AIM: To investigate the capacity of potentially probiotic strains from six bacterial genera to induce cytokine production alone or in combinations in order to identify potential enhancing or synergistic effects in order to select probiotic bacteria for in vivo purposes. METHODS: Cytokine production in human peripheral blood mononuclear cells (PBMC) in response to stimulation with eleven different potentially probiotic bacterial strains from streptococcus, Lactobacillus, Bifidobacterium, Lactococcus, Leuconostoc and Propionibacterium genera was analysed. Production and mRNA expression of TNF-alpha, IL-12, IFN-gamma and IL-10 were determined by ELISA and Northern blotting, respectively. RESULTS: All tested bacteria induced TNF-alpha production. The best inducers of Th1 type cytokines IL-12 and IFN-gamma were Streptococcus and Leuconostoc strains. All Bifidobacterium and Propionibacterium strains induced higher IL-10 production than other studied bacteria. Stimulation of PBMC with any bacterial combinations did not result in enhanced cytokine production suggesting that different bacteria whether gram-positive or gram-negative compete with each other during host cell interactions. CONCLUSION: The probiotic S. thermophilus and Leuconostoc strains are more potent inducers of Th1 type cytokines IL-12 and IFN-gamma than the probiotic Lactobacillus strains. Bacterial combinations did not result in enhanced cytokine production. (c) 2008 WJG. All rights reserved.