Resistance to trimethoprim in 1978-79 compared with 1973-75.

The incidence of resistance to trimethoprim among urinary isolates between October 1978 and November 1979 was 11.5%, more than double the figure found April 1973 and October 1975. Of the resistant strains, 60% had a minimum inhibitory concentration in excess of 1 mg/ml. Escherichia coli and Proteus mirabilis showed the greatest increase in resistance since the previous study. Rather wide fluctuations occurred in the incidence of resistance for various species when the figures were analysed over two-month periods, hence studies of short duration must be interpreted with caution. There are not yet enough data from this or other studies for the cause of the increased incidence of resistance to trimethoprim to be determined.