A prokaryote is an organism whose biological cells lack a membrane-bounded cell nucleus. The DNA of its chromosomes occupies a nucleoid within the cytoplasm. Bacteria and archaea have this cellular organization. Both contain ribosomes that perform biological translation and a selective cell membrane; neither is a bag of unorganized chemicals. See OpenStax Biology 2e, 4.2: prokaryotic organization.
The word prokaryotic describes organization, not a single exclusive clade. Grouping bacteria and archaea by the absence of a cell nucleus is useful for comparing biological cells, but does not establish that they share a more recent ancestor with one another than either does with eukaryotes. See three-domain system and two-domain system for the different ancestry models.
Prokaryotic does not mean membrane-free. Cyanobacteria contain internal thylakoid membranes for oxygenic photosynthesis, despite lacking chloroplasts and a cell nucleus. Many prokaryotes have circular chromosomes, but circular shape and a single chromosome are not defining requirements. See OpenStax Biology 2e, 22.2: bacterial and archaeal structures.
Bacteria are a major evolutionary lineage of organisms with prokaryotic cellular organization. Escherichia coli and cyanobacteria illustrate its variety: the same broad cellular plan supports different kinds of metabolism. Most bacteria have a cell wall containing peptidoglycan; archaea do not have peptidoglycan walls. A cell wall and a cell membrane are distinct structures, and some bacteria lack a cell wall. See OpenStax Biology 2e, 22.2: bacterial envelopes.
The usefulness of recognizing bacteria extends beyond naming: it identifies a shared evolutionary background for comparing genes, proteins and cellular mechanisms. A familiar shape or the absence of a cell nucleus alone cannot distinguish bacteria from archaea; molecular comparisons, including ribosomal RNA, supply additional evidence.
A group of bacteria capable of oxidative production of acetic acid from ethanol; particular cultures can contain several species.
A historical designation used for the vinegar-forming surface growth. It should not be assumed to identify every modern vinegar organism as one species.
Historical grouping of minute moving elongated or sinuous organisms; not coextensive with the modern genus Vibrio.
The bacterial species associated with tuberculosis. Bulk lesion chemistry does not by itself establish its presence.
The bacterial cause of typhoid fever.
A bacterial species that is a natural host of Bacillus virus phi29. The host identity matters when distinguishing that bacteriophage from unrelated Escherichia coli bacteriophages.
A functional grouping of bacteria able to establish nitrogen-fixing root-nodule symbioses with compatible legumes. Compatibility and fixation effectiveness vary between partners.
A bacterial species used as a model of respiratory extracellular electron transfer, including the MtrCAB pathway.
A microbial community associated with a self-produced extracellular matrix. Local exposure, growth state and population heterogeneity can change responses to antimicrobial treatment.
The bacterium responsible for American foulbrood.
Coxiella burnetii is a specialized intracellular bacterium whose replication was achieved in host-cell-free medium. Its natural intracellular niche must therefore be distinguished from an absolute biochemical requirement for a living host in all culture conditions.
Buchnera aphidicola is an intracellular aphid symbiont associated with nutritional complementation. Its reduced genome retains functions relevant to amino-acid provision, illustrating a host-dependent mutualism rather than simply a pathogenic strategy.
Rickettsia parkeri is a spotted-fever-group intracellular bacterium used to study Sca2-dependent actin assembly. Its motility machinery should not be generalized to every rickettsial species.
Chlamydia is a bacterial genus containing intracellular species with developmental transitions between infectious and replicative forms.
Rickettsia prowazekii is an intracellular bacterium with extensive biosynthetic losses but retained respiratory and citric-acid-cycle functions. Its cytosolic niche and metabolic inventory differ from those of inclusion-resident Chlamydia.
Chlamydia trachomatis is an intracellular bacterium that replicates within a modified host-cell inclusion and alternates between infectious and replicative forms.
Pseudomonas aeruginosa is a bacterium with flexible central carbon metabolism. Its isocitrate branchpoint includes two regulated dehydrogenase routes and a glyoxylate shunt.
Alphaproteobacteria are a bacterial group implicated in the ancestry of mitochondria. This relationship concerns common descent, not descent from a named modern member of Alphaproteobacteria.
Escherichia coli, often abbreviated E. coli, is a bacterial species widely used to study DNA replication and gene expression. Its chromosome is in a nucleoid, and its ribosomes work in the cytoplasm, outside any cell nucleus. A result from one laboratory strain should not automatically be generalized to every member of the species.
The original 1997 report by Blattner and colleagues presented the K-12 MG1655 genome sequence as a circular chromosome of base pairs. See Blattner laboratory: original K-12 sequence and 1997 paper. As a bookkeeping example, one complete round of DNA replication from one such chromosome produces two daughter sequence copies totaling base pairs. This is twice the copied material, not twice the length of the reference genome; it does not specify the instantaneous DNA content of a rapidly growing E. coli cell.
Cyanobacteria are bacteria that perform oxygenic photosynthesis: light-dependent reactions release oxygen from water. Their photosynthetic machinery operates on thylakoid membranes without a chloroplast or a cell nucleus. This is a particularly useful counterexample to equating photosynthesis with being a plant. Chloroplasts descend from an ancestral cyanobacterial partner through endosymbiosis; present-day cyanobacteria are independent descendants, rather than chloroplasts waiting to become plants. See Alberts et al., Molecular Biology of the Cell: chloroplasts and cyanobacteria.
A genus of oxygenic photosynthetic bacteria. The strain PCC 6803 is a common experimental system for photosynthesis and electron-export studies.
A marine cyanobacterial genus used in experiments on protection supplied by peroxide-removing microbes.
Typical Gram-positive bacteria have a relatively thick peptidoglycan layer outside the cell membrane, without the bacterial outer membrane of typical Gram-negative bacteria. This is a representative cell envelope pattern, not a complete description of every member of bacteria.
Typical Gram-negative bacteria have a relatively thin peptidoglycan layer between the cell membrane and a bacterial outer membrane.
Bacillus megaterium is the historical species name used for strain KM in the lysozyme wall-removal experiment.
Archaea are a major evolutionary lineage distinct from bacteria. The cellular comparison here concerns the prokaryotic members of archaea; see two-domain system for broader clade usage. Their biological cells have a nucleoid rather than a membrane-bounded cell nucleus, yet aspects of DNA replication, transcription and biological translation resemble those of eukaryotes. Their characteristic lipids differ chemically from the usual bacterial membrane lipids, and their cell walls, when present, lack peptidoglycan. See OpenStax Biology 2e, 22.2: archaeal distinctions.
Some archaea perform methanogenesis, producing methane as part of their metabolism. Archaea also inhabit ordinary soils and oceans: extreme habitats do not define the lineage. See OpenStax Biology 2e, 22.1: moderate and extreme environments. Woese and Fox's 1977 analysis of ribosomal RNA distinguished the methanogenic group they called archaebacteria from typical bacteria. Their proposed three primary lines of descent were a historical interpretation, rather than the final resolution of deep phylogenetics. See Woese and Fox, 1977: Phylogenetic structure of the prokaryotic domain.
Asgard archaea are an archaeal group important in studies of eukaryotic origins. Molecular comparisons connect their genes to the nuclear lineage of eukaryotes, rather than making modern Asgard archaea the direct ancestors of modern animals or plants. See the original analysis cited under two-domain system.
