Human microbiome - health and disease

Commensal microorganisms are known to colonize and form complex communities (microbiome) at various sites within the mammalian body. Because the human microbiome has the potential to affect so many aspects of human health, it has recently become the focus of a series of international human microbiome projects. Such studies are expected to lead to understanding of the impact of microbiota on human health and disease. Recent advances in sequencing technology have opened an entirely new arena in the research of diverse human microbiomes (the ecological community of commensal, symbiotic, and pathogenic microorganisms). In 2007 the National Institutes of Health launched the Human Microbiome Project to study the human microbiom broadly by examining at least four body sites i.e. gastrointestinal tract, the mouth, the vagina, and the skin. The primary goal of this project is to characterize the human microbiome and determine changes in the microbiome correlated to specific disease states. High-throughput sequencing is used to produce microbiome sequence data of samples from normal and diseased donors. Progress to date includes more than 1000 commensal bacteria genomes that have been completed and deposited in GenBank. In recent years, special attention has been paid to the ability of microbiota to modulate the expression of host genes. This phenomenon forms part of the "cross-talk process" that takes place between the host and its indigenous microbiota. Studies on human intestinal microbiota suggest that host epithelials cell can express specific glycoconjugates in response to the presence of bacteria. Therefore, the gut microflora is responsible for modifying potential sites for attachment. This could be a selective advantage when competing with other bacteria for a niche with limited resources. The mucosal immune system has developed specialized regulatory, anti-inflammatory mechanisms for eliminating pathogens and tolerating commensal microorganisms. Toll-like receptors mediate recognition of microbial patterns to eliminate pathogens. In contrast, commensal bacteria exploit the TLR pathway to actively suppress immunity in order to establish host-microbial symbiosis. Activating anti-inflammatory response in the host via pattern recognition receptor signaling maintains homeostasis. Moreover, it has been shown that the intestinal microbiota composition exerts an effect on the development of immune response to certain vaccine antigens. Thus microbiota along with host human cells form a complex ecosystem which, as a whole interactively performs various biological processes. Their genomes are tightly linked forming an integral part of common metagenome.