Proteobacteria and Alphaproteobacteria Overview

Study Notes

Largest and most diverse phylum of bacteria
Accounts for over a third of characterized bacterial species
All are gram-negative
Diverse energy-generating mechanisms (chemolithotrophic, chemoorganotrophic, phototrophic)
Six classes: Alpha, Beta, Gamma, Delta, Epsilon, and Zetaproteobacteria
Horizontal gene transfer likely influenced metabolic diversity

Second largest class of Proteobacteria (approx. 1000 described species)
Mostly obligate or facultative aerobes
Many are oligotrophic (prefer low nutrient environments)
Key orders include Rhizobiales, Rickettsiales, Rhodobacterales, Rhodospirillales, Caulobacterales, and Sphingomonadales
Includes phototrophs, chemolithotrophs, symbionts (rhizobia), free-living nitrogen-fixing bacteria, plant pathogens, and diverse chemoorganotrophs.
Rhizobia form root nodules in leguminous plants, fixing nitrogen
Agrobacterium tumefaciens causes crown gall disease in plants.
Methylobacterium are pink-pigmented, facultative methylotrophs, found on plants, soil, and water
Bartonella are intracellular human pathogens transmitted by arthropods
Pelagibacter ubique is an oligotroph, likely the most abundant bacterial species on Earth (up to 50% of bacteria in temperate seas)
Rickettsiales are obligate intracellular parasites or mutualists of animals, associated with arthropods

Third largest class of Proteobacteria (approx. 500 described species)
Diverse metabolic capabilities
Six orders: Burkholderiales, Hydrogenophilales, Methylophilales, Neisseriales, Nitrosomonadales, and Rhodocyclales
Includes diverse chemoorganotrophs, anoxygenic phototrophs, chemolithotrophs, nitrogen fixers, and pathogens
Burkholderia : metabolically versatile, some fix N2, some plant-promoting, some pathogenic
Rhodocyclales: purple nonsulfur bacteria, photoheterotrophs or photoautotrophs or by respiration. Found in illuminated anerobic environments
Zoogloea : produce gelatinous capsules promoting flocculation in wastewater treatment

Largest and most diverse class of Proteobacteria (over 1500 species, over 15 orders)
Show diversity in metabolic characteristics (phototrophic, chemoorganotrophic, chemolithotrophic): respiration or fermentation
Enterobacteriales are a key order within Gammaproteobacteria
Facultative aerobes; gram-negative; non-spore forming rods
Oxidase-negative; catalase-positive
Produce acid from glucose and reduce nitrate to nitrite
Include many important human pathogens and species of industrial importance
Fermentation patterns include mixed-acid and 2,3-butanediol

Rhodobacterales and Rhodospirillales: include purple nonsulfur bacteria, anoxygenic phototrophs, nitrogen fixers, denitrifiers, and methylotrophs
Caulobacterales: oligotrophic; strictly aerobic chemoorganotrophs; characteristic genus is Caulobacter.
Sphingomonadales: diverse; aerobic, facultatively aerobic chemoorganotrophs
Zymomonas: ferment sugars to ethanol
Neisseriales: diverse gram-negative; chemoorganotrophs including some human pathogens. Neisseria, Chromobacterium
Hydrogenophilales, Methylophilales, and Nitrosomonadales: chemolithotrophs and methylotrophs; most are obligate aerobes and many also autotrophs
Deltaproteobacteria: primarily sulfate-reducing bacteria, dissimilative iron-reducers, and bacterial predators
Epsilonproteobacteria: oxidize H2S; abundant at oxic-anoxic interfaces
Campylobacter, Helicobacter: gram-negative, oxidase and catalase positive, microaerophiles
Pseudomonas aeruginosa: Urinary and respiratory tract infections in humans
Pseudomonas syringae: well-known plant pathogen
Vibrionales: mostly aquatic; facultative aerobes
Vibrio cholerae: causes cholera
Vibrio parahaemolyticus: causes gastroenteritis

Account for nearly half of described bacteria species
Actinobacteria: filamentous soil bacteria with high GC content in DNA
Tenericutes: lack cell walls
Firmicutes: include endospore-forming bacteria, lactic acid bacteria; generally have low GC content in DNA