Trimethoprim antagonists: effect of uridine on thymine and thymidine uptake in mineral salts medium

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Abstract
Thymine is a potent antagonist of trimethoprim action on prototrophic organisms. Its presence in laboratory media invalidates sensitivity tests involving trimethoprim. Uridine has been shown to prevent the utilization of thymine by thymine requiring strains. Therefore, the action of uridine on prototrophic organisms, treated with trimethoprim, was examined to determine whether this base could prevent the antagonism by thymine. In higher concentrations of thymine, uridine was unable to prevent the incorporation of thymine by the trimethoprim-treated prototrophic strain. However, at lower thymine concentrations, uridine is as effective at preventing the uptake of thymine by the trimethoprim-treated prototroph as it was with a thymine-requtring strain.

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