Intestinal pathway of internalisation of lactic acid bacteria and gut mucosal immunostimulation.

The induction of the gut mucosal immune response is dependent on the antigen interacting with the M cells of Peyer's patches and with the immune cells associated with this lymphoid organ. In previous studies we showed that the mucosal immunostimulation by LAB varied depending upon the strain being studied. Some of them increased the inflammatory immune response and others enhanced the level of secretory antibody (S-IgA). Our aim was to determine the pathway of the internalisation of LAB strains in the intestine, to provide a basis for understing the different behaviour exhibited by them. The presence of LAB on Peyer's patches or in the immune cells associated with the villi of small intestine was determined using fluorescein-labelled bacteria and by transmission electron microscopy. Mice were dosed orally by intubation, with 10¹ cells of labelled single strains of Lactobacillus casei, Lactobacillus acidophilus, Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus. For electron microscopy studies unlabelled bacteria were used. Histological preparations of the small and large intestine were performed 1 hour after administration of the bacteria. Fluorescent bacteria of L. casei and S. thermophilus were found only in Peyer's patches while L. delbrueckii ssp. bulgaricus and L. acidophilus were observed in Peyer's patches and in the small intestine. L. acidophilus was also found in the large intestine. We confirmed these findings by electron microscopy. We determined that for L. casei the pathway of internalisation was via the M and FAE cells of Peyer's patches, while for S. thermophilus, L. acidophilus and L. delbrueckii ssp. bulgaricus the interaction with the immune cells of Peyer's patches was through the follicle associated epithelium (FAE). L. delbrueckii ssp. bulgaricus and L. acidophilus also interacted with the epithelial cells of the small intestine and L. acidophilus with epithelial cells of the large intestine. These results suggest that the different effects of LAB on the mucosal immunostimulation, are related to the different pathways of gut internalisation used to take contact with the immune cells associated with the lamina propria intestinal.