Bacteria (Listeni/bækˈtɪəriə/; singular: bacterium) constitute a large domain or kingdom of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most habitats on the planet. Bacteria inhabit soil, water, acidic hot springs, radioactive waste,[2] and the deep portions of Earth's crust. Bacteria also live in plants, animals (see symbiosis), and have flourished in manned space vehicles.[3]