According to this article, Navy stewards (from the Secret Service?) go around mopping up behind Obama, sanitizing or destroying anything he touched that might have bits of his DNA left on them.
The reason given is that technology is real close to enabling someone to whip up a bioweapon targeted at one person. Apparently the US has also been going around behind other world leaders and surreptitiously scooping up their DNA on the orders of Hillary Clinton, according to a Wikileak from 2010. Whether it is for this purpose, or whether it's just to figure out who their daddies were, or (if you think cloning and making someone grow quick is possible now) to make quik-grow doppelgangers of them, is anyone's guess.
I can think of another reason. They are protecting Mr. Obama from anyone who wants to test his DNA and see if he's really who he says he is. I have wondered why no birthers have attempted to snag something with his DNA on it, and now I know.
Or maybe it's something to do with the Lizard People. They're reputed to be obsessed with genetics. (Those silly lizard people! It's so easy to blame everything on them!)
Personally, I'm of the opinion that if you don't make so many enemies, you won't have to protect yourself so obsessively. But then, just like I wrote this morning, the sphere of power is a viper pit.