SUMMARYDuring a 3-year period Branhamella catarrhalis was isolated in significant numbers from 239 (1P3%) of 19488 specimens of sputum sent for routine microbiological examination at a 700-bed general hospital. The majority of patients (83 %) were over 60 years of age and 65 % were male. There was a distinct seasonal variation in isolations with a peak incidence during the winter and early spring, a pattern not found with other pathogens. Susceptibility to amoxycillin decreased by approximately 50 % over the 3 years, corresponding to an increased incidence of beta-lactamase-producing strains. There were minimal changes in susceptibility to other antimicrobial agents. Underlying pulmonary disease was the major factor predisposing to B. catarrhalis infection, and 71 % of patients were smokers or ex-smokers.

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Fifty strains of Branhamella catarrhalis were examined for susceptibility to sulphamethoxazole, trimethoprim and a combination of the two by determinating minimum inhibitory concentrations (MICs) and fractional inhibitory concentrations (FICs). All strains were susceptible to sulphamethoxazole and resistant to trimethoprim. On the basis of the MIC results it was predicted that greater synergy between sulphamethoxazole and trimethoprim would be observed with approximately equal proportions of each component. The lowest FIC values were obtained with a ratio of 1:1 and the greatest synergy was observed at this ratio with 39 strains (78%). Only seven strains were most synergistically inhibited at the ratio of 20:1 (sulphamethoxazole: trimethoprim) although this ratio was still synergic for most strains. Overall the 1:20 ratio was not synergic.

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