In vitro susceptibility ofHaemophilus influenzae to cefaclor,cefixime, cefetamet and loracarbef

The susceptibility of 2,212Haemophilus influenzae isolates cultured in UK clinical laboratories in 1991 was determined for four orally-administered -lactam drugs. These isolates included 1,893 ampicillin-susceptible, 191 -lactamase-positive and 128 ampicillin-resistant, -lactamase-negativeHaemophilus influenzae. While 150 (6.8 %) isolates were resistant to cefaclor (MIC 16 mg/l) and 85 (3.8 %) to loracarbef, all were inhibited by 2 mg/l cefetamet and 1 mg/l cefixime and were therefore susceptible to these agents. Ranges and modes of inhibition zone diameters and MICs indicated that the susceptibility of a variable proportion of the 191 -lactamase-positive isolates to cefaclor, loracarbef and cefetamet was reduced compared with the fully susceptible population. In contrast, a major reduction in susceptibility to all four antimicrobial agents was seen among the 128 ampicillin-resistant (MIC 1–64 mg/l) -lactamase-negative isolates such that these accounted for 53 % and 67 % of the total number of organisms resistant to cefaclor and loracarbef respectively. In addition, 23 of 25 isolates inhibited only by 1 mg/l cefetamet and all eight inhibited only by 0.5 mg/l cefixime showed this type of resistance to ampicillin. Results indicate the importance of detecting non--lactamase-mediated resistance to ampicillin and any concomitant diminished susceptibility to other -lactam drugs.