Pseudomonadota are a diverse group. Though some species may stain Gram-positive or Gram-variable in the laboratory, they are nominally
Gram-negative. Their unique
outer membrane is mainly composed of
lipopolysaccharides, which helps differentiate them from the Gram-positive species. Most Pseudomonadota are motile and move using flagella. Many move about using
flagella, but some are nonmotile, or rely on
bacterial gliding. Pseudomonadota have a wide variety of
metabolism types. Most are facultative or obligate
anaerobes,
chemolithoautotrophs, and
heterotrophs, though numerous exceptions exist. A variety of distantly related genera within the Pseudomonadota obtain their energy from light through oxygenic
photosynthesis or
anoxygenic photosynthesis. 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,
photoautotrophs, and generalist
heterotrophs. 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 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 neutrophilic chemolithoautotrophs, distributed worldwide in estuaries and marine habitats. The only confirmed type order for this class is the
Mariprofundaceae, which does not contain any known pathogenic species. ==Transformation==