The occurrence of human pathogens in raw vegetables is of great concern given the fact that these food products are not heated or treated to disinfect. Several outbrakes and cases of E. coli O157:H7 and Salmonella infection, which are traced back to the consumption of fresh vegetables, are known. We try to assess the risk of occurrence of E. coli O157:H7 and Salmonella typhimurium in iceberg lettuce. We therefore study the fate of these pathogens in different niche along the production chain of lettuce: survival in manure, manure amended soil and finally in or on the plant. We also conduct research on the possibility of entering the plant and the associated mechanisms. Moreover, we try to evaluate the mechanisms that lead to potential differences in pathogen survival and final risk of contamination between organic farming and conventional farming. Organic farming is thought to lead to a greater microbial diversity in manure, soil etc, which could function as a buffer against invasion of human pathogens.