Trimethoprim action and its analogy with thymine starvation.

In a minimal medium, trimethoprim is merely bacteriostatic on the prototroph Escherichia coli 114. The drug was bactericidal when the amino acids methionine and glycine, plus a purine or purine nucleoside, were also present. This response could be reversed completely when thymine and lysine were added to the culture. Methionine, glycine, and the purine are thought to maintain the integrity of the tetrahydrofolate pool under trimethoprim treatment and prevent the thymidylate synthetase reaction. Thus, the organism behaves phenotypically as a thymineless mutant. The mechanisms by which thymine and lysine reverse the bactericidal effect of trimethoprim in a minimal medium containing methionine, glycine, and adenine is discussed.