Actinomycetes are an extensive taxonomic group of gram-positive microorganisms capable of forming branching filaments resembling mushroom mycelium. In modern microbiology, actinomycetes are called actinobacteria (Actinobacteria). This taxon includes 130 genera of bacteria, combined on the basis of a high content of GC pairs (more than 55%) and the similarity of the 16-s-rRNA gene sequences.
Previously, actinomycetes were mistakenly considered lower fungi, then bacteria with elements of fungal morphology, and now even microorganisms that do not form mycelial hyphae at all are included in this group. This is due to the application of a phylogenetic approach to bacteriological taxonomy.
general characteristics
Actinobacteria are very diverse both physiologically and morphologically. The following bacterial forms are found in this group:
- Cocci.
- Sticks.
- Branching threads.
- Developed mycelium.
All actinomycetes are gram-positive and do not form endospores. However, the presence of exospores is their very common feature. Most representatives of this group are aerobes, but obligate and optional anaerobes are also found.
Actinobacteria populate various habitats and use almost all variants of energy and constructive metabolism for life. This taxonomic group contains representatives of all types of bacterial nutrition, except phototrophic.
Phenotypically actinobacteria are divided into 2 groups:
- Lower actinomycetes - do not form a spore. This group includes single bacteria of the mycelial and non-mycelial structure.
- Higher actinomycetes - form exospores and include only mycelial forms (both unicellular and multicellular).
The habitat for most actinobacteria is soil, as well as freshwater and marine silts. There are representatives living in extreme conditions with a high salt content or high temperatures (halophiles, thermophiles).
The habitat of some actinomycetes is the organism of animals and humans. Such representatives can be pathogenic to humans and therefore are the subject of medical microbiology. Actinomycetes can cause the following diseases:
- Endocarditis.
- Pharyngitis.
- Periodontal disease.
- Pulmonary nocardiosis.
- Actinomycetosis.
- Tuberculosis.
- Leprosy.
Among actinomycetes, plant pathogens are also found, for example, the genus Streptomyces, which causes potato scab. Some representatives are symbiotic bacteria.
In environmental microbiology, actinomycetes are considered as the most important participants in the cycle of substances. Most of this function is performed by actinomycetes that live in the soil.
Features of the structure of mycelial forms
Mycelium of actinomycetes form hyphae. These are thin branching filaments that lengthen as a result of apical (apical) growth. Hyphae contain a large number of nucleoids.
In the process of growth and branching of hyphae, cell division does not occur, however, septa may occur. Depending on this, the mycelium is divided into 2 types:
- Unsepted (has no partitions).
- Septic - contains partitions, which are usually located in the transverse direction.
The mycelium can be either stable or periodically fragmented into sticks or coca, as in the genus Nocardia. Branching of thin filaments (hyphae) in different groups of actinomycetes is expressed to varying degrees. Actinomycetes are capable of forming both substrate and aerial mycelium.
Distinctive properties of actinomycetes
In addition to the ability to form mycelium, actinomycetes are characterized by the following features:
- Lipophilic cell wall, allowing to tolerate drying well.
- Possibility of growth in the air.
Actinomycetes are bacteria with a very high producing ability. They are capable of producing a huge amount of biologically active compounds, including antibiotics. For this reason, the microbiology of actinomycetes occupies a special place in biotechnology. Most natural antibiotics were isolated from this group of microorganisms.
The ability to form thin filaments is not a unique sign of actinomycetes, since some representatives of other groups of microorganisms have the same structure. For example, the genus Hifomicrobium, belonging to the phylum of proteobacteria.
Variety of food types
Among actinomycetes, the following types of bacterial nutrition are found :
- Chemorganotrophs.
- Hemolithotrophs.
- Autotrophs.
- Heterotrophs for carbohydrate or nitrogen (or their combination).
Among chemoheterotrophs, both an aerobic type of metabolism and anaerobic type are found. A variety of compounds, including polymers, can serve as energy sources.
There is a variant of phenotypic classification in which the division of actinobacteria into groups is based on the types of nutrition. Among the lower actinomycetes, there are:
- Chemolithoautotrophs, oxidizing sulfur with oxygen.
- Chemorganorganoterotrophs characterized by obligate aerobic respiration, forming prostrates and multiplying by budding.
- Obligatory aerobic chemoorganeterotrophs - single cocci or rods.
- Chemorganorgan heterotrophs with obligate fermentation.
- Chemoorganeterotrophs with a respiratory or fermentation type of metabolism.
- Coryneform bacteria - they are characterized by aerobic respiration, as well as anaerobic fumarate respiration and fermentation.
- Mycobacteria - aerobic chemoorganeterotrophs.
Among the highest actinomycetes, practically the same groups are distinguished as in the classical phenotypic classification.
Life cycle
Reproduction of mycelial actinomycetes can occur in three ways:
- Fragmentation of gifs.
- Differentiation of the dispute.
- By budding.
Spores are cysts with a thickened cell wall, surrounded by a hydrophobic cover. They can be both mobile and motionless.
Phylogenetic classification of actinobacteria
Based on the analysis of the similarity of the 16-s-rRNA gene, 130 genera of actinobacteria phylum are combined into one class - Actinobacteria, in which 5 subclasses, 6 orders and 10 orders are distinguished.
The first four subclasses diverged greatly and contained only one genus. The fifth subclass, called Actinobacteridae, is divided into two orders:
- Actinomycetales (actinomycetes).
- Bifidobacteriales (Bifidobacteria).
The order of bifidobacteria includes the family of the same name with 3 genera, and the order of actinomycetes is divided into 10 orders:
- Actinomycinae.
- Micrococcineae.
- Corinebacterine
- Micromonosporineae.
- Propionibacterineae.
- Pseudonocardineae.
- Streptomycineae.
- Streptosporangineae.
- Francineae.
- Glycomycineae.
Many of these groups correspond to the phenotypic classification of actinobacteria. The latter subdivides actinomycetes on the basis of a combination of morphological, cultural, physiological and biochemical characters. Phenotypic groups of actinomycetes are described in the second volume of the determinant of the bacteria Bergey (2001).
Nocardial Actinomycetes
These include actinobacteria, the mycelium of which is periodically fragmented into sticks or cocci. Gram-positive aerobes do not form real spores. Able to form conidia. The percentage of HC pairs in this group varies from 63 to 79. A typical representative is the genus Nocardia.
Actinomycetes with multicellular sporangia
In such microorganisms, hyphae are divided in both directions (longitudinal and transverse), which leads to the formation of multicellular sporangia. Two genera are characterized by the absence of filamentous mycelium.
All representatives are chemoorganotrophs and mesophiles. Typical habitats are soil, water, and mammalian skin.
Actinoplanets
Actinoplanets adapted to life in the aquatic environment. They are characterized by the presence of a mobile stage in the life cycle. Mycelium form septic (mainly substrate, less often - air). The presence of sporangia is characteristic of all representatives except the genus Micromonospora.
The habitat for actinoplanets is fresh water, soil, and dead organic remains.
Streptomycetes
They are characterized by stable and well-developed aerial mycelium, which does not undergo fragmentation. The type genus Streptomyces includes approximately 500 species.
Maduroomycetes
This group is characterized by the formation of developed substrate mycelium, however, spores form only on air hyphae. Spores form short chains or are in sporangia.
Thermomonospores
Representatives of this group of actinomycetes are distinguished by their ability to grow at temperatures from 40 to 48 degrees. They are characterized by aerial mycelium with moving or motionless spores. A typical representative is the genus Thermomonospora.