Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Bacteria are microscopic single-celled prokaryotic organisms that reproduce by binary fission. Measuring only a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Despite their minute size, they are the most abundant organisms in the world, growing in nearly every environment - in soil, water, and deep in the Earth's crust, even in acidic hot springs and radioactive waste as well as in organic matter and the live bodies of plants and animals. There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water; in all, there are approximately five nonillion bacteria on Earth, forming a biomass that exceeds that of all plants and animals. Learn more about the most abundant form of life on Earth in this book.