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

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Abstract
The zones sizes and MICs of ampicillin, cefaclor and cephalexin were determined for 2458 clinical isolates of Haemophilus influenza 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 (< 20 mm) to a 2 .mu.g ampicillin disc showed a definite (MIC .gtoreq. 4 mg/l) or intermediate (MIC 1 or 2 mg/1) 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.