Resistance of Haemophilus influenzae to Trimethoprim

Out of 210 isolates of Haemophilus influenzae obtained from the sputum of 63 patients with chronic respiratory infections 109 (52%) were resistant to trimethoprim-sulphamethoxazole by the disc test. The minimal inhibitory concentrations of trimethoprim for 17 out of 18 strains recorded as resistant were 10 μg/ml or higher. Resistant strains were isolated from time to time from 32 (82%) out of 39 patients known to have been treated with trimethoprim-sulphamethoxazole, compared with only 1 (12·5%) out of 8 patients known not to have been treated with this drug combination. Resistant strains were isolated most frequently from patients who had received long-term treatment. Since sulphamethoxazole penetrates from the blood into the bronchial secretions less readily than does trimethoprim it seems likely that the ratio of the two drugs in the bronchial tree is far from ideal. This may be an important factor in the use of these drugs for chest infections.