Grapefruit seed extract is very bitter - I'm not sure if a cat will take it. Perhaps one or two drops in his/her water dish.
It is taught that it is a very powerful virus and bacterial killer. Just adding a few drops to a litre of water is supposed to kill any microbes that may be present!
It has a very potent sour taste! To me anything that tastes like that must kill potent germs! (ha ha)