Honestly? There's really nothing in coffee for bacteria to grow on. If you take your coffee black, I don't see this as being much of a public health problem. Unorthodox and slightly bleh, maybe, but not out and out unsafe.
Of course, if you adulterate it, all bets off.
Leave your coffee mug with a few ounces left for 4-5 days (long weekend). When you come back you'll see bacteria floating around. Leave it over a long vacation like Christmas and the entire surface will be green. My coffee is black.


People wash coffee mugs?!?!?! 