Proteobacteria and Alphaproteobacteria Overview

Proteobacteria

• 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

Alphaproteobacteria

• 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

Betaproteobacteria

• 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

Gammaproteobacteria

• 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

Other Bacterial Groups

• 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

Firmicutes, Tenericutes, and Actinobacteria

• 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

Explore the diversity and characteristics of Proteobacteria, the largest phylum of bacteria, including its subclasses and key metabolic functions. This quiz highlights the unique features of Alphaproteobacteria and their ecological roles, such as nitrogen fixation and plant interactions. Test your knowledge on these essential microorganisms!