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Bacteria display many cell morphologies and arrangements

Bacteria display a wide diversity of shapes and sizes, called morphologies. Bacterial cells are about one-tenth the size of eukaryotic cells and are typically 0.5–5.0 micrometres in length. However, a few species are visible to the unaided eye — for example, Thiomargarita namibiensis is up to half a millimetre long<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> and Epulopiscium fishelsoni reaches 0.7 mm.<ref name=Williams2011>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Among the smallest bacteria are members of the genus Mycoplasma, which measure only 0.3 micrometres, as small as the largest viruses.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Some bacteria may be even smaller, but these ultramicrobacteria are not well-studied.<ref name=Velimirov2001>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Most bacterial species are either spherical, called cocci (sing. coccus, from Greek kókkos, grain, seed), or rod-shaped, called bacilli (sing. bacillus, from Latin baculus, stick). Elongation is associated with swimming.<ref>Dusenbery, David B. (2009). Living at Micro Scale, pp. 20–25. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6.</ref> Some bacteria, called vibrio, are shaped like slightly curved rods or comma-shaped; others can be spiral-shaped, called spirilla, or tightly coiled, called spirochaetes. A small number of species even have tetrahedral or cuboidal shapes.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> More recently, some bacteria were discovered deep under Earth's crust that grow as branching filamentous types with a star-shaped cross-section. The large surface area to volume ratio of this morphology may give these bacteria an advantage in nutrient-poor environments.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> This wide variety of shapes is determined by the bacterial cell wall and cytoskeleton, and is important because it can influence the ability of bacteria to acquire nutrients, attach to surfaces, swim through liquids and escape predators.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

A biofilm of thermophilic bacteria in the outflow of Mickey Hot Springs, Oregon, approximately 20 mm thick.

Many bacterial species exist simply as single cells, others associate in characteristic patterns: Neisseria form diploids (pairs), Streptococcus form chains, and Staphylococcus group together in "bunch of grapes" clusters. Bacteria can also be elongated to form filaments, for example the Actinobacteria. Filamentous bacteria are often surrounded by a sheath that contains many individual cells. Certain types, such as species of the genus Nocardia, even form complex, branched filaments, similar in appearance to fungal mycelia.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Bacteria often attach to surfaces and form dense aggregations called biofilms or bacterial mats. These films can range from a few micrometers in thickness to up to half a meter in depth, and may contain multiple species of bacteria, protists and archaea. Bacteria living in biofilms display a complex arrangement of cells and extracellular components, forming secondary structures, such as microcolonies, through which there are networks of channels to enable better diffusion of nutrients.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> In natural environments, such as soil or the surfaces of plants, the majority of bacteria are bound to surfaces in biofilms.<ref name=Davey>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Biofilms are also important in medicine, as these structures are often present during chronic bacterial infections or in infections of implanted medical devices, and bacteria protected within biofilms are much harder to kill than individual isolated bacteria.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Even more complex morphological changes are sometimes possible. For example, when starved of amino acids, Myxobacteria detect surrounding cells in a process known as quorum sensing, migrate toward each other, and aggregate to form fruiting bodies up to 500 micrometres long and containing approximately 100,000 bacterial cells.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> In these fruiting bodies, the bacteria perform separate tasks; this type of cooperation is a simple type of multicellular organisation. For example, about one in 10 cells migrate to the top of these fruiting bodies and differentiate into a specialised dormant state called myxospores, which are more resistant to drying and other adverse environmental conditions than are ordinary cells.<ref name=autogenerated1>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Bacteria sections
Intro  Etymology  Origin and early evolution  Morphology  Cellular structure  Metabolism  Growth and reproduction  Genetics  Behavior  Classification and identification  Interactions with other organisms  Significance in technology and industry  History of bacteriology  See also  References  Further reading  External links  

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