I'm surprised no one has mentioned Zeniqin. I honestly don't think I've ever given anything but for a urinary infection. It's an excellent antibiotic with once a day dosing and it's very well tolerated. I've never had ANY side effects like upset tummy or any other GI issues.
When I had a cat that was prone to infections, we just gave her 2 weeks of Zeniquin every 3-4 months.