Molecular mechanisms of resistance to trimethoprim.

Resistance to inhibitors of dihydrofolate reductase arises from a variety of mechanisms involving enzyme alteration, cellular impermeability, enzyme overproduction, inhibitor modification, and loss of binding capacity. The mechanism of greatest clinical importance is the production of plasmid-encoded, trimethoprim-resistant forms of dihydrofolate reductase. At least two different types of these enzymes have been documented. The trimethoprim-resistant reductases differ from all other dihydrofolate reductases in molecular weight, subunit structure, kinetic properties, and binding of inhibitors. Colony hybridization techniques, developed for the detection of plasmid DNA coding for trimethoprim-resistant reductases, enable researchers to evaluate the prevalence and distribution of plasmid-borne resistance. Preliminary results obtained with a series of enzymatically characterized clinical isolates suggest that the colony hybridization technique may provide a convenient epidemiological tool for monitoring the dissemination of plasmid-borne resistance to trimethoprim.