Bacteria is a type of biological cell. Bacterium constitute a large domain of microscopic, single-celled organisms, that has neither a distinct nucleus with a membrane, nor other specialized structures. Bacteria typically only measure a few micrometres in length, and have a number of shapes. These shapes can range from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth. Consequently, bacterium are present in most of earths’ habitats. Furthermore, bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth’s crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. For the most part not all bacteria have not been characterized, and only about 27 percent of the bacterial phyla have species that can be grown in the laboratory. Virtually all animal life on earth is dependent on bacteria for their survival (see: Wikipedia).