One of our cats has been with us for about 9 months and has always had a bad tummy. We knew this when we got him and he is fed sensitivity food which helps but is not a total solution... He doesn't really like the food and would much rather eat normal cat food... Recently he was ill with sore gums and the vet diagnosed colici virus and took out all but one of his remaining teeth! To encourage him to eat I fed him normal cat food and I noticed that his tummy was fine.
Due to the Colici virus, the vet put him on a long course of anti biotics- about 6 weeks and the whole time he was on them he ate normal cat food and did normal poos. This was a first as even on a sensitivity diet his poos have never been solid (he has a litter tray- hence my in-depth knowledge of this!)
Anyway, I told the vet this and he said he has never heard of this as anti biotics are more likely to upset a stomach than stable it... However, he finished his course on Friday and sure enough the upset tummy is back with a vengence! This makes me think that whatever is wrong with his tummy is something that responds to anti biotics but what on earth could that be? I'll take him back to the vets to ask again but if anyone has any ideas I'de be very interested.
He is about 10 and is (apart from his tummy and mouth) a happy, healthy cat.