According to Meathead at AmazingRibs, ground beef is supposed to hit 160 for about 5 to 10 seconds to kill any e-coli that may be present. It will die at lower temps but takes more time. If you go reverse sear, cook indirect until you can get the internal burger temp at 145 or higher for 4 minutes, then sear to your crust liking.
http://amazingribs.com/tips_and_tech...ety.html#ecoli
Burger info is about half way down that page.