The effect of probiotics on host mucosal immune responses : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Nutritional Science at Massey University, Palmerston North, New Zealand
Lactic acid bacteria (LAB) are a group of Gram-positive anaerobic bacteria that convert carbohydrates and complex sugars into lactic acid as the end product through fermentation. Several species of LAB have been used as probiotics. Probiotics are mono- or mixed cultures of live microorganisms which, when orally administered to animals or man, benefit them by improving the balance of the indigenous microflora. Lactic acid bacteria are claimed to have several beneficial effects; one of them being stimulation of the immune system. Many studies have demonstrated the immunostimulatory effects of LAB and various mechanisms have been suggested as to how LAB stimulate the immune system. These include the ability of LAB to translocate to Peyer's Patches (PP) and other gut-associated lymphoid tissues (GALT) for immunological processing by immunocompetent cells and production of cytokines. There were three aims in our present studies. The first was to determine the effect of dose of an immunoenhancing probiotic strain L. rhamnosus on the mucosal and serum immune responses of mice to oral antigens cholera toxin (CT) and ovalbumin (OV). The second aim was to examine the effect of viability of L. rhamnosus on these responses. Various mucosal immune parameters were measured in these studies. Results indicate that the immunostimulatory effects of L. rhamnosus were dose-dependent and that the 1 x 109 cfu dose was the most appropriate dose for L. rhamnosus for its immunostimulatory effects. Viability also affects the immunostimulatory effects of L. rhamnosus as shown by the higher efficacy of viable L rhamnosus than non-viable L. rhamnosus in stimulation of several aspects of the mucosal immune system. In some other immune parameters, non-viable L. rhamnosus was found to be the same as, or more effective than the viable bacteria. These findings were significant in that they provide additional evidence of the dose- and viability-dependency of different LAB. This information will help those involved in the development of probiotic products to consider these factors when formulating their products so that the concentration of live LAB can be adjusted to ensure the product can convey the maximum beneficial effect to the consumer. The third aim of our studies was to examine the role played by the immune system in protecting against enteric infection. It was found that L rhamnosus increased the resistance of mice to S. typhimurium infection as demonstrated by the lower numbers of bacteria found in the liver and spleen, and a maintenance of liveweight. This was also accompanied by increased mucosal and systemic immune responses to S. typhimurium. This result suggests that the immune system may play an important role in mediating the protection against enteric infection. Various other mechanisms have also been postulated by which LAB protect against enteric infection, for example, production of antibacterial substances, competition for adherence to the gut wall and for nutrients. However, the precise role and relative importance of these mechanisms in mediating protection against enteric infection is unknown.