Trimethoprim is a dihydrofolate reductase inhibitor used to inhibit folate synthesis in susceptible bacteria. In microbiology workflows, it is employed to suppress growth of trimethoprim-sensitive populations and to select for trimethoprim-resistant clones in organisms such as E. coli and Staphylococcus aureus.

It is a white to off-white crystalline powder with poor aqueous solubility and good solubility in certain organic solvents (e.g., methanol). Typical laboratory applications include antimicrobial selection in bacterial culture, maintenance of plasmids or genomic inserts carrying trimethoprim resistance markers, and quality control testing in susceptibility assays.

Molecular formula: C14H18N4O3; molecular weight: 290.32 g/mol; CAS: 738-70-5. Store in a tightly closed container, protected from light and moisture, under controlled room temperature.