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Bacteria (/ b æ k ˈ t ɪər i ə / ⓘ; sg.: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats.
Bacteriology is the study of bacteria and their relation to medicine. Bacteriology evolved from physicians needing to apply the germ theory to address the concerns relating to disease spreading in hospitals the 19th century. [5] Identification and characterizing of bacteria being associated to diseases led to advances in pathogenic bacteriology.
Bacteria function and reproduce as individual cells, but they can often aggregate in multicellular colonies. [54] Some species such as myxobacteria can aggregate into complex swarming structures, operating as multicellular groups as part of their life cycle , [ 55 ] or form clusters in bacterial colonies such as E.coli .
High G+C Gram-positive bacteria Bacillati "Bipolaricaulota" Hao et al. 2018 KB1 "Acetothermia" (OP1) "Fraserbacteria" (RIF31) Bacillati: Deinococcota: Weisburg, Giovannoni & Woes 2021 Bacillati "Margulisiibacteriota" corrig. Anantharaman et al. 2016 "Saganbacteria" Bacillati: Cyanobacteriota: Oren, Mares & Rippka 2022 Blue-green algae ...
Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. [1] Their defining characteristic is that their cell envelope consists of a thin peptidoglycan cell wall sandwiched between an inner ( cytoplasmic ) membrane and an outer ...
Bacterial phyla constitute the major lineages of the domain Bacteria.While the exact definition of a bacterial phylum is debated, a popular definition is that a bacterial phylum is a monophyletic lineage of bacteria whose 16S rRNA genes share a pairwise sequence identity of ~75% or less with those of the members of other bacterial phyla.
The most commonly accepted definition is the polyphasic species definition, which takes into account both phenotypic and genetic differences. [90] However, a quicker diagnostic ad hoc threshold to separate species is less than 70% DNA–DNA hybridisation, [ 91 ] which corresponds to less than 97% 16S DNA sequence identity. [ 92 ]
Since bacteria ferment carbohydrates in patterns characteristic of their genus and species, the detection of fermentation products is commonly used in bacterial identification. Acids , alcohols and gases are usually detected in these tests when bacteria are grown in selective liquid or solid media.