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 greater than or equal to 2 micrograms/ml) to ampicillin by mechanisms other than beta-lactamase, also demonstrated reduced susceptibility to amoxicillin/clavulanate (MICs up to 8 micrograms/ml) and the second generation cephalosporins (MICs up to 32 micrograms/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 less than or equal to 0.25 micrograms/ml) to the two quinolones ciprofloxacin and pefloxacin.