Bacteria are small, single-celled organisms that are ubiquitous and often free-living. They are among the first life forms to appear on Earth and are present in most of its habitats, including soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earths crust. Bacteria can also live in symbiotic and parasitic relationships with plants and animals. Most bacteria are not harmful, and some are even beneficial to humans. Bacteria can be classified into five groups based on their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios), or corkscrew (spirochaetes) . They can exist as single cells, in pairs, chains, or clusters. Bacteria are prokaryotes, which means they have no nucleus, and their control center, containing the genetic information, is contained in a single loop of DNA. Bacteria can break down organic compounds, which is useful for activities such as waste processing and cleaning up oil spills and toxic waste. The pharmaceutical and chemical industries use bacteria in the production of certain chemicals, and scientists use bacteria in molecular biology, biochemistry, and genetic research because they can grow quickly and are relatively easy to manipulate.