Programming of host metabolism by the gut microbiota

The human gut harbors a vast ensemble of bacteria that has co-evolved with the human host and performs several important functions that affect our physiology and metabolism. The human gut is sterile at birth and is subsequently colonized with bacteria from the mother and the environment. The complexity of the gut microbiota is increased during childhood, and adult humans contain 150-fold more bacterial genes than human genes. Recent advances in next-generation sequencing technology and mechanistic testing in gnotobiotic mice have identified the gut microbiota as an environmental factor that contributes to obesity. Germ-free mice are protected against developing diet-induced obesity and the underlying mechanisms whereby the gut microbiota contributes to host metabolism are beginning to be clarified. The obese phenotype is associated with increased microbial fermentation and energy extraction; however, other microbially modulated mechanisms contribute to disease progression as well. The gut microbiota has profound effects on host gene expression in the enterohepatic system, including genes involved in immunity and metabolism. For example, the gut microbiota affects expression of secreted proteins in the gut, which modulate lipid metabolism in peripheral organs. In addition, the gut microbiota is also a source of proinflammatory molecules that augment adipose inflammation and macrophage recruitment by signaling through the innate immune system. TLRs (Toll-like receptors) are integral parts of the innate immune system and are expressed by both macrophages and epithelial cells. Activation of TLRs in macrophages dramatically impairs glucose homeostasis, whereas TLRs in the gut may alter the gut microbial composition that may have profound effects on host metabolism. Accordingly, reprogramming the gut microbiota, or its function, in early life may have beneficial effects on host metabolism later in life.