In vitro susceptibilities of 180 clinical isolates of Haemophilus influenzae to ampicillin, amoxycillin/clavulanate, cefaclor, cefuroxime, cefotaxime, clarithromycin, and azithromycin.

One hundred eighty consecutive, unduplicate isolates of Haemophilus influenzae from clinical specimens collected from November 1994 through February 1995 in nine general hospitals throughout Belgium were examined for beta-lactamase production using a nitrocefin-based test, and for their in vitro susceptibilities to ampicillin, amoxycillin/clavulanate, cefaclor, cefuroxime, cefotaxime, clarithromycin and azithromycin by means of the NCCLS agar dilution test. The isolates were all from respiratory tract specimens. The prevalence of capsular type b was 1.1%, and the overall rate of beta-lactamase production 16.7%. Rates of beta-lactamase production were higher in isolates from children (22.0%) than in those from adults (15.3%), and in isolates from upper respiratory tract specimens (22.0%) than in those from the lower respiratory tract (15.1%). Beta-lactamase-negative ampicillin resistance amounted to 1.1%. Cefotaxime had the highest activity on a weight basis [MIC (minimal inhibitory concentration) for 50% of the isolates tested (MIC50) < or = 0.06 microgram/ml], followed by ampicillin (MIC50 of 0.25 microgram/ml), amoxycillin/clavulanate and cefuroxime (MIC50 of 0.5 microgram/ml), azithromycin (MIC50 of 2 micrograms/ml), cefaclor (MIC50 of 4 micrograms/ml), and clarithromycin (MIC50 of 8 micrograms/ml). Cefotaxime was also the most active drug in terms of susceptibility rates of the isolates (100.0%), followed by amoxycillin/clavulanate and azithromycin (98.9%), cefuroxime (97.2%), cefaclor (89.4%), clarithromycin (82.8%), and ampicillin (82.2%). In conclusion, amoxycillin/clavulanate and cefuroxime retain an excellent activity against H. influenzae, while cefaclor lost some of its activity. The rate of susceptibility to azithromycin was markedly higher than that to clarithromycin; however, its ability to accumulate intracellularly while concentrations in serum and interstitial fluid remain low, should be considered, as it may represent a major drawback to its use in H. influenzae infections.