Microorganisms include all unicellular organisms and so are extremely diverse, living in almost every habitat, with some adapted to extremes such as very hot or very cold conditions, others to high pressure, and a few, such as Deinococcus radiodurans, which are adapted to high-radiation environments. Microorganisms also make up the microbiota found in and on all multicellular organisms. There is evidence that 3.45-billion-year-old Australian rocks once contained microorganisms, the earliest direct evidence of life on Earth. Microbes are important in human culture and health in many ways, serving to ferment foods and treat sewage, and to produce fuel, enzymes, and other bioactive compounds. This low-temperature electron micrograph shows a cluster of E. coli bacteria, magnified 10,000 times.Photograph credit: Eric Erbe; colorized by Christopher Pooley