Sorry to hear that your pet is sick! But it is wonderful that she is feeling better.
First of all lets talk a bit about Escherichia coli. This bacteria lives in the lower intestines of most animals, and yes.... that includes you and me. The most common methods of infection are fecal contaminated food or water and undercooked meat. Thus for dogs it includes ponds, other pets or wild animals feces, contaminated foods, and even toilet water.
Why do we all have E.coli in us but at the same time it makes us so sick? The illness can depend mainly on the strain of the E. coli and the amount that the pet is exposed to.
Can it be from the dog food? Yes this is possible. The major food recall that recently occurred was due to a artificial faux-protein called melamine but many dog and human food recalls are E.coli based. E.coli can be present in beef because the fecal bacteria get mixed in with the meat during the slaughtering process. (eww.....
Is interceptor involved? Nope.
I am sorry to hear that your vet has not answered these questions for you.
That must be frustrating. So often we vets become very busy.
Did that answer all the questions you had?