(A) What is one example of a mutualistic symbiosis between an animal and microorganisms?

1. (A) What is one example of a mutualistic symbiosis between an animal and microorganisms?  (B) What is one other example of a symbiosis between any type of organism and at least one microorganism?  For both (A) and (B): How does each organism benefit from the association?  Be sure to also give a detailed description the microorganism and give its full genus and species name.  The description might include mentioning anything about its cell wall (gram positive, gram negative, etc.) or other interesting structures and/or behaviors.