Evaluation of Hemophilus type B systemic isolates for β-lactamase and non-β-lactamase mediated ampicillin resistance and for susceptibility to other antimicrobial agents

Of 175 recent Minnesota Hemophilus influenzae type b isolates from systemic disease, 43 were found to be resistant to ampicillin (greater than or equal to 4 micrograms/mL [mg/L]), each of which produced beta-lactamase. Of the 132 ampicillin-susceptible isolates, 68 (52%), all beta-lactamase negative, had minimum inhibitory concentrations (MICs) of either 1 or 2 micrograms/mL (mg/L), indicating relative resistance if derived from cerebrospinal fluid (CSF) infections. From a review of the literature, and in agreement with the authors findings, ampicillin-resistant beta-lactamase-negative isolates are rare and are likely to be nontypeable, of respiratory origin, and with MICs in the low resistance range. For the 43 ampicillin-resistant isolates, percentages resistant to other agents were as follows: 0% chloramphenicol, 0% rifampin, 6% tetracycline, 0% trimeprim-sulfamethoxazole, 2% cefamandole, 5% cefaclor, 2% moxalactam, and 0% for the remaining third-generation cephalosporins cefotaxime, ceftazidime, and ceftizoxime. Unlike ampicillin-resistant isolates, 100% of ampicillin-susceptible isolates had relatively low cefaclor MICs of less than or equal to 4 micrograms/mL (mg/L), suggesting a relatively increased H. influenzae beta-lactamase effect on cefaclor in comparison with the other cephalosporins tested.