UTI from E.coli... Should I toss the food?

My cat Stella was just diagnosed with E.coli as the source of her urinary bacterial infection.  She's currently on amoxycillian, and a temporary diet of canned C/D Urinary Care.

My question is:  What's the probable cause of an E.coli infection in the urinary tract?  Could it be food-related?  

She currently eats a rotation of Only Natural Pet Power Pate and Fancy Feast Classic canned.  Should I toss her existing food as a potential source of the E.coli and start fresh?

Thanks!

E-coli comes from animal waste, essentially poop.  Our own could have it.  The bacteria could travel to the urinary tract especially given the way kitties clean themselves.  While it is true that food can be contaminated with it I don't think that is the most probable place where it is at.  Commercial foods could have initially had it but they are processed at a high heat that will kill it.  That is why it is recommended that ground meat be thoroughly cooked for human consumption.  Other cuts of beef may have it on the surface but not throughout the meat, grinding the meat would introduce e-coli from the surface of the cut throughout the meat.

I would say NO, my cat got an UTI in March-they explained to me-that E coli could come by in two ways-

one-the poo from the body and bacteria gets up into the urethra then the bladder-

or

digestion-she said sometimes the bacteria comes from the gut-it could have been traveling into her stomach from ingesting OR it traveled from the colon/small intestine then into the kidney/liver/bladder etc. and it got set off. I wish I could remember how she explained it to me-I may have screwed up how its explained-but pretty much isn't the food or anything I did.

Sometimes stress plays a part and makes them more susceptible to an infection-and E Coli is generally already present in the gut-but for some reason it made its way to her bladder/kidney-my vet used convenia- now I know most people don't like this med but she said that convenia works well for E Coli.

2 weeks later we did a urine screen and the infection has cleared up-

So no its not the food. Its just sometimes something that happens despite trying to keep things clean and neat.

did your vet do sub fluids for her? To help flush that bladder?