Non-β-lactamase-mediated insusceptibility to β-lactams was studied in Haemophilus influenzae. Mutants resistant to ampicillin were selected in a susceptible isolate, while phenotypically similar resistance was transformed from four wild-type isolates into a susceptible recipient. Ampicillin-selected mutants generally had reduced suscept ibility to penicillins, cephalosporins and aztreonam. Reduced susceptibility to mero penem was less common, and susceptibility to mupenem was unaffected. MICs of three cephalosporins and aztreonam were much higher for the ampicillin-resistant transformants than for the recipient strain, and were generally equal to, or within one doubling dilution of, the concentrations that inhibited the donor parents. In contrast, reduced susceptibility to imipenem observed with one parent (MIC of 4 mg/l) was not transferred to the recipient (MIC of 0·5 mg/l). Meropenem inhibited 22 of 24 transformants tested at ≤ 0·12 mg/l, compared with MICs of {succeeds or equal to} 0·5 mg/l for the donor parents and 0·03 mg/l for the recipient. These results indicated that the mechanisms that confer non-β-lactamase-mediated insusceptibility to penicillins, cephalosporins and aztreonam in H. influenzae have little or no effect on susceptibility to carbapenems. © 1990, by The British Society for Antimicrobial Chemotherapy.
