The human gut microbiota is an important environmental factor for human health with evolutionarily conserved roles in immunity, metabolism, development, and behavior of the host. Probiotic organisms are claimed to offer several functional properties including stimulation of immune system. The purpose of this study is to investigate the effects of a probiotic supplementation on adult volunteers who have contracted the common cold four or more times in the past year. This study is a single center, double-blind, randomized, controlled, prospective trial. Subjects received a probiotic drink containing Lactobacillus paracasei (at least 3 × 107 colony forming units (CFU) ml-1), Lactobacillus casei 431® (at least 3 × 107 CFU ml-1) and Lactobacillus fermentium PCC® (at least 3 × 106 CFU ml-1) or an identical placebo without probiotics for a 12-week study period. The consumption of probiotics significantly reduced the incidence of upper respiratory infection (p < 0.023) and flu-like symptoms with an oral temperature higher than 38 °C (p < 0.034) as compared to the placebo group. Subjects that consumed probiotics demonstrated a significantly higher level of IFN-γ in the serum (p < 0.001) and sIgA in the gut (p < 0.010) as compared to the placebo group and a significant higher level of serum IFN-γ (p < 0.001) and gut sIgA (p < 0.001) as compared to their baseline test results. In contrast, there were no significant differences in the serum IL-4, IL-10, IgA, IgG or IgM between the probiotics and the placebo groups. Results of this study demonstrated that probiotics were safe and effective for fighting the common cold and influenza-like respiratory infections by boosting the immune system.
Keywords: Human microbiota; IFN-γ; Probiotics; Upper respiratory infections; sIgA.