Future perspectives in Probiotic Use
Lactobacilli derived from the endogenous flora of normal donors are being increasingly used as probiotics in functional foods and as vaccine carriers. However, a variety of studies carried out with distinct strains of lactobacilli have suggested heterogeneous and strain-specific effects (Table IV).
To dissect this heterogeneity at the immunological level, Ibnou-Zekri et al. (137) selected two strains of lactobacilli that displayed similar properties in vitro and studied their impact on mucosal and systemic B-cell responses in monoxenic mice. Germ-free mice were colonized with L. johnsonii (NCC 533) or L. paracasei (NCC 2461). Bacterial loads were monitored for 30 days in intestinal tissues, and mucosal and systemic B-cell responses were measured.
Although both Lactobacillus strains displayed similar growth, survival, and adherence properties in vitro, they colonized the intestinal lumen and translocated into mucosal lymphoid organs at different densities. L. johnsonii colonized the intestine very efficiently at high levels, whereas the number of L. paracasei decreased rapidly and it colonized at low levels. They determined whether this difference in colonization correlated with an induction of different types of immune responses, and observed that colonization with either strain induced similar germinal center formation and IgA bearing lymphocytes in the mucosa, suggesting that both strains may activate mucosal B-cell responses.
However, clear differences in the patterns of immunoglobulins were observed between the two strains in the mucosa and in the periphery. Therefore, despite similar in vitro probiotic properties, distinct Lactobacillus strains may colonize the gut differently and generate divergent immune responses.
In another study, Coakley et al. (138), assessed strains of Lactobacillus, Lactococcus, Pediococcus and Bifidobacterium for their ability to produce the health-promoting fatty acid conjugated linoleic acid from free linoleic acid. Strains of Lactobacillus, Lactococcus, Pediococcus and Bifidobacterium were grown in medium containing free linoleic acid. Growth of the bacteria in linoleic acid and conversion of the linoleic acid to conjugated linoleic acid was assessed.
Of the bacteria assessed, nine strains of Bifidobacterium produced the cg, t11 CLA isomer from free linoleic acid. The cg, t11 conjugated linoleic acid isomer was also produced by some strains, but at much lower concentrations. Thus, the production of conjugated linoleic acid by bifidobacteria showed considerable interspecies variation.
Bifidobacterium breve and B. dentium were the most efficient producers of conjugated linoleic acid among the range of strains tested, with B. breve converting up to 65% linoleic acid to cg, t11 conjugated linoleic acid when grown in 0.55 mg/ml linoleic acid. Strains also varied considerably with respect to their sensitivity to linoleic acid. The production of conjugated linoleic acid by probiotic bifidobacteria offers a possible mechanism for some health-enhancing properties of bifidobacteria and provides novel opportunities for the development of functional foods.
Finally, it has been demonstrated that the therapeutic dose of IL-10 may be reduced by localized delivery of a probiotic genetically engineered to secrete the cytokine. Intra-gastric administration of IL10-secreting Lactococcus lactis caused a 50% reduction in colitis in mice treated with dextran sulfate sodium and prevented the onset of colitis in IL-10 gene-deficient mice (139). This observation has also opened the way for the use of probiotics as live vaccine delivery vectors (140).