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Biological classification is the method of grouping organisms into different categories in a hierarchical order based on their similarities and differences. Organisms are classified to make their study easier and more efficient. Over the years various changes have taken place in the system of classification as new characters are discovered and given priority. A few systems of classification are as follows:

  • The first scientific classification of organisms was done by Aristotle which is as follows
    Animals were divided based on whether they had RBCs or not into Anima (without RBC) and Enaima (with RBC). Plants were divided into Trees, Herbs, and Shrubs.
  • Two kingdom classification was proposed by Carolus Linnaeus and includes kingdoms Animalia and Plantae
  • Three kingdom classification of Ernst Haeckel introduced Protista which is a group of unicellular eukaryotic organisms in the previous system of classification (Animalia Plantae Protista)
  • Four kingdom classification was introduced by Copeland and he added the kingdom Monera which includes all the prokaryotic organisms.
    (Animalia, Plantae, Protista,and Monera)
  • Five kingdom classification was proposed by R.H. Whittaker in 1969 and introduced the kingdom Fungi
    (Animalia, Plantae, Protista, Monera, and Fungi)

Six kingdom classification was given by Carl Woese. He divided kingdom Monera further into two kingdoms, Archaebacteria and Eubacteria based on differences in the structure of their cell wall and membrane.


Monera kingdom includes the simplest prokaryotic organisms. They exhibit a diverse range of life processes in spite of possessing a very simple structure. Being prokaryotic, they lack a nuclear membrane and membrane-bound organelles. However, they possess nonmembrane-bound organelles like ribosomes (70S type). They can be autotrophic, heterotrophic, saprophytic, symbiotic, and parasitic. They are divided into two phyla: archaebacteria and Eubacteria.


They are found in extreme conditions due to the unique nature of their wall and are further of three types:

  1. Methanogens: They are found in marshy areas and in the guts of ruminants.
  2. Halophiles: They are found in extreme saline areas.
  3. Thermoacidophiles: They are found in extremely hot and acidic conditions.


They are aka ‘True bacteria’. Eubacteria are prokaryotic organisms (i.e. lacking a membrane-bound nucleus), predominantly unicellular and single cellular DNA chromosomes. They have peptidoglycan in their cell wall and usually have flagella if they are motile. It is one of the three domains in the three-domain system of classification proposed by Woese. They can be found in a variety of environments around the world.  Almost all kinds of bacteria fall under this, except archaebacteria. 


  • True bacteria is also the name given to eubacteria. These are prokaryotic organisms that are single-celled.
  • Eubacteria have stiff cell walls due to peptidoglycans.
  • For locomotion they have flagella.
  • The majority of Eubacteria are heterotrophic. However, a few are photosynthetic or chemosynthetic.
  • Some bacteria have pili, which are small appendages present on the surface of the cell which assists in sexual reproduction. Pili also helps in the attachment of pathogens to their hosts.
  • The size of these bacteria ranges from 0.2 to 50 micrometers.
  • Depending on the type of cell wall and the gram stain they take, they are classified as gram-positive or gram-negative. The gram-negative bacteria do not take the gram stain and are harmful to humans. Whereas the gram-positive bacteria take up the gram stain and are beneficial to human health.

Structure of Eubacteria

The structure of the eubacterial cell can be described as follows:



The outermost layer of the cell is the cell wall which is made up of peptidoglycan. In some bacteria, additionally, the capsule is present which is made up of sugars and proteins. Many eubacteria have a cellular appendage called flagella that allows them to move through rotational motion. These flagella are made up of flagellin proteins and extend out from the cell wall. The base of the flagella is fixed within the cell membrane. The cell wall is followed by the cell membrane and it is made up of a bilayer of phospholipids, proteins, and sugars. Plasma membrane is a selectively permeable i.e not all particles cross the membrane. Enclosed within the cell membrane is cytoplasm in which cellular contents are present. Eubacteria lack a well-developed nucleus and other membrane-bound organelles. DNA is present as the naked and coiled structure in the cytoplasm, such a structure is called the nucleoid. Eubacteria also have extrachromosomal circular DNA called plasmids which give them resistance to antibiotics and also provide pathogenicity. In the cytoplasm, enzymes and proteins are present to carry out metabolic activities. Sometimes, invaginations of the cell membrane are seen in the cytoplasm called mesosomes. These contain photosynthetic and respiratory enzymes.

Types of Eubacteria

On the basis of shape they are of four types:

  • Coccus: These bacteria have a spherical shape. Eg: Staphylococcus, Pneumococcus.
  • Bacillus: These are rod-shaped bacteria. Eg: Anthracis, B. megaterium and B. thuringiensis.
  • Vibrio: These are comma-shaped bacteria. Eg: V. cholera, V. parahaemolyticus, and V. vulnificus.
  • Spirillum: These are spiral-shaped bacteria. Eg: Spirochaete, Treponema, Borrelia, and Leptospira. 

On the basis of the stain they take during gram staining developed by Christian Gram, they are classified into two types:

  • Gram-positive bacteria: These bacteria take up the gram stain and are not pathogenic. Eg: Staphylococcus, Streptococcus, etc.
  • Gram-negative bacteria: These bacteria do not take up the gram stain and are pathogenic. Eg: E. coli, Salmonella 

Classification of Eubacteria

There are basically two main types of Eubacteria which are as follows:


They are aka BGA (Blue Green Algae). They are gram-negative bacteria. Cyanobacteria are the most primitive organisms to exhibit oxygenic photosynthesis. They possess chlorophyll similar to higher plants. They are mostly found in freshwater though a few are found in marine environments. They show symbiosis with almost all eukaryotic groups. Eg: Anabaena is found associated with the coralloid roots of Cycas. Their body structure is varied. They can be either unicellular, filamentous, or colonial. They may possess trichomes. However, flagella are strictly absent in them. Cyanobacteria help in nitrogen fixation as they contain nitrogenase-containing heterocysts eg Nostoc. They reproduce asexually. True sexual reproduction is absent in them.


They exist in various shapes and hence are called PPLO (Pleuropneumonia Like Organisms). They lack true cell walls and hence are called ‘bacteria with their coats off and ‘jokers of the plant kingdom’. PPLO are the smallest living organisms with a size of about 0.02-0.2 microns. They are facultative anaerobes. They exhibit a heterotrophic mode of nutrition and are generally parasitic. Some are saprotrophs. They are pathogenic to both plants and animals. Mycoplasma possesses resistance to antibiotics like Penicillin and Bacitracin. However, they lack resistance to antibiotics such as Streptomycin, Chloramphenicol, and Tetracycline. Mycoplasma can pass through bacteriological filters due to its extremely small size.

Difference between Archaebacteria and Eubacteria

                                       Archaebacteria                                          Eubacteria
Cell wall Their cell wall is made of pseudomurein (NAT and NAG) Their cell wall is made of peptidoglycan/murein (NAG and NAM)
Cell membrane Phospholipids of their cell membrane are present in a monolayer and are branched Phospholipids of their cell membrane are present in a bilayer and are unbranched
Complexity They are simple organisms. They are complex organisms.
Location They are found in extreme conditions. They are found everywhere.
Examples Examples: Pyrobaculum, Ferroplasma, Lokiarchaeum, and Thermoproteus Types of Eubacteria Examples: Clostridium, Bacillus, Pseudomonas, and Mycobacterium

FAQs on Eubacteria

Question 1: List the types of asexual reproduction exhibited by cyanobacteria.


The types of asexual reproduction exhibited by Cyanobacteria are Binary fission, Fragmentation, Heterocysts, Akinetes, Hormogonia

Question 2: Why does Mycoplasma show resistance to some antibiotics but lack it for other antibiotics?


Antibiotics like Penicillin attack the cell wall and destroy it. Mycoplasma lacks cell walls and hence is not affected by penicillin. However, antibiotics like Streptomycin inhibit other metabolic activities and hence have an effect on mycoplasma.

Question 3: Which BGA is commonly found in rice fields in India?


The commonly found BGA in rice fields of India is Aulosira 

Question 4: Nitrogenase requires an anaerobic condition for its functioning. How do cyanobacteria ensure this condition?


The thick cell wall of specialized cells of heterocysts with a mucilaginous coat provides the required anaerobic condition to the cyanobacteria for nitrogenase activity.

Question 5: Explain the three-domain system of classification.


The six kingdom system of classification is the three-domain system. Here organisms are grouped into three domains on the basis of genetic analysis of the 16S rRNA. The three domains are

    Archae  Bacteria Eukarya
Archaebacteria Eubacteria Protista

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Last Updated : 29 Sep, 2022
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