The gut is sterile at birth, but is rapidly colonised by faecal and vaginal bacteria of maternal origin. Over the succeeding weeks, months and years, a complex microbiota develops that plays a major role in host physiology. While the digestive tract is colonised to varying degrees by micro-organisms throughout its length, due to acid pH and the short retention time of gastric contents, bacterial numbers in the stomach are usually low. The rapid passage of digestive materials through the upper gut does not provide time for significant bacterial growth to occur, but cell numbers increase considerably in the distal ileum. The rate of movement of intestinal contents slows in the colon, which facilitates the development of complex bacterial communities. The large intestine is an intricate ecosystem that contains a complex microbiota composed of several hundred different types of bacteria. The growth and metabolism of microbial communities in the large intestine are determined by many factors, such as diet, environment and host physiological processes, as well as the anatomic structure of the digestive tract, disease, immunity, host genetics, drugs and ageing. Modifications in diet and host immune system activity, as well as physiological changes in the digestive tract affect microbiota composition in older people. The elderly have fewer bifidobacteria and higher numbers of enterobacteria and clostridia than young adults. Increased antibiotic use in older people and simply going into hospital have been shown to change bacterial community structure in the colonic microbiota, although the metabolic significance of this is unclear.