Bacteria are a large diverse group of microscopic, prokaryotic, unicellular organisms. They can be of various shapes (spherical, rod-like or spiral) and may be motile or non-motile. They include both autotrophic and heterotrophic species, and can be aerobic or anaerobic, and many species can thrive under either condition. They have relatively simple nutritional needs, and are easily adaptable and can readily change to suit their environment.