Distribution and transferability of plasmids in trimethoprim resistant urinary Escherichia coli

Urinary isolates of Escherichia coli that were resistant to trimethoprim were collected in Glasgow Royal Infirmary during 1979 and 1980. Eighty-eight were resistant to trimethoprim 1024 micrograms/ml and 80 (92%) were also resistant to sulphamethoxazole 1024 micrograms/ml; 73% were multiresistant. Plasmids were detected in 98% of strains and 60% carried two or more. Half of the isolates transferred trimethoprim resistance to E. coli K12 and 70% of these cotransferred resistance to sulphonamide although these markers were often not linked. Trimethoprim resistance was carried on 12 different plasmids, four of which also conferred sulphonamide resistance. All except two carried streptomycin resistance which suggests that Tn7 was probably present. The results are discussed in relation to current prescribing policy.