I have never had a diabetic foot ulcer, but if/when it happens, my first option would be Bee Propolis, as I have previously used them for tooth abcesses with great success, and also for cellulitis once, and they can both be pretty vicious.
I would take 2x1000mg twice a day for a few days, up to a week if there are no adverse effects. It's usually quite effective if you can't get to the doctor or dentist for any reason. It's a natural antibiotic, which comes from tree-resin, it's what the bees use to sterilise the hive.
The Down Side is that some people can develop quite a serious rash, but as it's ingested, you only notice it when it's leaving the body, so to speak, and it can be very irritating - if this happens (not always) I would give up the propolis and apply a small amount of Betnovate (or whatever you prefer) for a day or two, which clears it up. I actually didn't get the rash until I'd used it for 2/3 separate incidences, and don't always.
I've also been longing to try Honey, but not had the opportunity recently. I do use colloidal silver plasters for other small cuts/scrapes.
Good Luck