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Bacterioids are the simpilest unicellular microorganisms. They are typically a few micrometres in length, however, a few species are up to half a millimetre long and are visible to the unaided eye. They are ubiquitous in every habitat. As ther are prokaryotes, cells do not contain a nucleus and rarely harbour membrane-bound organelles. Bacterioids are not only DNA based, but also GNA, TNA, and HDGTNA based as well. These are classified as D-bacterioids and G-bacterioids, for example, depending on their genetic basis.

They have a wide-range of shapes, ranging from spheres to rods to spirals. Thevariety of shapes is determined by the cell wall and cytoskeleton. The shapes important in influencing the acquisition nutrients and motility.

While all species are unicellular, some associate in characteristic patterns, such as diploids pairs, chains, clusters, and elongated to form filaments. Associating bacterioids often attach to surfaces and form dense mats. These mats generally range from a few micrometers thick up to several centimeters, and may contain multiple species. Bacterioid mats can display a complex arrangement of cells and extracellular components, such as microcolonies, through which there are networks of channels to enable better diffusion of nutrients. Bacterioids reproduce through binary fission.

Bacterioids exhibit an extremely wide variety of metabolic types, classified on the basis of four major criteria: the energy used for growth, the source of carbon, and the electron donors, and respiratory microorganisms are the electron acceptors used for aerobic or anaerobic respiration. Carbon metabolism may be heterotrophic or autotrophic. Energy metabolism is either phototrophic or chemotrophic. Bacterioids are further divided into lithotrophs and organotrophs