The health benefits of kefir consumption have been well-known for hundreds of years. The objective of this study was to investigate the effect of kefir milk and the probiotic strain Lacticaseibacillus paracasei Ž2 isolated from kefir grains on the immune response and selected parameters of the lipid and liver enzymatic profiles of mice. Mice fed with kefir milk showed significantly increased phagocytic activity and percentages of B cells in the blood and increased gene expression for mucins and percentages of CD8+ lymphocytes in the gut. By applying kefir, we achieved a significant reduction in serum LDL cholesterol and an LDL/HDL ratio that favored an increase in HDL cholesterol. Regarding the hepatic enzymes, in particular a significant reduction in ALT activity was observed. L. paracasei Ž2 alone stimulated the immune response more markedly compared with kefir milk. Regarding the systemic level, we observed increases in the proportion of all T cells (CD3+), CD4+ lymphocytes and the ratio of CD4+:CD8+ cells, and regarding the local intestinal level we noted a significant increase in gene expression for mucins (MUC-1 and MUC-2) and IgA. Moreover, we confirmed the formation of a biofilm on the surface of the forestomach only after the application of L. paracasei Ž2 alone, but not after kefir administration. The results confirmed the hypothesis that the final effect of the probiotic does not correspond with the effect of the individual strain but is the result of mutual interactions of the microorganisms presented in a preparation, and therefore in the case of multi-strain probiotics, in vivo testing of the complex preparation is necessary.
Keywords: immunity; lactic acid; multi-strain; single-strain.