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Pseudomonadota (synonym " Proteobacteria") is a major phylum of Gram-negative bacteria. Currently, they are considered the predominant phylum within the realm of bacteria. They are naturally found as and free-living (non-Parasitism) genera. The phylum comprises six classes Acidithiobacillia, Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Hydrogenophilia, and Zetaproteobacteria. The Pseudomonadota are widely diverse, with differences in morphology, Metabolism processes, relevance to humans, and Ecology influence.
Previously, the Pseudomonadota phylum included two additional classes, namely Deltaproteobacteria and Oligoflexia. However, further investigation into the phylogeny of these taxa through genomic marker analysis demonstrated their separation from the Pseudomonadota phylum. Deltaproteobacteria has been identified as a diverse Taxonomy unit, leading to a proposal for its reclassification into distinct phyla: Desulfobacterota (encompassing Thermodesulfobacteria), Myxococcota, and Bdellovibrionota (comprising Oligoflexia).
The class Epsilonproteobacteria was additionally identified within the Pseudomonadota phylum. This class is characterized by its significance as chemolithotrophic primary producers and its metabolic prowess in deep-sea hydrothermal vent ecosystems. Noteworthy pathogenic genera within this class include Campylobacter, Helicobacter, and Arcobacter. Analysis of phylogenetic tree topology and genetic markers revealed the direct divergence of Epsilonproteobacteria from the Pseudomonadota phylum. Limited outgroup data and low bootstrap values support these discoveries. Despite further investigations, consensus has not been reached regarding the Monophyly nature of Epsilonproteobacteria within Proteobacteria, prompting researchers to propose its taxonomic separation from the phylum. The proposed reclassification of the name Epsilonproteobacteria is Epsilonbacteraeota, later revised to Campylobacterota in 2018.
The group Pseudomonadota is defined based on ribosomal RNA (rRNA) sequencing, and are divided into several subclasses. These subclasses were regarded as such for many years, but are now treated as various classes of the phylum. These classes are monophyly. (2025). 9780387950402, Springer. ISBN 9780387950402 The genus Acidithiobacillus, part of the Gammaproteobacteria until it was transferred to class Acidithiobacillia in 2013, was previously regarded as paraphyly to the Betaproteobacteria according to Phylogenomics. In 2017, the Betaproteobacteria was subject to major revisions and the class Hydrogenophilalia was created to contain the order Hydrogenophilales
Pseudomonadota classes with validly published names include some prominent genera: e.g.:
Pseudomonadota have a wide variety of metabolism types. Most are facultative or obligate anaerobes, chemolithoautotrophs, and Heterotrophic, though numerous exceptions exist. A variety of distantly related genera within the Pseudomonadota obtain their energy from light through conventional photosynthesis or anoxygenic photosynthesis.
The Acidithiobacillia contain only sulfur, iron, and uranium-oxidizing . The type order is the Acidithiobacillaceae, which includes five different Acidithiobacillus species used in the mining industry. In particular, these microbes assist with the process of bioleaching, which involves microbes assisting in metal extraction from mining waste that typically extraction methods cannot remove.
Some Alphaproteobacteria can grow at very low levels of nutrients and have unusual morphology within their life cycles. Some form stalks to help with colonization, and form buds during cell division. Others include agriculturally important bacteria capable of inducing nitrogen fixation in symbiosis with plants. The type order is the Caulobacterales, comprising stalk-forming bacteria such as Caulobacter. The mitochondria of eukaryotes are thought to be descendants of an alphaproteobacterium.
The Betaproteobacteria are highly metabolically diverse and contain chemolithoautotrophs, , and generalist . The type order is the Burkholderiales, comprising an enormous range of metabolic diversity, including opportunistic pathogens. These pathogens are primary for both humans and animals, such as the horse pathogen Burkholderia mallei, and Burkholderia cepacia which causes respiratory tract infections in people with cystic fibrosis.
The Gammaproteobacteria are one of the largest classes in terms of genera, containing approximately 250 validly published names. The type order is the Pseudomonadales, which include the genera Pseudomonas and the nitrogen-fixing Azotobacter, along with many others. Besides being a well-known pathogenic genus, Pseudomonas is also capable of biodegradation of certain materials, like cellulose.
The Hydrogenophilalia are thermophilic chemoheterotrophs and autotrophs. The bacteria typically use hydrogen gas as an electron donor, but can also use reduced sulfuric compounds. Because of this ability, scientists have begun to use certain species of Hydrogenophilalia to remove sulfides that contaminate industrial wastewater systems. The type order is the Hydrogenophilaceae which contains the genera Thiobacillus, Petrobacter, Sulfuricella, Hydrogenophilus and Tepidiphilus. Currently, no members of this class have been identified as pathogenic.
The Zetaproteobacteria are the iron-oxidizing Neutrophile chemolithoautotrophs, distributed worldwide in estuaries and marine habitats. This group is so successful in its environment due to their microaerophilic nature. Because they require less oxygen than what is present in the atmosphere, they are able to compete with the abiotic iron(II) oxidation that is already occurring in the environment. The only confirmed type order for this class is the Mariprofundales, which does not contain any known pathogenic species.
Classes Betaproteobacteria and Gammaproteobacteria are prevalent within the human oral cavity, and are markers for good oral health. The oral microbiome consists of 11 habitats, including the tongue dorsum, hard palate, tonsils, throat, saliva, and more. Changes in the oral microbiome are due to endogenous and Exogeny factors like host lifestyle, genotype, environment, immune system, and socioeconomic status. Considering diet as a factor, high saturated fatty acid (SAF) content, achieved through poor diet, has been correlated to increased abundance of Betaproteobacteria in the oral cavity.
As briefly mentioned in previous sections, members of Pseudomonadota have vast metabolic abilities that allow them to utilize and produce a variety of compounds. Bioleaching, done by various Thiobacillus species, are a primary example of this. Any iron and sulfur oxidizing species has the potential to uncover metals and low-grade ores that conventional mining techniques were unable to extract. At present, they are most often used for recovering copper and uranium, but researchers are looking to expand this field in the future. The downside of this method is that the bacteria produce acidic byproducts that end up in acid mine drainage. Bioleaching has significant economic promise if it can be controlled and not cause any further harm to the environment.
Because this phylum are able to form a symbiotic relationship with plant roots, incorporating Pseudomonadota into agricultural practices aligns with principles of sustainable farming. These bacteria contribute to soil health and fertility, promote natural pest management, and enhance the resilience of crops to environmental stressors.
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