What soap/preparation before rimming?

My wife loves to be on the receiving end of rimming, or “ass worship”, as we call it. We generally use Ivory bar soap, since it is pure and quite strong. We always bathe together before a session.  Since I am the giver, I also have the job of preparatory washing.  My wife kneels to present her bottom, and I wash the crack of her bottom with lots of soapy lather. I use both a wash cloth and my thumb to do a thorough cleaning.  I penetrate her anus very slightly with my soapy thumb in preparation for doing the same thing with my tongue afterwards. Then I rinse very thoroughly with lots of water.  Since bacteria could be transferred in both directions, I always brush my teeth and gargle with mouthwash before rimming. (I do the same thing afterwards). I have read that one should always use a dental dam to reduce the risk of a bacterial infection because no amount of washing can completely eliminate all bacteria.  However, we usually live dangerously because it is more pleasurable with no barrier.  I occasionally use dental dams when we travel and we can only prepare in the shower because there isn’t a decent bathtub in the room.  To increase my wife’s pleasure with a dental dam, I apply a water based lubricant to the crack of her bottom before putting the dental dam in place.