Dictionary > Eubacteria



n., singlular: eubacterium
Definition: true bacteria; bacteria excluding the archaea

Eubacteria are prokaryotic microorganisms consisting of a single cell lacking a nucleus and containing DNA is a single circular chromosome. Eubacteria can be either gram-negative or gram-positive, they have economic, agricultural, and medical importance. They include E. coli, Lactobacilli, and Azospirillum.

Eubacteria Definition

Eubacteria (biology definition): Literally means “true bacteria“. They include all bacteria (except for archaebacteria). These bacteria form the Domain Bacteria (previously, Domain Eubacteria). It is one of the three-domain systems of classification; the other two are Domain Archaeabacteria (now Archaea) and Domain Eukarya (the eukaryotes). Eubacteria are prokaryotic organisms (i.e. lacking a membrane-bound nucleus), predominantly unicellular, and with DNA in a single circular chromosome. Cell wall, when present, is made up of peptidoglycan. Examples are E. coli, Staphylococcus, Salmonella, Lactobacillus. Synonym:(true) bacteria. Compare: Archaebacteria.
bacterial cell
Figure 1: Bacterial cell – diagram with label.

All living organisms are classified into three major domains: Domain Eukaryota (eukaryotes), Domain Eubacteria (true bacteria), and Domain Archaea (archaebacteria). Domain Eubacteria includes the true bacteria. It is the largest domain that includes with the large group of organisms. What is eubacterial cell type? Eubacteria -as well as archaebacteria- are prokaryotes. Conversely, the Eukarya is composed only of eukaryotes. Unlike simple prokaryotic cells, eukaryotic cells tend to be more complex. They include the unicellular and multicellular protists, plants, algae, and animals.

Do prokaryotes have cell walls? Prokaryotic Eubacteria includes bacteria with cell walls consisting of peptidoglycan. However, not all bacteria have cell walls. But all eubacteria have a cell membrane. Bacterial cell membranes consist of glycerol and fatty acid combined together by an ester bond.

What is eubacteria? Eubacteria (sometimes called simply as “bacteria”) are small organisms that cannot be seen by naked eyes; thus, microscopes are used to visualize and study their morphology. To do so, bacteria are stained. Staining is an essential microbiological technique as it helps in highlighting the whole bacterial structure and cellular shape. Bacteria are classified according to Gram staining. Some of them are gram-negative whereas others are gram-positive.

Gram-positive bacteria have several layers of peptidoglycan in their rigid and thick cell wall, which is highlighted by gram staining. The cell wall of gram-positive bacteria contains lipoteichoic acid that is composed mainly of alcohol and phosphate in the plasmic space. Lipoteichoic acid function helps the bacterial cell to grow. It also protects the bacterial cell wall from lysis. Lipoteichoic acid provides the wall antigenic specificity to gram-positive bacteria; therefore, it helps in the grouping of gram-positive bacteria into several types.

The gram-negative bacterial cell wall consists of only one layer of peptidoglycan and has no lipoteichoic acid so gram stain cannot bind to their cell wall. As a result of its weak structure, the gram-negative bacterial cell wall is susceptible to mechanical damage. The outer membrane of the gram-negative bacterial cell wall consists of negatively-charged particles, such as lipopolysaccharides, phospholipids, and lipoproteins that enable gram-negative bacteria to evade immune cell detection, phagocytosis, and the action of complements produced by the immune system of the host. Moreover, this outer structure protects gram-negative bacteria from bile salts, heavy metals, digestive enzymes, detergents, and some antibiotics, such as penicillins.

gram negative and gram positive bacteria
Figure 2: Gram-positive vs Gram-negative. Credit: CNX OpenStax, CC 4.0.

How do eubacteria reproduce? Eubacteria reproduction usually includes dividing the parent cell into two daughter cells after the replication of genetic material in a process called binary fission. Some bacteria have the ability to form a spore in unfavorable conditions such as deficiency of nutrients, exposure to chemicals, or radiation. These spores cannot reproduce; however, they are highly resistant to toxins, radiation, heat, and dryness. Spore-forming bacteria such as Bacillus and Clostridium are considered to be virulent bacteria so sterilization techniques must eliminate bacterial spores. When environmental conditions become favorable again bacterial spores start to vegetate and reproduce again.

Diagram of binary fission in eubacteria
Figure 3: How eubacteria reproduce by binary fission. Credit: Watkinson, A., Researchgate.

Eubacteria Characteristics

What are the 3 characteristics of eubacteria? The bacteria are unicellular microorganisms of a prokaryotic cell. They contain a circular chromosome. Moreover, the eubacteria cell wall is composed of peptidoglycan. They greatly differ in terms of morphology and physiology. What is eubacteria cell structure? Eubacteria cell type is prokaryotic. Their cells lack nucleus and cell organelles present in eukaryotic cells. Their DNA is not inside a nucleus. (See Figure 1)

These structural differences between prokaryotes and eukaryotes are greatly important since the ability of antimicrobials to eliminate bacteria depends mainly on targeting these differences making them selective to bacterial cells without affecting the eukaryotic human cells.

Are eubacteria multicellular? The size of bacteria ranges from 0.2 to more than 50 micrometers. Bacteria are usually unicellular, however, some colonies of bacteria are found as filaments or aggregates in the form of surface biofilms.

Bacteria are unicellular prokaryotic microorganisms. Their cells contain carbohydrates, lipids, proteins, and nucleic acids. They produce energy and metabolites using different chemicals since their cells lack cellular organelles such as chloroplasts, nuclei, and mitochondria that perform these functions. Usually, the bacterial chromosome is circular, however, some species have two chromosomes like Vibrio cholera while other species have a linear chromosome. DNA of these cells is not linked with a histone. Rather, it is linked to different proteins.

Bacterial cells are cylindrical, spiral, spherical, or pleomorphic (irregular in shape). Some bacteria obtain their energy from the sunlight and use carbon dioxide for obtaining carbon to perform photosynthesis to create cellular biomass. These bacteria are called photosynthetic bacteria. Among these species are cyanobacteria (blue-green algae). Other eubacteria species get energy through organic and inorganic material metabolism (such as from sulfur and ammonia).

Eubacteria are mostly heterotrophs, which take food from an outer source. Most heterotrophs decompose dead material or parasites that live on or in a host. Other eubacteria are autotrophs by making their own food; they are either chemosynthetic or photosynthetic. The most important autotrophic eubacteria are cyanobacteria.

A diagram of a typical cyanobacterial cell showing parts.
Figure 4: A diagram of a typical cyanobacterial cell showing parts.

Respiration in eubacteria is either aerobic or anaerobic. Anaerobic bacteria undergo fermentation as an example of respiration.

Eubacteria Structure

Are eubacteria unicellular or multicellular? The eubacteria number of cells is only one. They are single prokaryotic cells. There is no such thing as eukaryotic bacteria. The structures found in eubacterial cells are either external or internal to the cell wall.

Structures external to the cell wall may be flagella, fimbriae, axial filaments, glycocalyx, or pili. Each of these structures has its distinctive function where some eubacteria have flagella to facilitate their movement.

Flagella are long filaments that facilitate the mobility of bacteria. Flagella consist of three main parts: a portion consisting of flagellin protein called the filament (i.e. the long outermost part), the middle hook, and the basal body that attaches the bacterial plasma membrane and cell wall.

Fimbriae and pili are hairlike structures similar to flagella but thinner. Pili or pilus are thin projections used for conjugation between bacteria for reproduction after the bacteria are brought together by pili, the DNA moves from one cell to another so new features as antibiotic resistance are transferred between cells.


Glycocalyx surrounds some eubacterial cells. It is a viscous polymer composed of polypeptides or polysaccharides and functions to protect the bacteria. It is commonly known as the capsule. The capsule is considered to be one of the virulence factors of some bacteria since it enables the bacteria to resist phagocytosis by the immune system. Glycocalyx coat can also help bacteria to attach firmly to the host cell such as Vibrio cholera that produces glycocalyx to attach to the intestinal cells.

The cell wall of bacteria consists chiefly of a network of peptidoglycan. It may be associated with other substances or present alone. The peptidoglycan network consists of disaccharide portions connected together by polypeptides forming a lattice to protect the bacteria. Some antibiotics such as penicillins and cephalosporins interfere with the structure of bacterial cell walls leading to cell lysis and rupture.

Structures internal to the cell wall include cell membrane, cytoplasm, DNA, plasmid, and ribosomes.

Like eukaryotic cells, prokaryotic internal structures, such as those of eubacteria, are surrounded by a cytoplasmic membrane consisting mainly of phospholipids. However, the cell membrane of prokaryotes is less rigid than that of many eukaryotes. That’s because they lack sterols (except for Mycoplasma that has sterol in its cell membrane). Some antimicrobial agents, such as quaternary ammonium and alcohol, act by disrupting the cell membranes of bacteria. Additionally, a group of antibiotics called polymyxins can also damage plasma membranes leading to the destruction of bacterial cells.

The cytoplasm is the substance inside plasma membranes and acts as a media for the internal structures of the cell. In eubacteria, the cytoplasm contains DNA that is not normally seen in the eukaryotic cytoplasm. Prokaryotic cells contain cytoskeletons in the cytoplasm that aids in the growth, reproduction, and maintenance of the cell’s shape.

The nucleoid or DNA of prokaryotic eubacteria differs from that of eukaryotes, it consists of a long, single, circular double-stranded DNA, which contains all the bacterial genetic information. The bacterial chromosome is not enveloped by a nuclear membrane like that of eukaryotes; it also lacks histone. Bacterial cells mostly contain a small structure of DNA double-strand called a plasmid that is distinct from the bacterial chromosome. Plasmids replicate independently from the bacterial chromosomes. Plasmids can be exchanged between different bacteria without causing any harm. Moreover, their presence is not essential for the functioning of a bacterial cell, however, the plasmid usually carries beneficial genes, such as genes of antimicrobial resistance, toxin production, resistance to toxicity by toxic metals, and enzymes synthesis. Plasmids are the main structures used in biotechnological applications recently because they can replicate independently, can be inserted or removed easily from the bacterial cells, and can be manipulated easily using different enzymes.

Like eukaryotes, prokaryotes have ribosomes for the synthesis of proteins. However, they are structurally different in the number of rRNA and proteins they contain. Consequently, several antibiotics act by inhibiting the synthesis of proteins by binding to bacterial ribosomes without affecting the host cell, such as chloramphenicol and erythromycin.

Eubacteria vs. Archaebacteria

What is the archaea definition in biology? The archaea are prokaryotic microorganisms that reproduce asexually by budding, binary fission, and fragmentation. They are highly specialized organisms known as ancient bacteria. Some archaea live in extreme environments such as extremely high temperatures (referred to as thermophiles) while other archaea can live in oxygen-free environments (methanogens). Halophiles are salt-loving archaea growing only in environments rich in salt, such as brines. Archaea bacteria can also be found in some parts of the human body such as the colon, mouth, and skin. Archaea bacteria are not usually pathogenic.

What is the difference between bacteria and eubacteria? Bacteria is a term that was previously used to include all bacteria. Soon, two groups emerged: eubacteria or true bacteria and archaebacteria or archaea. Eubacteria and archaebacteria are the only prokaryotes found on earth. They have a common progenitor cell but different evolutionary lines. Do bacteria have a nucleus? Both eubacterial and archaeal cells lack a nucleus and other membrane-bound organelles.

Even though the archaea are structurally similar to bacteria, they are different when examined at a molecular level. For example, bacterial cells usually have a peptidoglycan outer layer that varies in thickness in gram-negative or gram-positive bacteria. But do archaea have peptidoglycan? No. Some archaea, such as methanogens, have a pseudopeptidoglycan S layer that forms a layer to resist the internal high osmotic pressure.

Table 1: The main differences between archaebacteria and eubacteria

Eubacteria Archaebacteria
Complex microorganisms Simpler microorganisms
Live anywhere on earth Live mostly in extreme conditions
Peptidoglycan cell wall Pseudopeptidoglycan cell wall
Can obtain energy by Krebs cycle or glycolysis Cannot perform Krebs cycle or glycolysis
Contains L-glycerol phosphate in their membranes lipids Contains D-glycerol phosphate in their membranes lipids
Simple RNA polymerase Complex RNA polymerase
Contains L-glycerol phosphate in their membranes lipids Complex RNA polymerase
Examples: Clostridium, Bacillus, Pseudomonas, and Mycobacterium Examples: Pyrobaculum, Ferroplasma, Lokiarchaeum, and Thermoproteus

Types of Eubacteria

The domain bacteria are classified according to several characteristics including shape (bacillus, coccus, spirochete, or vibrio), the requirement of oxygen (facultative or obligate aerobes or anaerobes), nutrition (chemosynthetic or photosynthetic, and the composition of their cell wall (Gram-positive or Gram-negative).

Eubacteria usually have one shape. However, in some cases, their shape becomes altered due to environmental conditions. Some eubacteria are normally polymorphic, such as Corynebacterium and Rhizobium.

Rounded (cocci) bacteria may be elongated, flattened, or oval. After division and reproduction, they can remain attached to each other. Diplococci means that two cells remain attached to each other after reproduction whereas streptococci mean that they are attached to each other in a chainlike pattern. Those that remain in groups of four cells and divide into two planes are called tetrads. Staphylococci are divided into different planes forming grapelike clusters or sheets. Bacilli bacteria can divide in one direction only so they have fewer forms of grouping than cocci. Bacilli can either be single bacilli, diplobacilli (pairs) or streptobacilli (chains). Some bacilli are oval and look similar to cocci therefore they are called coccobacilli. Vibrio or spiral bacteria are twisted for one or more twists so they look like curved rods. Helical bacteria are called spirilla. They have rigid bodies and look like a corkscrew.


Eubacteria are classified into several phyla. Each bacterial phylum includes species characterized by specific features. Examples are as follows:

  • Proteobacteria includes most of the gram-negative bacteria. They are thought to have arisen from photosynthetic ancestors. Proteobacteria are classified into five classes: alphaproteobacteria, betaproteobacteria, gammaproteobacteria, deltaproteobacteria, and epsilonproteobacteria.
  • Cyanobacteria are characterized by a blue-green pigment. They perform photosynthesis as plants and algae. Many of these bacteria can fix nitrogen in the soil, therefore, they are important in agricultural fields.
  • Chlorobi phylum consists of photosynthetic bacteria. Members of this phylum are green sulfur bacteria. Chlorobi reduces carbon dioxide during photosynthesis using organic compounds such as carbohydrates and acids. Members of this phylum diverse greatly with their rods, spiral, cocci, or budding forms.
  • Chloroflexi members are green nonsulfur bacteria, such as Chloroflexi can perform photosynthesis.
  • Chlamydiae members are pathogenic gram-negative cocci that have a unique cycle of development. They are transmitted from human to human by direct contact or respiratory airborne routes.
  • Planctomycetes are budding gram-negative bacteria. Even though their DNA is similar to bacteria, their cell wall is similar to archaea. Additionally, some of them contain organelles similar to those of eukaryotes.
  • Bacteroidetes are anaerobic bacteria that inhabit the human intestinal tract or oral cavity or intestinal tract. They are present in feces and may cause infection due to surgery or puncture wounds.
  • Fusobacteria are anaerobic bacteria; their cell shape is either pleomorphic or spindle.
  • Spirochaetes are coiled resembling metal springs. They are flagellated. Their flagella facilitate their movement using axial filaments. Spirochaetes are usually present in the human mouth.

Evolution of Eubacteria

Three domains of life were proposed in the 1990s based on the fact that ribosomes are different in the three types of cells (Archaebacteria, Eubacteria, and Eukaryota) after comparing the nucleotides sequence in each cell. Even though Eubacteria and Archaebacteria are prokaryotes, the two domains were separated due to variation in the small rRNA subunit in both domains. Archaebacteria live in extreme environments, therefore, they are thought to be the first organisms to live on Earth. Molecular theories support the fact that genes were transferred horizontally between the three types of cells which consequently affected the evolutionary process of life.

According to the Endosymbiotic theory, some bacteria have evolved into mitochondria and chloroplasts by gene transmission. Accordingly, mitochondria and chloroplasts were genetically transferred between different domains to continue the evolutionary process.

Evolutionary tree of the three kingdoms
Figure 5: the evolutionary tree of the three kingdoms. Credit: Hug, L. A., et al. (11 April 2016). “A new view of the tree of life”. Nature Microbiology. 1 (5): 16048. DOI:10.1038/nmicrobiol.2016.48. PMID 27572647.

Biological Importance of Eubacteria

The world is filled with different eubacterial species and our bodies contain different species of eubacteria, which are biologically important in our life. Our body is only of the eubacteria habitats forming our normal flora. Normal flora causes no harm to us and they are beneficial to our bodies. For example, they defend our bodies against pathogenic eubacteria, others can produce biologically important substances such as B vitamins as well as vitamin K.

Several species of eubacteria are used in the mass production of chewable or tablet vitamins since bacterial species provide an inexpensive, safe, and non-toxic source of vitamins. For example, Propionibacterium and Pseudomonas species produce vitamin B12, whereas ascorbic acid (vitamin C) is produced by Acetobacter species using glucose.

Streptomyces hygroscopicus is of particular importance in the pharmaceutical industry since their different strains can produce about 200 different types of antibiotics.

Eubacteria decompose organic matter and dad leaves into carbon-dioxide and nutrients such as nitrogen They contribute to keeping the balance of all ecosystems.

Eubacteria in Ecology

Studying the relation between eubacteria and the environment is known as microbial ecology. It includes many branches that discuss how eubacterial and other microorganisms interact with their environment.

Eubacteria can convert forms of oxygen, carbon, nitrogen, and phosphorus that cannot be used by living organisms such as animals and plants into useful forms. They fix the nitrogen found in the air into the soil. Moreover, they return the natural carbon dioxide into the atmosphere through the decomposition of dead plants and organic wastes. Carbon dioxide is, then, used by plants, cyanobacteria, and algae to perform photosynthesis and obtain energy.

Beneficial eubacterium can be used in sewage treatment. It converts organic material and liquid harmful substances into organic unharmful substances such as carbon and nitrogen. Therefore, it helps in preserving water by controlling pollution.

Azospirillum is a bacterium that grows in the soil close to the roots of plants. It fixes nitrogen from the air into the soil and uses the excreted nutrients from plants as a source of nutrition. Like Azospirillum, Rhizobium and Bradyrhizobium, which are commonly known as rhizobia, fix nitrogen in the roots of plants especially leguminous plants such as peas and beans. Rhizobia are responsible for the formation of nodules in such plants.


Examples of Fascinating Eubacteria

Most people believe that bacteria are harmful organisms that cause diseases in humans, animals, and plants, but actually, only a few species of eubacteria are pathogenic. Many others are beneficial to all other living organisms. Eubacterial species are important in different fields such as medicine, agriculture, industry, and energy production. In this section, we are going to discuss examples of fascinating bacteria.

Nitrobacter and Nitrosomonas are species of bacteria that have the ability to use inorganic chemicals including carbon dioxide and energy sources as a source of carbon to produce complex chemicals which are reduced compounds of nitrogen. Nitrosomonas can oxidize ammonium to nitrite whereas Nitrobacter can oxidize nitrite to nitrates in a process known as nitrification. The product of nitrification, nitrates, is a mobile form of nitrogen of great importance in the agricultural field.


Zoogloea species contribute to the processes of treating sewage such as the activated sludge system. While growing, these bacteria form a slimy, fluffy mass which is important in operating such systems.

Xanthomonas campestris can be ingested by humans. It is capable of producing xanthan using lactose. Xanthan has a thickening effect, therefore, it is used in the production of salad dressing, dairy products, shampoos, cold creams, and cosmetics.



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  • Iwabe, N., Kuma, K. I., Hasegawa, M., Osawa, S., & Miyata, T. (1989). Evolutionary relationship of archaebacteria, eubacteria, and eukaryotes inferred from phylogenetic trees of duplicated genes. Proceedings of the National Academy of Sciences, 86(23), 9355-9359.‏
  • Levin, S. A. (2013). Encyclopedia of biodiversity. Elsevier Inc..‏
  • Ochman, H., & Wilson, A. C. (1987). Evolution in bacteria: evidence for a universal substitution rate in cellular genomes. Journal of molecular evolution, 26(1-2), 74-86.‏
  • Tortora, G. J., Funke, B. R., & Case, C. L. (2015). Microbiology: An Introduction, Global Edition. USA: Pearson Education.‏
  • Whitfield, C., Szymanski, C. M., & Aebi, M. (2017). Eubacteria. In Essentials of Glycobiology [Internet]. 3rd edition. Cold Spring Harbor Laboratory Press.‏

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