Probiotic bacteria and the human immune system

The human colon contains 10(14) bacteria/gram of luminal contents. The interactions between this flora and the host immune system has resulted in a state of controlled inflammation at mucosal surfaces. Pertubances of this delicate balance may result in disease and distress to the host. Probiotic bacteria have been reported to exert significant immune-modulatory effects. These effects include modulation of humoral responsiveness, viral resistance and proinflammatory activity within the gastrointestinal tract resulting in protection against infection and mucosal damage. These responses may result in alterations to host cell gene induction and protein expression (e.g, cytokines, antigen presentation, transcription factor activation) due directly to the physical binding of the probiotic strains or the presence of their secreted products. Research at University College Cork has identified a probiotic strain, Lactobacillus salivarius subsp. salivarius UCC118, which survives passage through the human gastrointestinal tract, modulates the enteric flora and adheres to human epithelial cells. The immune perception of this strain was examined in a human feeding trial. No alteration in serum cytokines or soluble receptor levels were observed between the control and test groups over the trial period. Also, there were no differences in the change from baseline concentrations of serum antibodies specific to UCC118 or total serum antibodies. However, mucosal antibody levels specific to UCC118 significantly increased. These results demonstrate that UCC118 is perceived by the human mucosal immune system but does not induce systemic reactivity. Currently, we are examining the mechanisms underlying the immune perception of probiotic bacterial strains both in vitro and in vivo, focusing on control as well as stimulation of the immune response. In summary, we have identified probiotic strains that exert specific immunomodulatory effects within the gastrointestinal tract.