Bacteria are unicellular microorganisms. They come in many shapes (spheres, rods, spirals). Bacteria live in every habitat on Earth. There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water. Bacteria are vital in recycling nutrients, and many important steps in nutrient cycles depend on bacteria, such as the fixation of nitrogen from the atmosphere. However, most of bacteria have not been cultivated or described.