Methinks I must have been a bacterium in another life: Pseudomonas putida. Many species in this genus persist in a variety of habitats and have a diverse diet. Some species of Pseudomonas are found feeding on a variety of substances including meat, decaying food, peteroleum oil, cyanide, toluene, geothermal features, and sometimes on humans and other beasts. One species produces the characteristic rainbow effect on deli cut roast beef. Don't worry though, as the critter is harmless to you or I (unless it is one of the parasitic species of Pseudomonas aeruginosa - the second most common infection in hospitals!). So, why not live on coffee? The little guy is a chemoautotroph that lives on the aromatic compounds in coffee including caffeine. This makes perfect sense as caffeine is a molecule that contains carbon, oxygen, hydrogen, and nitrogen. The prime ingredients in living systems. More on this newly discovered bacteria is at Scientific American.