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 in fluenzae from clinical specimens collected from November 1994 through February 1995 in nine general hospitals throughout Belgium were examin ed for beta-lactamase production using a nitrocefin-based test, and fo r their in vitro susceptibilities to ampicillin, amoxycillin/clavulana te, cefaclor, cefuroxime, cefotaxime, clarithromycin and azithromycin by means of the NCCLS agar dilution test. The isolates were all from r espiratory tract specimens. The prevalence of capsular type b was 1.1% , and the overall rate of beta-lactamase production 16.7%. Rates of be ta-lactamase production were higher in isolates from children (22.0%) than in those from adults (15.3%), and in isolates from upper respirat ory tract specimens (22.0%) than in those from the lower respiratory t ract (15.1%). Beta-lactamase-negative ampicillin resistance amounted t o 1.1%. Cefotaxime had the highest activity on a weight basis [MIC (mi nimal inhibitory concentration) for 50% of the isolates tested (MIC(50 )) less than or equal to 0.06 mu g/ml], followed by ampicillin (MIC(50 ) of 0.25 mu g/ml), amoxycillin/clavulanate and cefuroxime (MIC(50) of 0.5 mu g/ml), azithromycin (MIC(50) of 2 mu g/ml), cefaclor (MIC(50) of 4 mu g/ml), and clarithromycin (MIC(50) of 8 mu g/ml). Cefotaxime w as also the most active drug in terms of susceptibility rates of the i solates (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 c efuroxime retain an excellent activity against H. influenzae, while ce faclor lost some of its activity. The rate of susceptibility to azithr omycin was markedly higher than that to clarithromycin; however, its a bility to accumulate intracellularly while concentrations in serum and interstitial fluid remain low, should be considered, as it may repres ent a major drawback to its use in H. influenzae infections.