Actinobacteria: Morphology and Taxanomy

Deepa Gagwani, Catherine Paul
  • Author
    Deepa Gagwani

    Deepa is a postgraduate in Microbiology. She has more than 10 years experience of working in pharmaceutical industry and has taught elementary school (grades 3-5) environmental science and lifeskill for 2 years. She earned her best executive achievement during her teaching tenure.

  • Instructor
    Catherine Paul

    Catherine has taught high school science and has a master's degree in biology.

Explore the definition and characteristics of members of the actinobacteria phylum and learn about mycobacteria, streptomyces, and other types of actinobacteria. Updated: 12/10/2021

Actinobacteria

Ever wondered the reason for the distinct smell of soil when it rains? The answer is actinomycetes. But, what is actinomycetes? This bacteria is responsible for the earthy smell of the soil when it rains. Actinobacteria produces geosmin, an organic compound that gives out a distinctive odor from the soil when it rains.

Actinobacteria are gram-positive microorganisms and are considered to be the transitional forms of bacteria and fungi as they have characteristics from both organisms. They are distributed widely in several ecosystems and are mainly found in soil and aquatic niches. They can be found in extreme environments, including the cold arctic, hot springs, and salty seas. They play an important role in the ecosystem as saprophytic organisms, which means they recycle the biomaterials by decomposing the organic matter. Actinobacteria form one of the largest phyla under the domain of bacteria. This bacteria has a high guanine and cytosine content in the DNA. Actinobacteria are morphologically diverse and produce mycelia with filaments and form spores for reproduction. The mycelium splits into pieces and forms a separate mycelium. The bacteria get their name from their radiated growth. The word "aktin" comes from the Greek word which means "ray".

Actinobacteria Characteristics

There is a huge diversity of bacterial species. In order to help categorize bacteria, they can be separated into groups, such as gram-negative and gram-positive bacteria. Gram-negative bacteria have an outer membrane that encases their peptidoglycan layer in their cell wall, whereas gram-positive bacteria only have a thick peptidoglycan layer surrounding the cell. Another way that bacteria are grouped is by the ratio of guanine (G) and cytosine (C) DNA bases in their genome. Actinobacteria are typically gram-positive and are considered to have a high G and C DNA base ratio.

Actinobacteria are often found in soil and water. They are able to change their shape to adapt to their environment by extending out branches, or filaments. In fact, this group of bacteria got its name because of how their branches extend outward: Actino means radiate.' When trees grow roots, they are reaching out to find nutrients and water. These filaments serve the same purpose, allowing Actinobacteria to reach across dry gaps in the soil to access nutrients and water. And as a bonus, these branches increase the bacteria's surface area, allowing more nutrients to be absorbed.

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Actinobacteria Phylum

The taxonomy of Actinobacteria has evolved through time. According to the currently updated taxonomy, the Actinobacteria phylum is based on 16S rRNA trees. The phylum is now divided into six classes:

  • Actinobacteria
  • Rubrobacteria
  • Coriobacteria
  • Acidimicrobia
  • Thermoleophilia
  • Nitriliruptoria

The class Actinobacteria is further divided into 16 orders. This group of microorganisms is known to produce secondary metabolites like vitamins, enzymes, pigments, antibiotics, surfactants, and herbicides. The different genera of Actinobacteria include bacteria that are beneficial and harmful. Some are used for antibiotic production and have economic importance for humans as they maintain the quality of the soil. Others are known for causing a variety of diseases.

Several species of actinomycetes are found in the initial stages of composting. The "Thermomonospora" species are thermophilic actinomycetes that can tolerate high temperatures. These species have the potential to degrade waste and enhance the quality of compost, which can be used for plant growth promotion.

Mycobacteria

The family Mycobacteria is included in the order Corynebacteriales. The bacteria was discovered by Robert Koch and named it tubercle bacillus. All the species of bacteria in this order, including Mycobacteria, produce high amounts of lipids in their cell surfaces called mycolic acids, apart from the peptidoglycan and cytoplasmic membrane. The peptidoglycan and the outer waxy mycolic layer are referred to as mycomembrane. This unique cell envelope enhances the capability of the microbe to bear external stress and virulence. The pathogenic capabilities of M. tuberculosis are attributed to the mycomembrane, which allows the bacteria to change under conditions like hypoxia (poor oxygen) and nutrient deprivation.

The strains of Mycobacterium form compact and wrinkled colonies on solid media. Most mycobacteria are saprophytes. Few strains of mycobacteria are human pathogens like M. tuberculosis and M.leprae. M. tuberculosis is the causative agent of tuberculosis (TB), while M. leprae causes leprosy. The presence of mycolic acids gives them a distinct staining property referred to as acid-fastness. This unique staining property allows the identification of mycobacterium from tuberculosis lesions.

Mycobacterium tuberculosis from the sputum of tuberculosis patient and stained with acid-fast stain

Mycobacterium tuberculosis from the sputum of tuberculosis patient and stained with acid-fast stain

Tuberculosis is considered a high burden of disease. Approximately 1.5 million people died in 2020 due to TB. The causative agent "M.tuberculosis" affects the lungs and spreads when a healthy person inhales the germs from the sneezing, coughing, and spitting of TB infected patients. People with comorbid conditions like HIV are more likely to develop TB.

Streptomyces

Streptomyces is the most popular genus of the family "actinomycete." The genus Streptomyces consists of approximately 700 species. These bacteria are gram-positive. They form thread-like mycelium, which bears spores when mature. It is a diverse group of soil bacteria that produces secondary metabolites, potent antibiotics, antivirals, antifungals, and immunosuppressants. "Streptomycetes" species produce around 7600 secondary metabolites, which have made the species a primary organism for producing antibiotics. The species has been exploited for its antibiotics like streptomycin, actinomycin, streptothricin, and micromonosporin. These bacteria carry genes of resistance to their own antibiotic to survive in the soil.

The discovery of the natural secondary metabolites from "Streptomyces" species was pioneered by the microbiologist Selman Waksman. One of the first antibiotics produced from the genus, Streptomycin, was produced from Streptomyces griseus. Streptomyces grow mycelium made of many hyphae. These hyphae grow by tip extension and branch out in the soil for absorbing nutrients. When mature, the mycelium produces spores called conidiospores. Spore formation is also induced by several environmental factors like nutrient exhaustion and physiological stress. Spore formation is a dormant phase of "Streptomyces "that is used as a survival strategy.

Mycobacteria

Mycobacteria are an aerobic (requires oxygen to live) form of Actinobacteria which colonize soil and water. They are unique in that they have an outer membrane made of mycolic acids in addition to the typical peptidoglycan layer in their cell wall. Mycolic acids are distinctive to Mycobacterium and provide a waxy surface that acts as a sort of survival suit. This enables the bacteria to endure dry environments and repel antimicrobial chemicals.

Mycobacteria are a gram-positive bacteria, but because of their waxy surface, they resist staining, similar to that of gram-negative bacteria. These bacteria grow very slowly in culture due to nutrients passing slowly through the mycolic acid outer layer, yet can still be very pathogenic. Mycobacteria tuberculosis are responsible for tuberculosis, while Mycobacteria leprae cause leprosy.

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Video Transcript

Actinobacteria Characteristics

There is a huge diversity of bacterial species. In order to help categorize bacteria, they can be separated into groups, such as gram-negative and gram-positive bacteria. Gram-negative bacteria have an outer membrane that encases their peptidoglycan layer in their cell wall, whereas gram-positive bacteria only have a thick peptidoglycan layer surrounding the cell. Another way that bacteria are grouped is by the ratio of guanine (G) and cytosine (C) DNA bases in their genome. Actinobacteria are typically gram-positive and are considered to have a high G and C DNA base ratio.

Actinobacteria are often found in soil and water. They are able to change their shape to adapt to their environment by extending out branches, or filaments. In fact, this group of bacteria got its name because of how their branches extend outward: Actino means radiate.' When trees grow roots, they are reaching out to find nutrients and water. These filaments serve the same purpose, allowing Actinobacteria to reach across dry gaps in the soil to access nutrients and water. And as a bonus, these branches increase the bacteria's surface area, allowing more nutrients to be absorbed.

Mycobacteria

Mycobacteria are an aerobic (requires oxygen to live) form of Actinobacteria which colonize soil and water. They are unique in that they have an outer membrane made of mycolic acids in addition to the typical peptidoglycan layer in their cell wall. Mycolic acids are distinctive to Mycobacterium and provide a waxy surface that acts as a sort of survival suit. This enables the bacteria to endure dry environments and repel antimicrobial chemicals.

Mycobacteria are a gram-positive bacteria, but because of their waxy surface, they resist staining, similar to that of gram-negative bacteria. These bacteria grow very slowly in culture due to nutrients passing slowly through the mycolic acid outer layer, yet can still be very pathogenic. Mycobacteria tuberculosis are responsible for tuberculosis, while Mycobacteria leprae cause leprosy.

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Frequently Asked Questions

Is Actinobacteria harmful?

Not all species of bacteria under the phylum Actinobacteria are harmful. Some species like Mycobacterium tuberculosis, Mycobacterium leprae, Propionibacteria acnes, and Actinomyces israelii cause severe diseases in humans.

Where is Actinobacteria found?

Actinobacteria are commonly found in soil and aquatic ecosystems. They play a significant role in recycling biomaterials by complex decomposing polymers found in dead and decaying plant or animal material.

Are actinomycetes fungi or bacteria?

Actinomycetes share the characteristics of both bacteria and fungi. This group of bacteria forms filaments and spores, which is a common characteristic found in fungi.

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