Flow cytometry showed that all three strains were internalized by THP-1 cells but in contrast to the M-cell translocation results, L. salivarius was internalized by THP-1 cells at a higher rate (54%) than E. coli (31%) or B. fragilis (22%; Fig. 6a). Confocal laser scanning microscopic analysis confirmed this observation, (Fig. 6b). In addition, THP-1 cells that were co-incubated with L. salivarius had significantly less production of the pro-inflammatory cytokines IL-1β, IL-6 and TNF-α (P < 0·01 and P < 0·001)

than click here THP-1 cells incubated with B. fragilis or E. coli (Fig. 6c–e). In contrast, THP-1 cells co-incubated with L. salivarius had increased production of the chemokine IL-8 compared with THP-1 cells that were co-incubated with B. fragilis or E. coli (P < 0·05; Fig. 6f). The aim of this study was twofold: (i) to assess the translocation of different commensal bacteria across M cells and (ii) to assess the capacity of M cells for immunosensory discriminatory responses to these same bacteria. Although many studies have examined the rate of translocation of pathogens, fewer studies have examined translocation of non-pathogenic commensal bacteria, which are constantly MK-8669 ic50 sampled

by M cells within the gut and may even reside in Peyer’s patches under normal physiological conditions.10,20–22 As the normal gut flora belong predominantly to two phyla; the Firmicutes and the Bacteroidetes, we chose L. salivarius and B. fragilis to represent second each of these phyla and a non-pathogenic E. coli as a second common commensal bacterium.23 This study demonstrates that these three different commensal bacteria translocate in vitro across an M-cell monolayer with varying efficiencies. An unexpected finding was that B. fragilis translocated with the greatest efficiency, as previous in vivo studies have shown that it is the least efficient commensal at translocating across

M cells to the mesenteric lymph nodes.24 This discrepancy may be accounted for in part by species differences in M-cell surface properties and function between human cells in culture and gnotobiotic mice as used in the original study. Some M-cell receptor/microbe ligand interactions have been characterized, including β1 integrin/Yersinia spp., α(2,3) sialic acid/reovirus and GP2/FimH-positive bacteria, but it is likely that many more remain to be discovered.25–28 For example, Chassaing et al.29 recently observed that the presence of long polar fimbriae enhances adherent-invasive E. coli translocation in M-cell monolayers, although the respective receptor in this instance was not identified. Microarray analysis of the C2-M cells revealed that each commensal bacterium induced different gene expression patterns in M cells, with E. coli and B. fragilis inducing the most similar gene expression changes.