The incidence and epidemiology of β-lactam resistance in Haemophilus influenzae

Consecutive isolates of Haemophilus influenzae were collected by the Public Health Laboratory, Bath between 1 June 1992 and 31 May 1993. Of 379 apparently distinct isolates, 216 originated from the respiratory tract, 102 from eyes and 61 from other sites. The minimum inhibitory concentrations of amoxycillin, amoxycillin/clavulanate, cefaclor, cefuroxime, cefotaxime and cefpodoxime were determined for each isolate. Forty strains (10.6%) were β-lactamase producers. MIC50 and MIC90 values and the range of MICs were determined for all isolates. The overall resistance rates were: amoxycillin (MIC>1.0 mg/L), 22.7%; amoxycillin/clavulanate (MIC>1.0 mg/L), 14.8%; cefuroxime (MIC>1.0mg/L), 18.5%, (MIC>4.0mg/L), 5.5%; cefaclor (MIC>8 mg/L), 15.6%; cefpodoxime (MIC>1.0mg/L), 0.3%; cefotaxime (MIC>1.0 mg/L), 0%. Twenty non-β-lactamase producing but β-lactam resistant strains (cefuroxime MIC>4.0 mg/L) were matched with 20 susceptible strains on the basis of patient age, sex, and specimen type. The strains were characterised by outer-membrane protein (OMP), random amplified polymorphic DNA (RAPD) and ribotyping patterns. Eleven of the 20 resistant strains were indistinguishable by the methods used, suggesting spread of a single β-lactam resistant, non-β-lactamase producing clone. The distribution of resistant strains within the local community was plotted geographically.