Actinobacteria
Bifidobacterium adolescentis stained by Gram's method, light microscopy
The Actinobacteria constitute one of the richest tribes within the domain of the bacteria. As a " Germanized " spelling is also found the names of Actinobacteria and Actinobacteria. Together with the Firmicutes they form the large group of gram-positive bacteria. It is a distinguishing feature that the representatives of the Actinobacteria have a rather high content of the DNA bases guanine and cytosine in their DNA. However, this supplement is not always applicable to the definition.
- 3.1 Historical Development
- 3.2 Current systematics
- 3.3 Sequenced organisms
- 5.1 Literature
- 5.2 Notes and references
Features
Appearance
The Actinobacteria are gram -positive rods. Some are filamentous and multicellular, many can form endospores. They form together with the Firmicutes the large group of gram-positive bacteria. On suitable culture media, the cells grow into visible colonies. Here, many representatives of the Actinobacteria fall on already, as their colonies do not look smooth but wrinkled.
Chemotaxonomic features
The Actinobacteria are bacteria with high GC content, ie a high proportion of the nucleic bases (guanine and cytosine) in the bacterial DNA. Specifically refers to including a GC content of more than 55 mole percent. This applies to the majority of the representatives, but not so that this definition is not enough for all.
Growth and metabolism
Actinobacteria have diverse metabolic pathways, only photosynthetically active agents are not known. Some metabolic pathways are limited to the Actinobacteria and does not occur in other prokaryotes. The metabolism of the most representative of the Actinobacteria is marked as chemoorganotroph and heterotrophic, they use organic compounds as an energy source as well as for the construction of cellular materials. They use either aerobic respiration deplete the substrates or perform a fermentation by. Accordingly, their behavior can vary the oxygen contained in the air: The representatives may strictly aerobic, facultative anaerobic, microaerophilic or strictly grow anaerobically.
The Actinobacteria are exceptionally well in terms of certain metabolites. Thus we find, among others, the representatives of the family of Mycobacteriaceae ( mycobacteria), a high proportion of lipid in the peptidoglycan of bacterial cell wall. The mycolic acids contained therein are a feature of some families within the class of Actinobacteria and cause the cell wall is hydrophobic, which is also referred to as acid-fast cell wall. And the synthesis of these lipids is unusual involved are enzyme complexes that are referred to as polyketide synthases.
A special feature of Actinobacteria is the production of Mycothiol instead of glutathione. Mycothiol is not subject to the degree of autoxidation, as is the case with glutathione. It acts as an antioxidant ( antioxidant) of the amino acid cysteine. Two cysteine residues may also form a disulfide bridge, which is found in proteins in the formation of tertiary and Quartärstrukturbeteiligt. Further Mycothiol is due to the thiol group contained as a reservoir of thiol, which helps to maintain a reducing environment within the cell, and thus to protect against oxidative stress.
The Streptomycetaceae family with the genus Streptomyces are another interesting example of the phylum Actinobacteria, since they are known for unusual metabolic pathways. They also produce a variety of secondary products, such as antibiotics and Geosmine that cause the typical earthy smell.
Occurrence and significance
Actinobacteria are widespread in the environment, soil and water are typical habitats. Among the Actinobacteria are found numerous representatives who are mining the nature of pollutants and complex compounds or play an important role as symbionts or pathogens. Examples include species of the genera Mycobacterium and Actinomyces.
System
Historical development
The systematics of the Actinobacteria or to representatives counted has changed several times. From the generic name Actinomyces, which was used by Corda in 1839 for the bacteria Actinomyces bovis in cattle discovered, the term actinomycetes, which was not clearly defined group of morphologically similar microorganisms developed. The generic names of the representatives were subject to numerous changes. The actinomycetes formerly known as actinomycetes this is, inter alia, the fact that they, even though they are bacteria that are similar in appearance forth the mushrooms. The genus name Streptomyces ( Waksman and Henrici 1943) combines the names used for the first group and actinomyces Streptothrix.
From the first not clearly defined group of actinomycetes, the order of Actinomycetales, which was described by Buchanan 1917,. In the 1950s and 1960s, many new species were discovered, which were called Sporoaktinomyzeten and Coryneforme and showed similarity with the actinomycetes. However, the known agent showed a broad phenotypic heterogeneity, so that the non-uniform classification systems for a taxonomic classification more difficult. This was improved in the 1980s by the internationally agreed classification of bacteria. Increasingly chemotaxonomic features for classification were used. While this facilitated the assignment to a genus, but led to higher taxa (such as the order or class) appeared as a heterogeneous composite group.
In the 1990s, in addition to the order Actinomycetales, the class Actinomycetes was performed, an allocation of other gram- positive bacteria with high GC content only on the basis of morphological and chemotaxonomic characteristics became increasingly difficult. The option (eg DNA sequencing ) to carry out genetic studies brought new insights for the systematics of bacteria and helped in the elucidation of the phylogeny and the family relationships of microorganisms with each other. In particular, this applies to the examination of the 16S rRNA, a typical representative for prokaryotes the ribosomal RNA in bacteria. Based on these results suggested Stackebrandt 1997, inter alia, a new classification system before. The class Actinobacteria was introduced with the Actinobacteridae as a subclass. The current system still complies with this proposal, but still the phylum Actinobacteria was of Cavalier -Smith introduced in 2002 as the parent taxon.
Current systematics
Currently (as of 2013) contains the order Actinomycetales next to the family Actinomycetaceae ( with the type genus Actinomyces ), numerous other families, such as the Nocardiaceae that Streptomycetaceae that Corynebacteriaceae or Mycobacteriaceae.
The phylum Actinobacteria contains currently (as of 2013) only one of the same class, which in turn is divided into five sub-classes. The subclasses contain different orders, sometimes even sub-orders, each of which families are then assigned. Numerous well-known representatives of the Actinobacteria are in the order Actinomycetales contain ( see there).
Class Actinobacteria
- Subclass Acidimicrobidae
- Order Acidimicrobiales
- Subordination " Acidimicrobineae "
- Family Acidimicrobiaceae
- Family Iamiaceae
- Subclass Actinobacteridae
- Order Actinomycetales (...)
- Order Bifidobacteriales
- Family Bifidobacteriaceae
- Subclass Coriobacteridae
- Order Coriobacteriales
- Subordination " Coriobacterineae "
- Family Coriobacteriaceae
- Subclass Nitriliruptoridae
- Order Euzebyales
- Family Euzebyaceae
- Order Nitriliruptorales
- Family Nitriliruptoraceae
- Subclass Rubrobacteridae
- Order Gaiellales
- Family Gaiellaceae
- Order Rubrobacterales
- Subordination " Rubrobacterineae "
- Family Rubrobacteraceae
- Order Solirubrobacterales
- Family Conexibacteraceae
- Family Patulibacteraceae
- Family Solirubrobacteraceae
- Order Thermoleophilales
- Family Thermoleophilaceae
Sequenced organisms
The already genomsequenzierten members of the Actinobacteria can be brought by examining orthologous genes in a lineage relationship. The depicted phylogenetic tree showing the relationships of Actinobacteria (as of 2010 ).
Selection of a few representative
The Acidimicrobiaceae family contains the genus Acidimicrobium, of which only the type Acidimicrobium ferrooxidans is known. It is rod-shaped bacteria that grow rather thermophilic ( optimum temperature 45-50 ° C). The GC content of the bacterial DNA is 69 mol percent. They are in accordance with the generic name of acidophilic, so preferred acidic pH levels for growth, the optimum is at a pH of 2.0. From the species name can be inferred that they are able to iron ( chemical symbol Fe, Latin ferrum ) to oxidize, in fact they oxidize using oxygen as an oxidant Fe2 ions to Fe3 ions. A. ferrooxidans was isolated, inter alia, from ferruginous mineral pyrite.
The Coriobacteriaceae family includes several genera, such as Atopobium and Coriobacterium. The type Coriobacterium glomerans was isolated from the gastrointestinal tract of Commons Firebug ( Pyrrhocoris apterus ). The cells have a pear- like shape and are strung together to form long chains. They grow under obligately anaerobic conditions do not form endospores as Überdauerungsformen and are not motile. The GC content of the bacterial DNA is 60 mol percent. Glomerans form as well as Coriobacterium Atopobium species in a fermentation lactic acid and acetic acid, they were initially erroneously attributed to the genera Streptococcus and Lactococcus. Although these are also gram- positive, but belong to the Firmicutes division. In Atopobium species is still remarkable that their GC - content in the bacterial DNA at only 35-46 mole percent, this shows that the original definition of Actinobacteria as gram-positive bacteria with a high GC content is not always true.
The Rubrobacteraceae family contains only one genus called Rubrobacter. The types Rubrobacter radiotolerans (formerly Arthrobacter radiotolerans ) and R. xylanophilus are thermophilic, they grow optimally at temperatures of 48-60 ° C. R. radiotolerans was isolated from a hot spring in Japan, where the water was previously exposed to gamma radiation. Only a radiation dose of 10,000 Gray inactivates the bacteria, while for a human cell is already sufficient a radiation dose of 5 Gray.