My urine culture showed an e coli resistant to penicillin and ampicillin, why did the doc prescribed me cefixime that is related to penicillin

Don't know: Your dr obviously checked drug sensitivities from turine culture. Cefixime is a 3rd generation cephalosporin & a very effective antibiotic, not related to penicillin. However, cross resistance between penicillin & 1st generation cephalosporins like Keflex is common. Wonder why dr didn't prescribe macrodantin, Bactrim or cipro, (ciprofloxacin) all 3 v effective & much cheaper. Possibly your e. Coli v resistant?

Different class: Cefixime is related, but a different class and generation; it was specifically created to kill things like e coli resistant to the original penicillin and ampicillin.