Class γ-proteobacteria

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  • Class γ-proteobacteria
    • Order Acidithiobacillales
      • Family Acidithiobacillaceae (Acidothiobacillus)
      • Family Thermithiobacillaceae (Thermothiobacillus)
    • Order Aeromonadales
      • Family Aeromonadaceae (e.g. Aeromonas, Oceanomonas)
      • Family Succinivibrionaceae (e.g. Ruminobacter, Succinivibrio)
    • Order Alteromonadales
      • Family Alteromonadaceae (e.g. Alteromonas, Glaciecola)
      • Family Colwelliaceae (Colwellia, Thalassomonas)
      • Family Ferrimonidaceae (Ferrimonas)
      • Family Idiomarinaceae (e.g. Idomarina)
      • Family Moritellaceae (Moritella)
      • Family Pseudoalteromonadaceae (Algicola, Pseudoalteromonas)
      • Family Psychromonadaceae (Psychromonas)
      • Family Shewanellaceae (Shewanella)
    • Order Cardiobacteriales
      • Family Cardiobacteriaceae (e.g. Cardiobacter)
    • Order Chromatiales
      • Family Chromatiaceae (e.g. Chromatium, Thiocapsa, Thiocystis)
      • Family Ectothiorhodospiraceae (e.g. Ectothiorhodospira, Nitrococcus)
      • Family Halothiobacillaceae (e.g. Halobacillus, Thiovirga)
    • Order Enterobacteriales
      • Family Enterobacteriaceae (e.g. Buchnera, Escherichia, Yersina)
    • Order Legionellales 
      • Family Coxiellaceae (e.g. Coxiella, Rickettsiella)
      • Family Legionellaceae (e.g. Legionella)
    • Order Methylococcales
      • Family Methylococcaceae (e.g. Methylobacter, Methylococcus)
    • Order Oceanospirillales
      • Family Alcanivoraceae (e.g. Alcanivorax)
      • Family Hahellaceae (e.g. Hhella, Halospina)
      • Family Halomonadaceae (e.g. Chromohalobacter, Halomonas)
      • Family Oceanospirillaceae (e.g. Marinomonas, Oceanospirillum)
      • Family Oleiphilaceae (Oleophilus)
      • Family Saccharospirillaceae (Saccharospirillum
    • Order Pasteurellales
      • Family Pasteurellaceae (e.g. Actinobacillus, Haemophilus, Pasteurella)
    • Order Pseudomonadales
      • Family Moraxellaceae (e.g. Acinetobacter, Psycrobacter, Moraxella)
      • Family Pseudomonadaceae (e.g. Azotobacter, Pseudomonas)
    • Order Thiotrichales
      • Family Francisellaceae (Francisella )
      • Family Piscirickettsiaceae (e.g. Thiomicrospira, Methylophaga)
      • Family Thiotrichaceae (e.g. Beggiatoa, Thiothrix, Thiomargita)
    • Order Salinisphaerales
      • Family Salinisphaeraceae (Salinsphaera)
    • Order Vibrionales
      • Family Vibrionaceae (e.g. Photobacterium, Vibrio)
    • Order Xanthomonadales
      • Family Xanthomonadaceae (e.g. Stenotrophomonas, Xanthomonas)

About this Class


The γ-proteobacteria is a very large and diverse Class. As mentioned at the beginning of the Chapter, most of the familiar Gram-negative Bacteria are members of the proteobacteria, but even within this large and diverse Phylum, a large fraction of familiar Gram-negative organisms are specifically members of the Class γ-proteobacteria. This Class contains 15 orders, 35 recognized families, and about 250 genera.


Members of this Class span the metabolic gamut. There are obligate aerobes, facultative anaerobes, microaerophiles and obligate anaerobes; heterotrophs, chemoautotrophs and photoautotrophs; deadly pathogens, opportunistic pathogens and life-sustaining symbionts; and cryophiles, mesophiles and moderate thermophiles. Most metabolic possibilities have examples in this Class. This being said, most familiar γ-proteobacteria are heterotrophic.


Most of the organisms in this group fall into the stereotypical bacterial morphologies and sizes; rods, cocci and some spirilla. Curved rods (vibrios) are also common, and a few are filamentous. An occasional organism will fall outside of this range, an extreme example being the sulfide and nitrate oxidizing Thiomargarita, a spherical organism with cells nearly a millimeter in diameter.


The γ-proteobacteria are ubiquitous in nature, very often making up the largest fraction of the population. They are not known to exist in hyperthermophilic or strongly alkaline environments, but the γ-proteobacteria can be found almost anywhere except the fringes of the limits of life.


The Enterobacteria (enterics) are probably the best studied group of Bacteria. They are easy to isolate, grow and manipulate, very common, and are important symbionts (and pathogens) of plants and animals. Enterics are mesophilic facultatively anaerobic heterotrophs, growing respiratively in aerobic conditions and fermentatively under anaerobic conditions. Many can also grow by anaerobic respiration using nitrate as the terminal electron acceptor, producing nitrite. Motility is by peritrichous flagella; a few are non-motile. Acid and gas are commonly produced from carbohydrate fermentation. They are oxidase negative but catalase positive with few exceptions.

Most or all of the enterics are at least opportunistically pathogenic, and this group contains many well-known important pathogens; Salmonella, Shigella, Escherichia, Yersina, and Klebsiella are good examples. Plant pathogens in this group are responsible for a wide range of plant wilts and blights in commercial and wild plants.

Example: Escherichia coli

Escherichia coli : Biology, 1989 Worth Publishers, Inc., ISBN 0-87901-394-X, Laura Riley/Bruce Coleman

Certainly the most well-known and understood of Bacteria. It is generally a commensalistic inhabitant of mammalian colon. Because it is much easier to grow than are the far more abundant obligately-anaerobic Clostridium and Bacteroides, it is routinely used as an indicator of fecal contamination of food and water. It is the standard workhorse of molecular biology because it is easy to grow, safe, and readily manipulated genetically. Nevertheless, it is an opportunistic pathogen; even otherwise benign strains can cause urinary tract infections. More virulent strains cause gastroenteritis ranging from mild to life-threatening.

Example: Buchnera aphidicola

Aphid : Mary Harris, Reiman Gardens, Iowa State University

B. aphidicola is an endosymbiont of aphids. These animals are sap-sucking parasites of plants, and so their diet is rich in minerals and plant sugars, but essentially devoid of essential vitamins and amino acids. They have a unique organ, the bacteriome, consisting of about 70 bacteriocytes. These are specialized cells containing vacuoles filled with B. aphidicola. These endosymbiotic bacteria provide the insect with the vitamins and essential amino acids it needs for survival; aphids fed antibiotics stop growing and reproducing, and die prematurely. The endosymbionts are transmitted to offspring in utero, and like Wolbachia (see the above discussion of α-proteobacteria) has had a dramatic impact on the reproductive biology of the insect. Aphids are parthenogenic, producing live pregnant offspring without the need for fertilization.


These are gliding filamentous sulfur-oxidizers, commonly found in marine and freshwater sediments, and cold sulfur springs. Sulfide is oxidized first to elemental sulfur, which is stored in intracellular granules. When sulfide is depleted, these sulfur granules are oxidized to sulfate. These organisms are microaerophilic, facultative autotrophs, and most are capable of nitrogen fixation. Few have been grown in pure culture, and species identification is based on morphology.

Example: Beggiatoa alba

Beggiatoa alba and B. minimus : James W. Brown

Beggiatoa alba is found in on the surface of freshwater and marine sediments, but is most conspicuous in freshwater sulfide-rich springs, where it can form spectacular white fuzzy mats covering all submerged surfaces. B. alba is the only formally recognized species of this genus, but a wide range of species have been defined informally. The primary distinction between species is filament diameter; B. alba is 2-3μm is diameter. Unlike other filamentous colorless sulfur Bacteria, no holdfasts or sheaths are present.


These organisms are very common in most aerobic environments. They are obligately aerobic except for a few that can grow anaerobically by nitrate reduction, and are straight or slightly curved (not helical) rods with polar flagella. "Pseudo-monas", or "false unit", is an apt name - until recently, this was a huge group with species that turned out to be unrelated gamma and beta proteobacteria. The genus has recently been divided up phylogenetically, and only organisms related to the “fluorescent” Pseudomonas species remain in this genus. “Fluorescent” refers to diffusible pigments produced by these organisms that are siderophores; they bind iron with very high affinity and are used by the organism to scavenge trace quantities of this essential mineral. These organisms are common in lab distilled water, because they are experts at extracting trace amounts of nutrients from sparse environments. Some are opportunistic pathogens (P. aeruginosa is usually the proximal cause of death for cystic fibrosis and burn patients), but most are free-living oligotrophic aquatic species. This group also contains the free-living nitrogen-fixers, the Azotobacteria.

Example: Azotobacter vinelandii

Azotobacter vinlandii : Microbiology Video library,

The azotobacteria are free-living nitrogen fixers, distinguished from most species of the genus Pseudomonas only by the ability to fix nitrogen, and distinguished from the rhizobia by the fact that they do not infect plants (although some are external symbionts of roots). Like other members of the genus Azotobacter, A. vinelandii differentiates into resting spore-like microcysts in stationary phase. A. vinelandii is a common soil, freshwater and marine inhabitant, preferring slightly alkaline (pH 7.5-8.0) conditions. Unlike other motile pseudomonads, A. vinelandii has peritrichous flagella.

Purple sulfur Bacteria

The purple sulfur Bacteria (Chromatia) are all phylogenetically and metabolically much alike. They are anaerobic photosynthetizers that require sulfide for growth, and so in some ways resemble the green sulfur Bacteria (Chlorobi), and are often found in the same environment. In these environments, the purple sulfur bacteria are often found overlying the green sulfur bacteria because they require more light and less sulfide. Photosynthesis is by cyclic photophosphorylation, and reducing power (NADH) for autotrophic carbon fixation is generated by reverse electron flow using sulfide as the electron donor. Elemental sulfur or polysulfides generated by sulfide oxidation is stored in intracellular globules; when environmental sulfide is depleted, these globules are oxidized first to sulfite and then sulfate. Most purple sulfur Bacteria are also capable of heterotrophic growth in the absence of light. These organisms appear in Winogradsky columns as pastel purple blotches in the sulfide-rich anaerobic regions of the column.

Example: Chromatium vinosum

Chromatium vinosum : The Prokaryotes pp3210 fig 1b

Chromatium vinosum is a large (ca. 2 x 3-6μm) rod-shaped specie that accumulates many small sulfur globules per cell. Motile by polar flagella, and cells grow individually, not in clumps as do most other purple Bacteria of other genera. Species in this genus are usually distinguished by cell size and absorption spectra. Unlike most other purple sulfur Bacteria, C. vinosum can use hydrogen in place of sulfide as an electron donor for reverse electron flow.