Joint Trimethoprim and Sulphamethoxazole Resistance in Bacteria Infected with R Factors

SUMMARY Gram-negative bacteria, harbouring R factors that confer resistance to sulphonamides, mutate to trimethoprim resistance at an appreciable rate and are then able to grow in the presence of both trimethoprim and a sulphonamide. All trimethoprim-resistant organisms isolated in this study were thymine requiring, and it is suggested that the frequency of isolation of organisms resistant to trimethoprim, or combinations of trimethoprim and a sulphonamide, may be increased if adequate thymine or thymidine is included in the media used for their isolation.