At the end of an ad set to air during Super Bowl XLIX, a scantily-clad supermodel takes a bite out of Carl's Jr.'s new "All-Natural" hamburger. Bold letters flash across the screen: No Antibiotics. The risqué nature of the commercial has triggered some controversy, but perhaps not as impassioned as the debate that continues over the widespread use of antibiotics by livestock producers -- and the role the practice may play in rising rates of antibiotic-resistant infections in humans. "This ad tells you that Carl's Jr.