The role of gut microbiome in the maintenance of host homeostasis

Recent study on intestinal microbiome irrevocably altered the view that mammalian metabolism is solely influenced by their genome. Intestinal microbiota harbor a repertoire of protein encoding genes that by far exceed the gene pool found in the host genome. This has established the importance of the gut microbiome, because part of the responsibility for host metabolic regulation is devolved to the microbial symbionts. Subtle changes in co-metabolic profiles in response to physiological perturbations or environmental factors lead to many diverse disease processes including inflammatory bowel diseases, colorectal cancer, obesity, circulatory disease, and others. In most mammals, the gut microbiome is dominated by four phyla: Firmicutes, Bacteroidetes, Actinobacteria and Proteobacteria. The host has evolved to establish many processes that sustain unresponsiveness toward the commensal bacteria while at the same time maintaining responsiveness toward pathogens. The intestinal microbiome and mucosal tissues are intertwined by multiple interactions influencing host health or disease. Microbes, in response to environmental or host cues, form highly coordinated, multi-cellular networks trough intercellular cross-species and cross-domain signaling pathways, resulting in potent expansion of adaptive response to environmental changes. Similarly, the host is constantly sampling and assessing colonizing organisms and regulates defense mechanism. Under physiological conditions, the intestinal community serves the host via several ways including maturation and regulation of intestinal immune system, energy metabolism, intestinal response to epithelial cell injury and others. Changes to the intestinal milieu influence this advantageous balance is seriously injured, as benign commensals sensing danger rapidly switch to feared pathogens and initiate a coordinated program to invade the succumbed tissues. Molecular mechanisms responsible for recognizing the intestinal microflora are diverse, including numerous pathways like Toll-like receptors (TLRs), formylated peptide receptors (FPRs), nucleotide binding oligomerization-like receptors (NODs) and others with corresponding signal transduction routs. NF-kappa B depending signaling induce the inflammatory and proapoptotic response. Gastric and mucosal mucosa is engaged, with the ability to respond to inflammatory signals via production of different mediators, i.e. TNF alpha, IL-1, Il-6, IL-8 and IL-12. Many commensal bacteria have the ability to activate anti-inflammatory responses inducing expression of target genes mediating anti-inflammatory and antiapoptotic effects i.e. IL-10 and TGF-beta. Under physiological circumstances, these host-microbiome interactions are considered to be placed at the exquisitely equilibrated state between pro-inflammatory and anti-inflammatory responses. 1. Introduction. 2. The role of the gut microbiome in etiopathogenesis of some systemic diseases. 3. Factors influencing gut microbiome composition. 4. Bidirectional relationship between gut microbiome and host-cross talk process. 5. Microbiom in the maintenance of host homeostasis. 6. Concluding remarks