Owing to its high affinity for bacterial DHFR, trimethoprim (TMP) has been one of the most widely used antibiotics for the treatment of bladder infections.1–3,5 However, multiple strains of E. coli have developed resistance to TMP through chromosomal and point mutations which ultimately modulate the structure of DHFR and allow retention of function even in the presence of previously successful antibiotics.1–3,6–8. Here, DHFR is linked to cystitis.