In BE style, "Mr", "Mrs", "Dr" do not usually have full stops, because they are contractions (they end with the same letter as the word they are abbreviating). Some people don't adhere to this anymore, but a lot of people still do.
As far as I'm aware, the word "acronym" usually refers to an abbreviation that is pronounced as one word ("NATO", "scuba", "radar"), so U.S.A./USA and F.B.I./FBI are initialisms, not acronyms in the strict sense of the word.
I write "USA" without full stops, but one of my American proofreaders has told me to put them in, so I thought that was the accepted AE style. Maybe that's no longer the case (this particular proofreader is quite old-fashioned, I suppose).