Reaction to federal health officials’ warning of a new E. coli outbreak linked to romaine lettuce was swift Tuesday afternoon at the Mellow Mushroom on Denver’s 16th Street Mall. Less than two hours after the U.S. Food and Drug Administration warned Americans to avoid eating romaine lettuce until further notice, the popular pizza restaurant — alerted by its Atlanta-based corporate staff — had stopped serving romaine on its salads. “We are aware of it.