To help with the gut bacteria? Plain unflavored is ok as a treat once in while but cats can't really digest the milk proteins in yogurt. Maybe try a probiotic instead.
I had to take one of the cats into the vet last year for "gastritis" and they gave us Clavamox drops - on the written after care instructions they gave me, they recommended giving him some (I think 1-2 tsp) plain unflavored organic greek yogurt with his canned food for a few days to help replenish the gut flora. A probiotic would work too. Either way, it's safe and a good idea since the antibiotic will wipe out ALL the intestinal bacteria - both good AND bad.