I. Introduction
E. coli O157 can be transmitted to humans by foods and vegetables contaminated by animal fecal materials.Although undercooked ground beef meat has been identified as a leading food vehicle of E .coli O157, fresh raw vegetables are also becoming increasingly important vehicles of food born transmission [6]. Many outbreaks of E .coli O157 infections were associated with contaminated leafy lettuce [7], radish sprout [8], alfalfa sprout [9], potatoes[10].