A doctor who worked for UC Berkeley's health center for more than 20 years pleaded not guilty Thursday to charges that he sexually assaulted patients. Robert Martin Kevess, 52, of Oakland assaulted six male patients since 2006 while working at the University Health Services just south of the campus, according to Alameda County prosecutors. Investigators declined to detail the alleged crimes but said the incidents involved patients, students ranging in age from 18 to 42, who had come to Kevess for examinations. All were inappropriately touched under the guise that the contact was medically necessary, said Senior Deputy District Attorney Susan Torrence. During a brief arraignment in an Oakland courtroom, Kevess pleaded not guilty to eight counts of sexual exploitation of a patient, seven counts of sexual battery involving false professional purpose and four counts of sexual penetration by a foreign object. All the charges are felonies.