Classification, Characteristics and Examples

Acidobacteria is a phylum that consists of a wide variety of organisms (bacteria) that are widely distributed worldwide. According to studies, this type is also one of the most abundant groups on earth. 

While the name suggests that members of this group are acidophiles, it's worth noting that some of the species can be found in neutral and slightly alkaline environments. 

* It's estimated that Acidobacteria make up about 52 percent of the total bacteria community in the soil and about 20 percent of all microorganisms in the soil.

* In the environment, Acidobacteria plays an important role in the carbon cycle. This is achieved through the degradation of cellulose and lignin. 


Classification of Acidobacteria

·       Kingdom/Domain: Bacteria - Acidobacteria belong to the kingdom Bacteria. As such, they are prokaryotic cells that lack a true nucleus and membrane-bound organelles.

·       Subkingdom: Negibacteria - According to a model that was proposed in 2002 by Cavalier Smith, members of the phylum Acidobacteria were placed under the subkingdom Negibacteria. This is an ancient group whose members lack the outer membrane. 

·       Phylum: Acidobacteria - As mentioned, the phylum Acidobacteria consists of a wide variety of organisms that can be found in different environments across the world (metal-contaminated soils to hot springs, etc.).

Members of the group also display a range of characteristics which has led to their division into several subgroups. 

·       Classes: Though some studies have suggested that there are over 18 classes of the phylum Acidobacteria, only three classes are well known. These are Acidobacteriia, Blastocatellia, and Holophagae (the class Holophagae is described as an Order in some literature). Characteristics of these groups will be discussed below in detail.


Examples (Subdivisions)

Based on genetic studies, researchers have been able to identify 26 subdivisions of the phylum Acidobacteria. Over time, studies have identified various characteristics of these groups allowing them to be placed within the same classes. 

Before looking at the three classes, it's important to learn about some of these subdivisions: 


Subdivision 1 - Member of subdivision 1 belong to the class Acidobacteriia. Some species within this group include Acidobacterium capsulatum and Telmatobacter bradus.

Subdivisions 2,4, and 8 - Members of these subdivisions belong to the class Holophagae. Some of the species within these subdivisions include Chloracidobacterium thermophilum, Pyrinomonas methylaliphatogenes, and Acanthopleuribacter species.

Subdivision 3 - Some species in this group include thermophilic Thermoanaerobacter species.

Subdivisions 10 and 23 - Thermotomaculum species belong to subdivision 10 while Thermoanaerobaculum belong to subdivision 23. These bacteria are heterotrophic in nature.


* Thorough studies have identified about 26 subdivisions, some of these groups still don't have valid taxonomic names and are yet to be properly classified.



Class Acidobacteriia


The class Acidobacteriia consists of several representatives that include:


Genus Acidobacterium

Members of the genus Acidobacterium are commonly found in acidic habitats. Acidobacterium capsulatum, which is one of the most popular members of the group (it consists of about 8 strains).

Like other members of this class, Acidobacterium capsulatum is a Gram-negative bacterium which means that it does not retain crystal violet stain during Gram staining. This is due to the fact that the bacterium has a thin peptidoglycan layer. 

Acidobacterium bacteria are also aerobic and therefore need oxygen to grow. Like many other bacteria, they are mesophilic in nature; growing well in moderate temperatures (between 20 and 45°C). 

Some of the other characteristics associated with these bacteria include:


·       Do not form/produce spores 

·       The majority of species are rod-shaped (elongated)

·       They break down sugars for energy (they are Saccharolytic) - Some of the sugars used as sources of energy include glucose, maltose, cellobiose, and starch, etc.

·       Capsulated

·       Move by means of flagella (peritrichous flagella)

·       Found in habitats with a pH range between 3.0 and 6.0


* The genus Silvibacterium is closely related to Acidobacterium. 


Genus Terracidiphilus

The genus Terracidiphilus consists of organisms that are known to produce extracellular enzymes used for breaking down chitin and oligosaccharides. One of the most popular species in the group is Terracidiphilus gabretensis. The bacterium is commonly found in coniferous forests where it plays an important role in carbon sink. 

Like other members of the genus, the species Terracidiphilus gabretensis is a Gram-negative bacterium that moves by means of flagella and does not produce spores. It's also rod-shaped (or ovoid) and only grows in aerobic habitats. 

As an acidophilic organism, it can be found in acidic environments with a pH range between 3 and 6. It's also a mesophile bacterium with optimal growth between 20 and 24°C.



Some of the other characteristics of these bacteria include:


·       Measure between 0.6 and 1.2 um in length (and 0.5 to 0.8 um in diameter)

·       Form colonies that are transparent and measure between 0.6 and 1.2 mm in diameter (after 7 days of growth)

·       Some of the sugars used as a source of energy include D-glucose, D-mannose, D-cellulose, Xylan, and Xylose, etc.

·       Have a G+C base composition of about 57.3 mol%

Genus Terriglobus

The genus Terriglobus is closely related to the genera Granulicella and Adaphobacter. Members of this group are aerobic chemo-organoheterotrophs commonly found in the soil.

Currently, there are 5 well-known species in this genus which include; T. roseus, T. tenax, T. albidus, T. saanensis, and T. aquaticus.

While these organisms are commonly found in soil (rhizosphere soils), they have also been detected in freshwater habitats. Unlike some of the other Acidobacteria, Terriglobus bacteria are mildly acidic and grow best at pH range between 5 and 6. They are also mesophilic, preferring temperature range between 25 and 30 degrees C.

Unlike Terracidiphilus, Terriglobus form circular colonies that may have a pink pigmentation. Apart from carbon sources (sugars for energy), they also need nitrogen for survival (obtained from amino acids and ammonium chloride). 

Some of the other characteristics associated with these bacteria include:


·       Non-motile - They do not use flagella to move like some of the other Acidobacteria 

·       Gram-negative bacteria

·       Vary in size from 0.8 to 3um in length and 0.4 to 0.9um in diameter 


Class Blastocatellia

The class Blastocatellia consists of Acidobacteria that belong to subdivision 4. They are commonly found in soil with pH ranging from neutral to slightly basic. Currently, there are only a few known groups within the class Blastocatellia. 

These include:


Genus Brevitalea


Members of the genus Brevitalea belong to the family Pyrinomonadaceae and can be found in the soil. One of the most common species in this group is Brevitalea aridisoli; first isolated from savanna soil in Namibia.

It's a Gram-negative, rod-shaped bacterium that is non-motile and non-capsulated. It's also strictly aerobic and therefore needs oxygen for growth and proliferation (cell division is through binary fission). 

Like most other Acidobacteria, Brevitalea aridisoli is a chemoorganoheterotrophic organism. As such, it depends on organic substrates (carbon sources) for growth and development. However, compared to some of the other bacteria in the class, it shows a strong preference for substrates with complex proteins as a source of energy. 

Some of the other characteristics of the bacterium include:


·       It forms soft and slimy colonies on solid media. These colonies are usually circular in shape and about 1mm in diameter. They can be smooth and shiny with whitish margins

·       Vary in size from 1.0 to 2.2 um in length - They are rod-shaped 

·       Are mesophiles and prefer temperature ranges of between 35 and 45°C

·       Unlike some of the other Acidobacteria, the bacterium can tolerate sodium chloride concentrations of about 1.0 percent

·       Preferred sources of energy include yeast extracts, cellulose, peptone, and casein hydrolysate (they do not exhibit growth in such simple sugars like glucose, fructose, mannose, and fucose etc.)


* Brevitalea deliciosa which is also a member of the genus Brevitalea exhibits many of the characteristics associated with Brevitalea aridisoli. However, there are several differences. 

Some of these differences include:


·       Ranges from 0.9 to 1.8 um in length

·       Prefers habitats with pH range of between 4.7 and 9.0

·       Optimal temperature is between 32.37 to 37.2°C

·       Tolerates sodium chloride concentration of about 0.5% 

Genus Arenimicrobium

Arenimicrobium is also a genus in the Phylum Acidobacteria that belongs to the family Pyrinomonadaceae. The bacterium shares a number of characteristics with other Acidobacteria including a rod-shaped morphology as well as reproduction through binary fission. 

Members of this genus are also Gram-negative, non-motile organisms that do not produce spores. The bacterium Arenimicrobium luteum is the most common member of the group. While it forms colonies that are similar to those of other Acidobacteria, they have been shown to be orange-yellowish in color. 

Some of the other characteristics of the bacterium include:


·       Ovoid/rod shaped - The cells range from 1.0 to 2.3 um in length 

·       Cells prefer temperature range of between 28 to 44.6 degrees C 

·       Optimal pH range of between 5.5. and 7.0 

·       It can tolerate sodium chloride of up to 0.25% 

·       Preferred sources of energy (carbon sources) include peptone, starch, casamino acids, and maltose etc. The cells do not grow in most simple sugars (glucose, fucose, erythrose, and arabinose etc.)


Class Holophagae

The class Holophagae consist of three known genera each of which belongs to subdivisions 3, 4, and 8. Members of this group are all Gram-negative bacteria. However, there are various differences between the different groups. 

Genus Holophaga

The genus Holophaga consists of only a single species; Holophaga foetida. First describe in 1994, the bacterium is a strict anaerobe. As such, can only grow in the absence or in the presence of very little oxygen concentration (less than 5 Micro M dissolved oxygen). 

Like most Acidobacteria, the bacterium is rod-shaped that measures between 1 and 3 um in length and 0.5 to 0.7 um in diameter.

Based on microscopic studies, the cells have been shown to be non-motile (the cells do not have flagella for locomotion). The optimum growth temperature is between 28 and 32°C while the optimum pH range is between 6.8 and 7.5. 

Some of the other characteristics of this bacterium include:


·       Cells can grow in salinity range of between 1 to 15 g/l

·       Bacterium can be found in black anoxic freshwater 

·       It can degrade aromatic compounds anaerobically - A unique metabolic pathway allows the bacterium to produce volatile sulfur compounds (e.g. dimethylsulfide)

Genus Geothrix

Like the genus Holophaga, the genus Geothrix also consists of a strictly anaerobic species (Geothrix fermentans). The bacterium is non-motile and has a rod-shaped morphology.

Though it does not have a flagellum (or flagella), the bacterium has hair-like structures on its surface. The cells vary in size from 2 to 3 um in length and grow optimally at 35 °C.


Some of the other characteristics associated with the bacterium include:


·       Cells may occur singly or form short chains 

·       Bacterium can reduce iron anaerobically 

·       Some of the material used as alternative electron donors include acetate, fumarate, and palmitate, etc. (Iron is the primary electron acceptor)

·       Cells can grow in organic acids like citrate


Genus Acanthopleuribacter

The third genus is known as Acanthopleuribacter and belongs to the family Acanthopleuribacteraceae. Like the other genera, this genus consist of a single species; Acanthopleuribacter pedis. Unlike the other two species, Acanthopleuribacter pedis is an obligate aerobe and thus requires oxygen for growth. 

Compared to other members of the class Holophagae, Acanthopleura pedis is a motile organism that moves by means of peritrichous flagella. The Gram-negative, rod-shaped cells measure between 2.4 and 4.7 um in length and  0.7 and 1.0 um in diameter. Optimum growth occurs at 30°C and H range of between 7 and 8.0. 

Some of the other characteristics associated with the bacterium include:


·       Cells form circular colonies which are smooth and yellow in color 

·       They exhibit catalase and oxidase activities 


Ecological Roles

Acidobacteria are involved in a number of important ecological roles that include:


Carbon cycle - By degrading various carbon sources (sugars and proteins etc.), these bacteria return carbon to the environment. This carbon is then used by plants and other organisms for various functions and the cycle continues.

Nitrogen cycle - Currently, the species Geothrix fermantans is the only organism within the Phylum Acidobacteria is known to play some role in the nitrogen cycle. This is achieved through the reduction of nitrates. 

Sulfur cycle - The bacterium Chloroacidobacterium thermophilium is an example of Acidobacteria that require sulfur for growth and development. Here, sulfur metabolism contributes to sulfur cycle.


Bacteriology as a field of study

Bacteria under a Microscope

Bacterial TransformationConjugation 

Bacteria Vs Virus

How do Bacteria cause Disease?

Bacteria - Size, Shape and Arrangement - Eubacteria

What does Phylum mean in Biology?

Return from Acidobacteria to MicroscopeMaster home


John D. Coates. (2010). Phylum XVII. Acidobacteria phyl. nov.


Miriam Gonçalves de Chaves. (2019). Acidobacteria Subgroups and Their Metabolic Potential for Carbon Degradation in Sugarcane Soil Amended With Vinasse and Nitrogen Fertilizers. 


Sadaf Kalam et al. (2020). Recent Understanding of Soil Acidobacteria and Their Ecological Significance: A Critical Review. 


Anna M. Kielak et al. (2016). The Ecology of Acidobacteria: Moving beyond Genes and Genomes.


Svetlana N Dedysh, Jaap S Sinninghe Damsté. (2018). Acidobacteria.


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