Review/Oorsig Volume 23, Issue 01 - Page 15

Volume 23 • Issue 01 • 2019 in gut microbiota homeostasis. It has been shown that EcN can stimulate the production of human β-defensin 2, which can protect the mucosal barrier against adhesion and invasion by pathogenic commensals [68, 69]. In addition, several in vivo and in vitro studies have shown that EcN has a protective function against Salmonella, Shigella, Candida, and some other invasive commensals and may restore damaged epithelium by modulation of tightjunction and zonula occludens proteins [70]. However, outer membrane vesicles (OMVs) released by gram- negative bacteria play a vital role in the signalling process of the intestinal gut mucosa. The release of OMVs begins a mechanism to deliver some active compounds and microbial proteins to the host body without intercellular contact. It was recently demonstrated that OMVs trigger the host immune and defense responses of the probiotic strain EcN, which entered intestinal cells via clathrin-mediated endocytosis. In fact, in vitro and ex vivo studies have demonstrated expression of antimicrobial peptides and modulation of the cytokine/chemokine response of gut epithelial and gut immune cells when the probiotic strain EcN induced OMVs. Moreover, these OMVs promote the upregulation of the tight-junction proteins of zonula occludens and claudin-14, but down-regulation of claudin-2 reduces gut permeability and supports intestinal barrier functions in intestinal epithelial cell lines [71, 72]. Finally, the probiotic strain EcN is also involved in the intestinal microbiota immune response, including macrophages, epithelial cells, dendritic cells, and upregulation of proinflammatory cytokines (IL-6, IL-8, and IL-1β) [71]. Enterococcus are gram-positive bacteria in the lactic acid bacteria family. Some strains of Enterococcus exert antibioticinduced dysbiosis and act as antitumor or anticancer agents and modulate the immune system. It has been found that culture of E. faecium strain from human intestinal epithelium increased the bactericidal effects against enteroaggregative E. coli, membrane damage, and cell lysis [73, 74]. Fusco et al. [74] characterized intestinal cytokine expression in epithelia cells and reported that intestinal cytokines play a key role in the host inflammatory response to damage by Salmonella typhimurium. It has been revealed that E. faecium increases the expression of proinflammatory and anti-inflammatory cytokines without appearing as a pathogen. Furthermore, E. hirae exerts the gut epithelial barrier function by inducing T17 [30]. Saccharomyces is well-known nonpathogenic selective probiotic yeast that has been used commercially in the production of probiotic foods. Over the past few decades, S. cerevisiae and S. boulardii have demonstrated extensive promise as a probiotic treatment [28]. Several studies have demonstrated that S. cerevisiae and S. boulardii were associated with an increased proportion of Bacteroidetes in the gut microbiota composition and decreased the relative abundance of Firmicutes and Proteobacteria. In addition, this yeast has ability to prevent infammation by promoting proinflammatory immune function and increasing the production of short-chain fatty acids [28, 29, 75, 76]. 5. Conclusions Probiotic bacteria species form a reproducible gut microbiota population in various host bodies and diseases. Various probiotic species have been reported to prevent many degenerative diseases, including obesity, diabetes, cancer, cardiovascular disease, malignancy, liver diseases, and IBD. An imbalance of the gut microbiota composition can lead to several diseases. Probiotics have been proved to modulate gut microbiota composition imbalance by increasing bacteria population, gut epithelium barrier function, and cytokine production. Meanwhile, diets and different nutrients have been reported to productively and markedly shape gut microbiota communities [77–83], further studies should be performed to elucidate the metagenomic relationship between alteration of the gut microbiota composition and probiotic species under different diets or nutrients. A well-designed and appropriate experimental model (in vivo, in vitro, or ex vivo) is suggested to provide insights into the gut microbiota composition and potential commensals for host health. Furthermore, the identification of new probiotics and isolation from microbiome and mixture of probiotic species would be a key pathway for future studies to promote host health. Abbreviations GIT: Gastrointestinal tract CVD: Cardiovascular disease LAB: Lactic acid bacteria HFD: High-fat diet LPS: Lipopolysaccharide Treg: T regulatory cell IL: Interleukin T: T helper cell DC: Dendritic cell IBD: Intestinal bowel disease WSD: Western-style diet EcN: Escherichia coli Nissle OMVs: Outer membrane vessels. References available online: 15