Practically all surfaces of the human body exposed to the environment are normally inhabited by micro-organisms. The intestine constitutes an especially rich and diverse microbial habitat. Approximately 800–1000 different bacterial species and .7000 different strains inhabit the gastrointestinal tract. These bacteria act together in many physiological processes and also interact with human cells, including those of the immune system. The diversity of the gut microbiota is relatively simple in infants but becomes more complex with increasing age, reaching a high degree of complexity in adults. Lifestyle factors and diet can also affect the diversity and composition of the gut microbiota. Interestingly, the relative proportions of the two most dominant bacterial phyla i.e. Bacteroidetes and Firmicutes, were found to be correlated with obesity in mice and humans, respectively, but a study by Duncan et al. did not see this same correlation in obese and lean humans. Molecular analyses have also revealed that the composition of the human intestinal microbiota is host-specific and relatively stable over time. Recent metagenome sequencing data of 124 individuals suggest the existence of a common core human gut microbiome, but this core may exist more at the level of shared functional genes rather than shared taxa.