Bacteria, unicellular (one-celled) microorganisms of the class Schizomycetes, existing either as free-living organisms or as parasites. Bacteria may be divided into 3 groups: aerobes, which require atmospheric oxygen to live; anaerobes, which cannot live when exposed to it; and facultative anaerobes, which can live with or without it. They also come in 3 main shapes: rod, round, and spiral, called bacillus, coccus, and spirillum respectively. Generally a bacterium has an exterior cell wall within which a membrane encloses the soft cytoplasm, where enzymes digest and assimilate food. The DNA in which genetic information is encoded is in a portion of the cytoplasm, but unlike that of most other cells, is not separately enclosed in a nucleus. Bacteria reproduce asexually, by fission (mitosis), with each cell dividing evenly in two. In certain bacteria DNA is also sometimes transferred between 2 cells (conjugation). Bacteria cause many different chemical reactions in their hosts. Some aid in digestion and after processes within animals, and others break down dead plant and animal material in soil to provide nutrients for new growth. Bacteria that cause disease are called pathogens.