I am currently reading a book about sex. There is a section dedicated to anal stimulation. According to research for the book, by the time a couple is together for two years they have about the same anal make-up. What that means is they both have the same types of bacteria, etc., in their anal canal. So if you have been together for that long you will not get anything from her anus that you don't already have in your own system.
I will stress here that anal cleanliness is very important. The bacteria found in the anal canal can cause severe infection in the vagina. Never put anything into the vagina that has been in the anal canal. It is suggested that if you play with the anus to use a condom, or a latex glove. They can be removed when finished with no worries about then infecting the vagina with the bacteria. If a condom, or glove, is not used then you should clean thoroughly with soap and water before going any further with sex.