examples of potentially harmful bacteria found in the GIT. These bacteria are capable of producing harmful substances, including amines, indole, hydrogen sul fide, and phenols, from food components. Harmful bacteria in the intestine have been linked to a number of clinical disorders such as cancer, inflammatory disease, and ulcerative colitis and also to an increase in the host's susceptibility to infection by enteropathogens such as Salmonella, Campylobacter Escherichia coli, and Listeria." For example, Clostridium difficile has been implicated as the primary cause of pseudomembraneous colitis. Some gut microflora undoubtedly protect the host animal from disease; for example, germ-free guinea pigs can be killed by only 10 cells of Salmonella enteritidis, while it requires up to 109 cells to kill an animal with a normal gut microflora. In addition, it is important to stress that the correct"balance" of bacteria must be maintained in order to allow the intestine to operate optimally. Lactobacillur and Bifidobacterium species are the most commonly used probi- otics in foods for human consumption; for this reason, this chapter will deal only