The Role of Lactic Acid Bacteria in Colon Cancer Prevention

Probiotics, in the form of dairy foods with lactic acid bacteria, have been consumed for centuries by humans. Over the past decade there has been increased interest in bacterial food supplements, which we now call probiotics. The term 'probiotic' was first used for growth-promoting animal feeds in the 1970s. Fuller has now defined the term as 'a live microbial feed supplement which beneficially affects the host animal by improving its microbial balance'. The probiotic preparations currently on the market are in the main based on lactic acid bacteria (lactobacilli, bifidobacteria, and streptococci). There are also other microorganisms used as probiotics. Effects and modes of action of probiotics can differ. The most evident effects involve changes in viable counts of microorganisms in the intestinal flora after ingestion. These effects can, according to Fuller be caused by competition for adhesion sites and nutrients between the ingested microorganisms and potential pathogens. Another mode of action for the probiotic can be production of antibacterial substances. However, the influence of a probiotic supplement is not always that pronounced. There can also be an alteration of microbial metabolism in the gut, which can be detected as, for instance, altered bacterial enzyme activities, changed pH, or influence on levels of cholesterol. There are several characteristics that are of importance for organisms that are to be used as probiotics. These include: the organisms should be normal inhabitants of the intestinal tract, should survive the upper digestive tract, should be capable of surviving and growing in the intestine, should produce beneficial effects when in the intestinal tract, and should maintain viability and activity in the carrier food before consumption. It is also important that the organism is non-pathogenic and non-toxic. The most common media for probiotics in lyophilized form include tablets and capsules and in live form fermented and non-fermented milk. Even products like candy bars and ice cream have been used as vehicles. For further information, the reader is referred to some excellent recent reviews on the subject. Healthful effects of lactic cultures that have been suggested include improved absorbability of certain nutrients, alleviation of lactose intolerance symptoms, metabolism of some drugs, serum cholesterol reduction, improvement of intestinal motility, stimulation of the immune system, anticancer effects, creation of an antagoni