In-vitro activity of cefaclor, cephalexin and ampicillin against 2458 clinical isolates of Haemophilus influenzae.

The zone sizes and MICs of ampicillin, cefaclor and cephalexin were determined for 2458 clinical isolates of Haemophilus influenzae collected in 1986 during a national survey of the prevalence of resistance in this species. Cefaclor showed greater in-vitro activity than cephalexin against all isolates, with a modal MIC of 4 mg/l compared with 8 mg/l. MIC50 values for the 2201 ampicillin-sensitive and 157 beta-lactamase-positive resistant strains were similar for both cephalosporins (4 mg/l for cefaclor, and 8 mg/l for cephalexin). A further 100 strains with a reduced zone (less than 20 mm) to a 2 micrograms ampicillin disc showed a definite (MIC greater than or equal to 4 mg/l) or intermediate (MIC 1 or 2 mg/l) degree of intrinsic resistance to ampicillin. Both cephalosporins showed a marked and significant rise in MIC values for this group. Cefaclor is the more active agent against ampicillin-sensitive and beta-lactamase-positive H. influenzae but strains with an intrinsic mechanism of resistance to ampicillin are markedly less sensitive to both cefaclor and cephalexin.