The normal gut microbiota plays a crucial role in maintaining human health. It is now well established that a healthy gut flora is largely responsible for the overall health of the host. The normal human gut microbiota consists of two major phyla, Bacteroidetes and Firmicutes. Although the gut microbiota in an infant appears haphazard, it starts resembling the adult flora by the age of 3 years. However, there are temporal and spatial variations in the microbial distribution from the esophagus to the rectum throughout an individual's life. Advances in genome sequencing technologies and bioinformatics have enabled scientists to study these microorganisms and their functions in detail, both in health and disease. The normal gut microbiota performs specific functions in host nutrient metabolism, xenobiotic and drug metabolism, maintenance of the gut mucosal barrier, immunomodulation, and protection against pathogens. Several factors shape the normal gut microbiota, including the mode of delivery, diet during infancy and adulthood, and the use of antibiotics or antibiotic-like molecules. A major concern of antibiotic use is the long-term alteration of the normal healthy gut microbiota and the horizontal transfer of resistance genes that could result in a reservoir of organisms with a multidrug-resistant gene pool. The normal gut microbiota is predominantly composed of the phyla Firmicutes and Bacteroidetes, followed by Actinobacteria and Verrucomicrobia. The gut microbiota exhibits both temporal and spatial differences in distribution at the genus level and beyond. The composition of the normal gut microbiota is influenced by factors such as age, diet, and antibiotic use. The gut microbiota plays a significant role in nutrient metabolism, xenobiotic and drug metabolism, immunomodulation, and protection against pathogens. The gut microbiota also contributes to the structural development of the gut mucosa and the integrity of the gut barrier. The normal gut microbiota is affected by various factors, including age, diet, and antibiotic use. The gut microbiota is also influenced by geographic and seasonal variations in diet. The use of antibiotics can have significant short and long-term implications on the ecology of the normal gut microbiota. The normal gut microbiota is essential for maintaining the structure and function of the gastrointestinal tract. The gut microbiota is also involved in the production of antimicrobial peptides and the regulation of immune responses. The gut microbiota plays a crucial role in the development and function of the immune system. The gut microbiota is also involved in the production of vitamins and other essential nutrients. The gut microbiota is also involved in the metabolism of dietary polyphenols and other compounds. The gut microbiota is also involved in the metabolism of xenobiotics and drugs. The gut microbiota is also involved in the protection of the gut barrier and the maintenance of gut homeostasis. The gut microbiota is also involved in the modulation of the immune system and the regulation of immune responses. The gut microbiota is also involved inThe normal gut microbiota plays a crucial role in maintaining human health. It is now well established that a healthy gut flora is largely responsible for the overall health of the host. The normal human gut microbiota consists of two major phyla, Bacteroidetes and Firmicutes. Although the gut microbiota in an infant appears haphazard, it starts resembling the adult flora by the age of 3 years. However, there are temporal and spatial variations in the microbial distribution from the esophagus to the rectum throughout an individual's life. Advances in genome sequencing technologies and bioinformatics have enabled scientists to study these microorganisms and their functions in detail, both in health and disease. The normal gut microbiota performs specific functions in host nutrient metabolism, xenobiotic and drug metabolism, maintenance of the gut mucosal barrier, immunomodulation, and protection against pathogens. Several factors shape the normal gut microbiota, including the mode of delivery, diet during infancy and adulthood, and the use of antibiotics or antibiotic-like molecules. A major concern of antibiotic use is the long-term alteration of the normal healthy gut microbiota and the horizontal transfer of resistance genes that could result in a reservoir of organisms with a multidrug-resistant gene pool. The normal gut microbiota is predominantly composed of the phyla Firmicutes and Bacteroidetes, followed by Actinobacteria and Verrucomicrobia. The gut microbiota exhibits both temporal and spatial differences in distribution at the genus level and beyond. The composition of the normal gut microbiota is influenced by factors such as age, diet, and antibiotic use. The gut microbiota plays a significant role in nutrient metabolism, xenobiotic and drug metabolism, immunomodulation, and protection against pathogens. The gut microbiota also contributes to the structural development of the gut mucosa and the integrity of the gut barrier. The normal gut microbiota is affected by various factors, including age, diet, and antibiotic use. The gut microbiota is also influenced by geographic and seasonal variations in diet. The use of antibiotics can have significant short and long-term implications on the ecology of the normal gut microbiota. The normal gut microbiota is essential for maintaining the structure and function of the gastrointestinal tract. The gut microbiota is also involved in the production of antimicrobial peptides and the regulation of immune responses. The gut microbiota plays a crucial role in the development and function of the immune system. The gut microbiota is also involved in the production of vitamins and other essential nutrients. The gut microbiota is also involved in the metabolism of dietary polyphenols and other compounds. The gut microbiota is also involved in the metabolism of xenobiotics and drugs. The gut microbiota is also involved in the protection of the gut barrier and the maintenance of gut homeostasis. The gut microbiota is also involved in the modulation of the immune system and the regulation of immune responses. The gut microbiota is also involved in