Effect of trimethoprim or trimethoprim/sulphamethoxazole usage on the emergence of trimethoprim resistance in urinary tract pathogens.

The frequency of trimethoprim (TMP)-resistant bacteria isolated from urine was studied in hospitals, in Stockholm, Sweden, and Turku, Finland. TMP has been in clinical use in Finland since 1973, whereas in Sweden TMP was available only in combination with sulphamethoxazole (SMZ) at the time for this study. All samples were collected solely from in-patients at 4 different occasions during a 20-month period 1977-78. The frequency of TMP-resistant strains (MIC greater than or equal to 8 micrograms/ml) in Turku increased from 31 to 49%, whereas it remained at a low level (1.6-3.6%) in Stockholm during the whole period. Also the frequency of TMP/SMZ-resistant strains (MIC greater than or equal to 64 micrograms/ml) increased in Turku from 24 to 45% but remained at the same level, 0-4% in Stockholm. In Turku there was also an increased frequency of resistance to SMZ (MIC greater than or equal to 512 micrograms/ml), ampicillin (MIC greater than or equal to 32 micrograms/ml) and nitrofurantoin (MIC greater than or equal to 64 micrograms/ml), while these frequencies remained at their initial levels in Stockholm. The distribution of bacteria was different in the two areas. In Turku there was also a change towards more nosocomial strains during the observation period. Since antibacterial agents are used more frequently in Turku than in Stockholm, this might be one explanation to the increase of resistance in Turku.