When the Human Genome Project revealed that human chromosomes encode a paltry 21 500 genes, far fewer than anticipated, scientists began to think outside the genome to find additional factors that could be involved in the complexities of human health and disease.
They didn't have to look far. The human microbiome, the constellation of microbes living in and on the body, harbors millions of additional genes that contribute to the well-being of their host.
Although much is known about how pathogens contribute to disease, the roles played by the more abundant beneficial and benign members of the microbiome are less well understood. Often this is because it is very difficult or impossible to culture these organisms outside of the human host. Through research in metagenomics, in which genomic analyses of entire microbial communities are carried out, and investigations into host-microbe interactions, researchers are studying the microbial contributions to health and disease and using what they are learning to develop new diagnostic and therapeutic strategies.
Friedrich MJ. Genomes of Microbes Inhabiting the Body Offer Clues to Human Health and Disease. JAMA. 2013;309(14):1447-1449. doi:10.1001/jama.2013.2824