A microbiologist who advocates the use of irradiation to kill food-borne pathogens was nominated by President Bush on Tuesday to oversee the Agriculture Department's food-safety programs. Elsa Murano, 41, is director of the Center for Food Safety at Texas A&M University, where she has done research on a variety of pathogens, including E. coli O157:H7, and methods for destroying them including irradiation.