Susceptibility of multiply resistant Haemophilus influenzae to newer antimicrobial agents

One hundred and six isolates of Haemophilus influenzae from a national antimicrobial surveillance study demonstrated resistance to two or more of 10 primary antimicrobial agents by mechanisms other than or in addition to beta-lactamase. Of particular note were strains multiply resistant to ampicillin (by beta-lactamase production), chloramphenicol, trimethoprim/sulfamethoxazole, and tetracycline in various combinations. All of the aforementioned strains were shown to be highly susceptible to amoxicillin/clavulanate, the second generation cephalosporins cefuroxime and cefonicid, and the third generation cephalosporins cefotaxime, ceftizoxime, ceftriaxone, ceftazidime, moxalactam, and cefixime. However, 68 strains that demonstrated resistance or marginal susceptibility (MIC ≥ 2 μg/ml) to ampicillin by mechanisms other than beta-lactamase, also demonstrated reduced susceptibility to amoxicillin/clavulanate (MICs up to 8 μg/ml) and the second generation cephalosporins (MICs up to 32 μg/ml). While the latter strains were susceptible to the third generation cephalosporins, MICs were often 10-fold higher than MICs of ampicillin susceptible isolates or of beta-lactamase producing isolates. All of the multiply antimicrobial-resistant strains were highly susceptible (MIC ≤ 0.25 μg/ml) to the two quinolones ciprofloxacin and pefloxacin.