Pseudomonadota (synonym Proteobacteria) is a major phylum of Gram-negative bacteria. 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 pathogenic genera, such as Escherichia, Salmonella, Vibrio, Yersinia, Legionella, and many others. Others are free-living (nonparasitic) and include many of the bacteria responsible for nitrogen fixation. 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, a Greek god of the sea capable of assuming many different shapes (not after the Proteobacteria genus Proteus). In 2021 the International Committee on Systematics of Prokaryotes designated the synonym Pseudomonadota. Characteristics All Pseudomonadota (Proteobacteria) are diverse. They are nominally Gram-negative, although in practice some may actually stain Gram-positive or Gram-variable. Their outer membrane is mainly composed of lipopolysaccharides. Many move about using flagella, but some are nonmotile, or rely on bacterial gliding. Pseudomonadota have a wide variety of metabolism types. Most are facultatively or obligately anaerobic, chemolithoautotrophic, and heterotrophic, but numerous exceptions occur. A variety of genera, which are not closely related to each other, convert energy from light through conventional photosynthesis 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, 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, photoautotrophs, and generalist heterotrophs. The type order is the Burkholderiales, comprising an enormous range of metabolic diversity, including opportunistic pathogens. The Gammaproteobacteria are the largest class in terms of species with validly published names. The type order is the Pseudomonadales, which include the genera Pseudomonas and the nitrogen-fixing Azotobacter. The Zetaproteobacteria are iron-oxidizing neutrophilic chemolithoautotrophs, distributed worldwide in estuaries and marine habitats. The type order is the Mariprofundales. The Hydrogenophilalia are obligate thermophiles and include heterotrophs and autotrophs. The type order is the Hydrogenophilales. The Acidithiobacillia contain only sulfur, iron, and