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. 2013 Sep 1;121(9):A276–A281. doi: 10.1289/ehp.121-a276

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False-color scanning electron micrograph shows the surface of the colon mucosa with pink clusters of rod-shaped bacteria, possibly Escherichia coli, attached. The genomes of the bacteria and viruses of the human gut alone are thought to encode 3.3 million genes, which could supplement the human genome in determining how the body functions.

© P.M. Motta & F. Carpino/Sapienza University of Rome/Science Source