Dysbiosis of human microbiome and infectious diseases.

Since birth, the human body gets colonized by various communities of symbiotic or commensal microorganisms and they persist till the death of an individual. The human microbiome is comprised of the genomes of microorganisms such as viruses, archaea, eukaryotes, protozoa, and, most remarkably, bacteria. The development of "omics" technologies gave way to the Human Microbiome Project (HMP) which aimed at exploring the collection of microbial genes and genomes inhabiting the human body. Eubiosis, i.e., a healthy and balanced composition of such microbes contributes to the metabolic function, protection against pathogens and provides nutrients and energy to the host. Whereas, an imbalance in the diversity of microorganisms, termed dysbiosis, greatly influences the state of health and disease. This chapter summarizes the impact of gut bacteria on the well-being of humans and highlights the protective role played by the human microbiota during bacterial and viral infections. The condition of dysbiosis and how it plays a role in the establishment of various infections and metabolic disorders such as Clostridioides difficile infection (CFI), inflammatory bowel disease (IBD), cancer, periodontitis, and obesity are described in detail. Further, treatments such as fecal transplantation, probiotics, prebiotics, phage therapy, and CRISPR/Cas system, which target gut microbiota during digestive diseases are also discussed.