The human body consists of approximately 10 trillion cells. The number of microorganisms residing in our gut is perhaps 10 times that number. Bacteria comprise the majority of gut flora, somewhere between 300 and 1000 species. To some degree, the relationship between intestinal bacteria and the human body is commensal – a non-harmful co-existence – but many bacterial species are symbiotic: They perform a variety of useful functions, from training the immune system to producing vitamins to benignly occupying space that might otherwise be taken up by harmful, pathogenic bacteria.
A mouse’s gut is no different. In the image above, produced by Ivaylo Ivanov, Dan Littman, Eric Roth and Alice Liang at the New York University Skirball Institute and OCS Microscopy Core, a filament of Segmented Filamentous Bacteria interacts with an intestinal epithelial cell in the small intestine of a mouse. The interaction helps the mouse’s immune system maintain homeostasis by triggering differentiation of cells that produce cytokines. The colors are false: Orange designates mitochondria.