Present significance of resistance to trimethoprim and sulphonamides in coliforms, Staphylococcus aureus, and Streptococcus faecalis.

The incidence of trimethoprim resistance in coliforms and multiresistant strains of Staphylococcus aureus isolated in Bristol from 1970 to 1972 is low-2.3 and 1.0% respectively. The resistance is probably intrinsic; there is no evidence that it is R-factor or plasmid mediated. A single mechanism that confers resistance to both trimethoprim and sulphamethoxazole has not been detected. Normal growing one-step mutants of S. aureus and Escherichia coli resistant to trimethoprim could not be isolated in vitro. For these reasons cotrimoxazole should retain its usefulness against these bacteria for some years. However, contrimoxazole was found not to be bactericidal against many coliforms. The usefulness of cotrimoxazole against Streptococcus faecalis seems limited because mutants resistant to trimethoprim occurred at high frequency in one step.