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Pseudomonadota (synonym Proteobacteria) is a major
phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature ...
of
Gram-negative bacteria Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...
. The renaming of phyla in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature. The phylum Proteobacteria includes a wide variety of
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...
ic genera, such as ''
Escherichia ''Escherichia'' () is a genus of Gram-negative, non- spore-forming, facultatively anaerobic, rod-shaped bacteria from the family Enterobacteriaceae. In those species which are inhabitants of the gastrointestinal tracts of warm-blooded animals, ...
'', ''
Salmonella ''Salmonella'' is a genus of rod-shaped (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two species of ''Salmonella'' are '' Salmonella enterica'' and '' Salmonella bongori''. ''S. enterica'' is the type species and is ...
'', ''
Vibrio ''Vibrio'' is a genus of Gram-negative bacteria, possessing a curved-rod (comma) shape, several species of which can cause foodborne infection, usually associated with eating undercooked seafood. Being highly salt tolerant and unable to survive ...
'', ''
Yersinia ''Yersinia'' is a genus of bacteria in the family Yersiniaceae. ''Yersinia'' species are Gram-negative, coccobacilli bacteria, a few micrometers long and fractions of a micrometer in diameter, and are facultative anaerobes. Some members of ''Yer ...
'', '' Legionella'', and many others.Slonczewski JL, Foster JW, Foster E. Microbiology: An Evolving Science 5th Ed. WW Norton & Company; 2020. Others are free-living (non
parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
) and include many of the
bacteria Bacteria (; singular: 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 am ...
responsible for
nitrogen fixation Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmo ...
. Carl Woese established this grouping in 1987, calling it informally the "purple bacteria and their relatives". Because of the great diversity of forms found in this group, it was later informally named Proteobacteria, after
Proteus In Greek mythology, Proteus (; Ancient Greek: Πρωτεύς, ''Prōteus'') is an early prophetic sea-god or god of rivers and oceanic bodies of water, one of several deities whom Homer calls the "Old Man of the Sea" ''(hálios gérôn)''. ...
, a Greek god of the sea capable of assuming many different shapes (not after the Proteobacteria genus ''
Proteus In Greek mythology, Proteus (; Ancient Greek: Πρωτεύς, ''Prōteus'') is an early prophetic sea-god or god of rivers and oceanic bodies of water, one of several deities whom Homer calls the "Old Man of the Sea" ''(hálios gérôn)''. ...
''). In 2021 the International Committee on Systematics of Prokaryotes designated the synonym Pseudomonadota.


Characteristics

All Pseudomonadota (Proteobacteria) are diverse. They are nominally
Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wa ...
, although in practice some may actually stain Gram-positive or Gram-variable. Their outer membrane is mainly composed of
lipopolysaccharide Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O- antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the out ...
s. Many move about using
flagella A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have fro ...
, but some are nonmotile, or rely on bacterial gliding. Pseudomonadota have a wide variety of
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ...
types. Most are facultatively or obligately
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...
,
chemolithoautotrophic A lithoautotroph is an organism which derives energy from reactions of reduced compounds of mineral (inorganic) origin. Two types of lithoautotrophs are distinguished by their energy source; photolithoautotrophs derive their energy from light while ...
, and
heterotrophic A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
, but numerous exceptions occur. A variety of genera, which are not closely related to each other, convert energy from light through conventional
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
or anoxygenic photosynthesis. Pseudomonadota are associated with the imbalance of the microbiotic community in the lower reproductive tract of women. These species are associated with inflammation. Some Alphaproteobacteria can grow at very low levels of nutrients and have unusual morphology such as stalks and buds. Others include agriculturally important bacteria capable of inducing nitrogen fixation in symbiosis with plants. The type order is the
Caulobacterales ''Caulobacterales'' is an order of gram-negative pseudomonadota Pseudomonadota (synonym Proteobacteria) is a major phylum of Gram-negative bacteria. The renaming of phyla in 2021 remains controversial among microbiologists, many of whom contin ...
, comprising stalk-forming bacteria such as '' Caulobacter''. The
mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...
of eukaryotes are thought to be descendants of an alphaproteobacterium. The Betaproteobacteria are highly metabolically diverse and contain
chemolithoautotroph A lithoautotroph is an organism which derives energy from reactions of reduced compounds of mineral (inorganic) origin. Two types of lithoautotrophs are distinguished by their energy source; photolithoautotrophs derive their energy from light while ...
s, photoautotrophs, and generalist
heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
s. The type order is the Burkholderiales, comprising an enormous range of metabolic diversity, including opportunistic pathogens. The
Gammaproteobacteria Gammaproteobacteria is a class of bacteria in the phylum Pseudomonadota (synonym Proteobacteria). It contains about 250 genera, which makes it the most genera-rich taxon of the Prokaryotes. Several medically, ecologically, and scientifically imp ...
are the largest class in terms of species with validly published names. The type order is the
Pseudomonadales The Pseudomonadales are an order of Pseudomonadota. A few members are pathogens, such as species of ''Pseudomonas'', ''Moraxella'', and ''Acinetobacter'', which may cause disease in humans, animals and plants. ''Pseudomonas'' The bacterial genu ...
, which include the genera ''
Pseudomonas ''Pseudomonas'' is a genus of Gram-negative, Gammaproteobacteria, belonging to the family Pseudomonadaceae and containing 191 described species. The members of the genus demonstrate a great deal of metabolic diversity and consequently are able t ...
'' and the nitrogen-fixing ''
Azotobacter ''Azotobacter'' is a genus of usually motile, oval or spherical bacteria that form thick-walled cysts (and also has hard crust) and may produce large quantities of capsular slime. They are aerobic, free-living soil microbes that play an impo ...
''. The
Zetaproteobacteria The class Zetaproteobacteria is the sixth and most recently described class of the Pseudomonadota. Zetaproteobacteria can also refer to the group of organisms assigned to this class. The Zetaproteobacteria were originally represented by a single ...
are iron-oxidizing neutrophilic chemolithoautotrophs, distributed worldwide in estuaries and marine habitats. The type order is the
Mariprofundales ''Mariprofundus ferrooxydans'' is a neutrophilic, chemolithotrophic, Gram-negative bacterium which can grow by oxidising ferrous to ferric iron. It is one of the few members of the class Zetaproteobacteria in the phylum Pseudomonadota. It is ty ...
. The
Hydrogenophilalia The class Hydrogenophilalia in the '' Bacteria'' was circumscribed in 2017 when it was demonstrated that the order '' Hydrogenophilales'' was distinct from the ''Betaproteobacteria'' on the basis of physiology, biochemistry, fatty acid profiles, ...
are obligate thermophiles and include heterotrophs and autotrophs. The type order is the
Hydrogenophilales The Hydrogenophilaceae are a family of the Hydrogenophilalia, with two genera – '' Hydrogenophilus'' and '' Tepidiphilus''. Like all ''Pseudomonadota'', they are Gram-negative. All known species are thermophilic, growing around 50 °C and ...
. The Acidithiobacillia contain only sulfur, iron, and uranium-oxidising
autotroph An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Wo ...
s. The type order is the Acidithiobacillales, which includes economically important organisms used in the mining industry such as '' Acidithiobacillus'' spp.


Taxonomy

The currently accepted taxonomy is based on the
List of Prokaryotic names with Standing in Nomenclature List of Prokaryotic names with Standing in Nomenclature (LPSN) is an online database that maintains information on the nomenclature, naming and Taxonomy (biology), taxonomy of prokaryotes, following the taxonomy requirements and rulings of the In ...
(LSPN) and the
National Center for Biotechnology Information The National Center for Biotechnology Information (NCBI) is part of the United States National Library of Medicine (NLM), a branch of the National Institutes of Health (NIH). It is approved and funded by the government of the United States. Th ...
(NCBI). The group is defined primarily in terms of
ribosomal RNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from riboso ...
(rRNA) sequences. The Pseudomonadota are divided into several classes. These were previously regarded as subclasses of the phylum, but they are now treated as classes. These classes are
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gr ...
. The genus '' Acidithiobacillus'', part of the Gammaproteobacteria until it was transferred to class Acidithiobacillia in 2013, was previously regarded as
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...
to the ''Betaproteobacteria'' according to multigenome alignment studies. In 2017, the Betaproteobacteria was subject to major revisions and the class
Hydrogenophilalia The class Hydrogenophilalia in the '' Bacteria'' was circumscribed in 2017 when it was demonstrated that the order '' Hydrogenophilales'' was distinct from the ''Betaproteobacteria'' on the basis of physiology, biochemistry, fatty acid profiles, ...
was created to contain the order
Hydrogenophilales The Hydrogenophilaceae are a family of the Hydrogenophilalia, with two genera – '' Hydrogenophilus'' and '' Tepidiphilus''. Like all ''Pseudomonadota'', they are Gram-negative. All known species are thermophilic, growing around 50 °C and ...
Pseudomonadota classes with validly published names include some prominent genera: e.g.: * Acidithiobacillia: '' Acidithiobacillus'', '' Thermithiobacillus'' * Alphaproteobacteria: '' Brucella,
Rhizobium ''Rhizobium'' is a genus of Gram-negative soil bacteria that fix nitrogen. ''Rhizobium'' species form an endosymbiotic nitrogen-fixing association with roots of (primarily) legumes and other flowering plants. The bacteria colonize plant cells ...
,
Agrobacterium ''Agrobacterium'' is a genus of Gram-negative bacteria established by H. J. Conn that uses horizontal gene transfer to cause tumors in plants. ''Agrobacterium tumefaciens'' is the most commonly studied species in this genus. ''Agrobacterium ...
, Caulobacter,
Rickettsia ''Rickettsia'' is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The term "ricke ...
,
Wolbachia ''Wolbachia'' is a genus of intracellular bacteria that infects mainly arthropod species, including a high proportion of insects, and also some nematodes. It is one of the most common parasitic microbes, and is possibly the most common reproducti ...
'', etc. * Betaproteobacteria: '' Bordetella, Ralstonia, Neisseria, Nitrosomonas'', etc. *
Gammaproteobacteria Gammaproteobacteria is a class of bacteria in the phylum Pseudomonadota (synonym Proteobacteria). It contains about 250 genera, which makes it the most genera-rich taxon of the Prokaryotes. Several medically, ecologically, and scientifically imp ...
: ''
Escherichia ''Escherichia'' () is a genus of Gram-negative, non- spore-forming, facultatively anaerobic, rod-shaped bacteria from the family Enterobacteriaceae. In those species which are inhabitants of the gastrointestinal tracts of warm-blooded animals, ...
,
Shigella ''Shigella'' is a genus of bacteria that is Gram-negative, facultative anaerobic, non-spore-forming, nonmotile, rod-shaped, and genetically closely related to '' E. coli''. The genus is named after Kiyoshi Shiga, who first discovered it in ...
,
Salmonella ''Salmonella'' is a genus of rod-shaped (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two species of ''Salmonella'' are '' Salmonella enterica'' and '' Salmonella bongori''. ''S. enterica'' is the type species and is ...
,
Yersinia ''Yersinia'' is a genus of bacteria in the family Yersiniaceae. ''Yersinia'' species are Gram-negative, coccobacilli bacteria, a few micrometers long and fractions of a micrometer in diameter, and are facultative anaerobes. Some members of ''Yer ...
, Buchnera,
Haemophilus ''Haemophilus'' is a genus of Gram-negative, pleomorphic, coccobacilli bacteria belonging to the family Pasteurellaceae. While ''Haemophilus'' bacteria are typically small coccobacilli, they are categorized as pleomorphic bacteria because of ...
,
Vibrio ''Vibrio'' is a genus of Gram-negative bacteria, possessing a curved-rod (comma) shape, several species of which can cause foodborne infection, usually associated with eating undercooked seafood. Being highly salt tolerant and unable to survive ...
,
Pseudomonas ''Pseudomonas'' is a genus of Gram-negative, Gammaproteobacteria, belonging to the family Pseudomonadaceae and containing 191 described species. The members of the genus demonstrate a great deal of metabolic diversity and consequently are able t ...
'', etc. *
Zetaproteobacteria The class Zetaproteobacteria is the sixth and most recently described class of the Pseudomonadota. Zetaproteobacteria can also refer to the group of organisms assigned to this class. The Zetaproteobacteria were originally represented by a single ...
: ''
Mariprofundus ''Mariprofundus ferrooxydans'' is a neutrophilic, chemolithotrophic, Gram-negative bacterium which can grow by oxidising ferrous to ferric iron. It is one of the few members of the class Zetaproteobacteria in the phylum Pseudomonadota. It is ty ...
''


Transformation

Transformation, a process in which genetic material passes from one bacterium to another, has been reported in at least 30 species of Pseudomonadota distributed in the classes alpha, beta, and gamma. The best-studied Pseudomonadota with respect to natural genetic transformation are the medically important human pathogens ''
Neisseria gonorrhoeae ''Neisseria gonorrhoeae'', also known as ''gonococcus'' (singular), or ''gonococci'' (plural), is a species of Gram-negative diplococci bacteria isolated by Albert Neisser in 1879. It causes the sexually transmitted genitourinary infection gon ...
'' (class beta), and ''
Haemophilus influenzae ''Haemophilus influenzae'' (formerly called Pfeiffer's bacillus or ''Bacillus influenzae'') is a Gram-negative, non-motile, coccobacillary, facultatively anaerobic, capnophilic pathogenic bacterium of the family Pasteurellaceae. The bact ...
'' (class gamma). Natural genetic transformation is a sexual process involving DNA transfer from one bacterial cell to another through the intervening medium and the integration of the donor sequence into the recipient genome. In pathogenic Pseudomonadota, transformation appears to serve as a
DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA d ...
process that protects the pathogen's DNA from attack by their host's phagocytic defenses that employ oxidative free radicals.


See also

* List of bacteria genera * List of bacterial orders


References


External links


Pseudomonadota information
from Palaeos.
Pseudomonadota.
– J. P. Euzéby: List of Prokaryotic names with Standing in Nomenclature. {{Taxonbar, from=Q130999 Gram-negative bacteria Bacteria phyla