64 microgram/ml for the 19 beta-lactamase-positive strains; all but two strains required 8 microgram/ml or more for inhibition. The capillary beta-lactamase test is an easy, rapid and reliable test for the detection of H. influenzae resistance to ampicillin. It is suitable for routine use in the clinical microbiology laboratory. The MIC of carbenicillin was higher for ampicillin-resistant than for ampicillin-susceptible strains, but the highest MIC (32 microgram/ml) was within achievable serum concentrations. Both cefamandole and chloramphenicol were active against all strains.