DHFR and escherichia coli infection: The use of SXT to treat intestinal E coli infection has occasionally enhanced the development of high-level resistance to trimethoprim in enterobacteria populations due to changes in cell permeability, loss of drug-binding capacity, and alterations in dihydrofolate reductase, which are mediated by mutation of the chromosomal DHFR gene (dfrA and dfrB) [50].