what causes a uti
1 Infection of the bladder (cystitis). 2  This type of UTI is usually caused by Escherichia coli (E. coli), a type of bacteria commonly found in the gastrointestinal (GI) tract. 3  However, sometimes other bacteria are responsible.4  Sexual intercourse may lead to cystitis, but you don't have to be sexually active to develop it. This type of UTI can occur when GI bacteria spread from the anus to the urethra. 2  Also, because the female urethra is close to the vagina, sexually transmitted infections, such as herpes, gonorrhea, chlamydia and mycoplasma, can cause urethritis.