Gammaproteobacteria is a large and diverse group with an estimated 250 genera. The majority of species in this class are unicellular organisms found in terrestrial and aquatic ecosystems.
While about 50 species in the group can cause diseases in human beings, many others exist as symbionts, commensals, and free-living species in various ecological niches. Moreover, they have several applications including the treatment of wastewater and serving as a source of energy.
Some examples of species within the class Gammaproteobacteria include:
Domain: Bacteria - They have a cell wall and lack membrane-bound organelles
Class: Gammaproteobacteria - Discussed below
The class Gammaproteobacteria is divided into 14 major Orders.
The following is a highlight of these Orders and examples of species in each group:
Enterobacteriales are a large group of rod-shaped, Gram-negative bacteria in CLASS . Members of this order are facultative anaerobes and do not have spores. They are also diverse and can be found in different habitats as free-living, symbiotic, or parasitic organisms.
For instance, members of the genus Enterobacter can be found living freely in the soil or in association with plants. They can also be found on human skin (as commensals), in sewage, and intestinal tract of various animals.
Others like Yersinia pestis and Escherichia coli are parasitic and cause disease in human beings. Other species can be found in aquatic habitats or in association with various insects.
* Currently, the Order Enterobacteriales consists of over 250 species.
* The phylogeny of Enterobacteriales species is largely based on the 16S rRNA gene.
Some of the other characteristics of Enterobacteriales include:
Examples of species within this group include:
Pseudomonadales is a fairly large group of Gram-negative bacteria IN CLaa Gammaproteobacteria. Pseudomonadales species are also diverse with different morphologies. For instance, whereas some of the species are spherical (e.g. Rugamonas ruba, Azomonas, and some Rhizobacteria), others may be straight rods, slightly curved rods (e.g. Pseudomonas), or spiral in shape.
Some of the species are parasitic and cause diseases in human beings and animals (e.g. Pseudomonas syringae and some Acinetobacter species) while others like Azotobacter can be found in soil or in association with plants.
Some like Azotobacter move by means of flagella (peritrichous flagella) and form cysts in the event of unfavorable conditions. However, others like Azomonas do not form cysts and lack flagella for movement.
* Some species have been shown to contain photosynthetic pigments while others like Pseudomonas fluorescens promote the process in some plants.
Some of the other characteristics associated with pseudomonadales include:
Example of Pseudomonadales species include:
Alteromonadales is a large group with over 400 species. Like many other Gammaproteobacteria, Alteromonadales are rod-shaped (straight or slightly curved), Gram-negative species many of which move by means of a single flagellum.
A great majority of species are marine organisms and can be found in various marine and coastal niches. However, others can be found in freshwater, sandy sediments, and various terrestrial habitats.
With regard to nutrition, many of the species have been shown to be chemoheterotrophs. As such, they obtain energy from a variety of organic materials in their surrounding (anaerobically or aerobically).
Whereas some of the members are facultative anaerobes (can survive in the presence or absence of oxygen, E.g. Shewanella), others are strict aerobes and need oxygen for growth.
Here, however, it's worth noting that growth by fermentation is not common even in anaerobic conditions. While the majority of species are aquatic organisms, some of the species can be found in extreme habitats.
For instance, Colwellia and Shewanella species (normal flora of fish) can be found in niches with very low temperatures (Psychrophile) and high pressure (piezophilic).
Some of the other characteristics of Alteromonadales include:
Some example of Alteromonadales species in class Gammaproteobacteria include:
Members of the Order Vibrionales in class Gammaproteobacteria are Gram-negative species characterized by a rod-shaped morphology. Here, most of the species are curved while a few are straight or slightly spiral. They move by means of a polar flagellum. However, a few species have been shown to produce lateral flagella under some conditions (in culture).
The majority of Vibrionales species are aquatic organisms where they exist as pathogens. In these environments, they have been shown to infect and cause diseases to many invertebrates. However, some of the species (E.g. bioluminescent species) form a mutually beneficial relationship with various sea animals. On land, some (e.g. some Vibrio and Proteus species) are pathogens of human beings and animals.
Some of the other characteristics associated with Vibrionales include:
Example of Vibrionales species include:
The Order Pasteurellales in class Gammaproteobacteria is made up of a single family (Pasteurellaceae). Members of this group are Gram-negative and exist as commensals or potential parasites of many birds and animals (vertebrates). Depending on the species, they vary from rod-shaped (e.g. Actinobacillus) to ovoid and coccibacilli (e.g. Haemophilus and Pasteurella species).
Many of the species are non-flagellated and thus immotile. They are also facultative anaerobes and therefore can survive in the presence or absence of oxygen. As mentioned, the majority of species in this group exist as pathogens, potential pathogens, or commensals. As such, they are commonly found living in their respective hosts.
For instance, some Pasteurella species, commonly associated with animals like cats and dogs, can cause skin disease (cellulitis) in human beings. The bacterium Pasteurella multocida is part of the normal flora in the digestive and respiratory tract of various animals.
Some of the other characteristics of Pasteurellales include:
Examples of species in this group include:
Members of the Order Chromatiales in class Gammaproteobacteria are Gram-negative bacteria that exhibit significant diversity in morphology and lifestyle. Whereas some are spherical, others are rod-shaped while some of the species appear vibroid. For instance, members of the genus Chromatium are generally rod-shaped (slightly curved or straight) while Ectothiorhodospira species are spiral-shaped.
With regard to motility, some of the species can move by means of polar flagella (E.g. Chromaium species) while others like Thiohalocapsa are non-motile. The majority of Chromatiales are Purple sulfur bacteria and are capable of photosynthesis.
They can be found in various anoxic , anaerobic and microaerobic conditions, including hot springs, intermedial zones, and microbial mats where they use sulfide for photosynthesis (sulfide acts as the electron donor).
Other characteristics of Chromatiales include:
Some species of the Order Chromatiales include:
The order Xanthomonadales in class Gammaproteobacteria is a relatively large group that consists of Gram-negative rods. Many of the species grow in areas with moderate temperatures and can be found in both terrestrial and aquatic environments.
Here, however, some of the species (especially members of the genera Xylella and Xanthomonas) are phytopathogens and infect a variety of crops including banana, rice, almond, maple, tomatoes, and mulberry, etc.
In human beings, members of the genus Stenotrophomonas are opportunistic pathogens and can cause cystic fibrosis, especially among immunocompromised patients.
* Many of the species form colonies that are yellowish in color.
Other characteristics of Xanthomonadales species include:
Examples of species in the Order Xanthomonadales include:
* The bacterium Thiomargarita namibiensis belongs to the Thiotrichales. It's one of the largest bacteria on earth that can grow to over 0.3 mm in diameter.
The Order Thiotrichales in class Gammaproteobacteria consists of a number of species distributed within the families Francisellaceae, Thiotrichaceae, and Piscirickettsiaceae. They are Gram-negative bacteria with variations in size and shape between different species.
Whereas Francisella species range from small rods to coccibacilli, many Piscirickettsiaceae (e.g. members of the genus Cycloclasticus) are rod-shaped. However, some of the species may appear ovoid under the microscope.
Some of the species in the Order can move by gliding or by means of flagella (E.g. Thiospira species) while others are non-motile (E.g. Francisella species).
Many Thiotrichales are aerobic and therefore need oxygen for growth. They can be found in land and aquatic environments where they exist as free-living, symbionts, or parasites of various animals.
Various Piscirickettsiaceae species are parasites of fish and cause disease in Salmon while Francisellaceae species like F. tularensis cause disease in mammals.
As well, Thiothrix, in the family Thiotrichaceae can be found living freely in wastewater and water containing sulfur while some of the species live harmoniously with different organisms in their surroundings.
Examples of Thiotrichales species include:
Legionellales are rod-shaped Gram-negative bacteria many of which are facultative intracellular parasites. In the environment, some of the species can be found living freely in soil and sediments and some water systems. Here, however, they can be found in association with amoeba.
In some organisms (e.g. ticks), species like Coxiella burnetii have been shown to live as endosymbionts. However, they cause disease in human beings (Q fever) when they are transmitted from insects. Others, in the genus Legionella are responsible for pneumonia and have been shown to cause large outbreaks.
Some of the other characteristics associated with Legionellales include:
Examples of Legionellales species in class Gammaproteobacteria:
As the name suggests, members of the Order Methylococcales (methanotrophs) are methanotrophs. As such, they metabolize methane and play an important role in the global carbon cycle. They vary from cocci to rod-shaped and exist singly or in pairs. In rare cases, however, some species form short chains or exist in groups of four (tetrads).
Like many other Gammaproteobacteria, methanotrophs are Gram-negative bacteria. However, unlike many, they have a cyst-like stage which allows them to survive harsh environmental conditions.
Methylococcales can be found in a number of habitats in terrestrial and aquatic environments. These include the soil, marine sediments, groundwater, and natural gas sewage sludge, etc. They are also catalase and oxidase-positive and strictly aerobic.
Example of species in this group include:
The order Oceanospirillales in class Gammaproteobacteria consists of several families including Oleiphilaceae, Hahellaceae, Oceanospirillaceae and Halomonadaceae among a few others. They are all Gram-negative bacteria with variations in shape from rod-shaped to ovoid.
Many species move by means of polar flagella and live in aerobic environments (some are facultatively anaerobic). As heterotrophs, they survive on organic material obtained from their surroundings.
Some species in the Order Oceanospirillales include:
Acidithiobacillales are Gram-negative rods found in acidic and moderately thermophilic niches. In nature, they can be found in a number of habitats including acid mine drainage, coal waste, and thermal sulfidic spring water, etc.
In these habitats, they oxidize iron or sulfur and thus live a chemoautotrophic lifestyle.
Some examples of species in the Order Acidithiobacillales in class Gammaproteobacteria include:
Cardiobacteriales are Gram-negative rods that measure between 1 and 6 um in length. Some of the species are facultative anaerobes while others are strict anaerobes that only grow in the absence or presence of very little oxygen. They are also chemoorganotrophs and therefore need organic material to survive.
While some Cardiobacterium species are part of the normal microbiota in the mouth and throat, others like Dichelobacter species are pathogen and can be found in ovine.
Example of species in the Order Cardiobacteriales in class Gammaproteobacteria include:
Aeromonadales species are Gram-negative rods some of which move by means of a polar flagellum. Others like Tolumonas species are nonmotile. They are commonly found in aquatic environments including freshwater and brackish water.
Some of the species (e.g. members of the genus Aeromonas) can be found in dairy products and meat while others exist as parasites of vertebrate and invertebrate animals (snails, human beings, amphibians, etc.).
Depending on the species, they may exist as strict aerobes or facultative anaerobes (capable of surviving in the presence or absence of oxygen).
Example of species in this group include:
Beile Gao, Ritu Mohan and Radhey S. Gupta. (2009). Phylogenomics and protein signatures elucidating the evolutionary relationships among the Gammaproteobacteria.
Geert Huys. (2014). The Prokaryotes: The Family Aeromonadaceae.
Hafiz Sohail Naushad and Radhey S. Gupta. (2013). Phylogenomics and Molecular Signatures for Species from the Plant Pathogen-Containing Order Xanthomonadales.
Mobolaji Adeolu, Seema Alnajar, Sohail Naushad, and Radhey S. Gupta. (2016). Genome-based phylogeny and taxonomy of the ‘Enterobacteriales’: proposal for Enterobacterales ord. nov. divided into the families Enterobacteriaceae, Erwiniaceae fam. nov., Pectobacteriaceae fam. nov., Yersiniaceae fam. nov., Hafniaceae fam. nov., Morganellaceae fam. nov., and Budviciaceae fam. nov. Free.