Germ expert: Coffee mug on office desk a germ machine TEMPE, Ariz., Feb. 23 (UPI) -- Reusing a coffee mug at the office is great for the environment, but a U.S. germ expert advises taking the mug home for dishwasher washing. Professor Charles P. Gerba of the University of Arizona's Department of Soil, Water and Environmental Science suggested when drinking a mug of coffee at the office, don't nurse the cup for longer than an hour or so. Colonies of germs are living in your favorite cup. Twenty percent of office mugs carry fecal bacteria, and 90 percent are covered in other germs," Gerba told Men's Health magazine. That's because in an office, most people tend to clean their cups with bacteria-laden sponges or scrub brushes instead of in a dishwasher. That bacteria transfers to the mug and can live there for three days. Take the mug home daily to be washed in the dishwasher, and make sure it goes through the dry cycle, which uses the hottest temperatures and zaps every last germ, Gerba advised. At the very least, wash it with hot water, soap and a paper towel. If it sits unwashed on your desk after being used, germs will start reproducing immediately -- and bacterial colonies grow even when the cup contains nothing more than a coffee ring," Gerba said. Gerba said his earlier research found in the office, the phone was the object in the office with the most germs, followed by the desktop, keyboard, mouse, fax machine and photocopier, while the object with the least amount of germs was the the toilet seat.