In vitro resistance of haemophilus influenzae to co-trimoxazole and trimethoprim.

Over a period of five months Haemophilus influenzae isolated from 123 patients were tested for resistance to co-trimoxazole and trimethoprim by the disc method of sensitivity testing and isolates from four patients were found to be resistant. However, the minimum inhibitory concentrations of trimethoprim lactate were estimated for the isolates from these four patients only one was found to be resistant. This H. influenzae strain was from one of the two patients in the survey who had a history of repeated courses of co-trimoxazole therapy. The discrepancy between the results of the disc sensitivity tests and the minimum inhibitory concentration estimations of the resistance of H. influenzae to co-trimoxazole and trimethoprim. The appearance of co-trimoxazole and trimethoprim resistance in H. influenzae isolated from a patient who has had prolonged treatment with co-trimoxazole is in agreement with the findings of other authors.