Bacteria - Definition, Diagram and Classification

Oct 21, 2022, 16:45 IST

The first organism to evolve on Earth was probably a single-celled organism, similar to modern bacteria. Since then, life has evolved into many forms of life over many millennia. However, we can still trace our ancestors back to this single-celled organism. Today, bacteria are considered the oldest life forms on Earth.

Although most bacteria make us sick, they have a long-term relationship with humans and are very important to our survival. But before we elaborate on its use, let us know in detail the structure of bacteria, their classification, and the bacteria diagram.


"Bacteria are the unicellular organisms belonging to the prokaryotic group, where the organisms lack several organelles and a true nucleus."

Bacteria Diagram

The diagram of bacteria mentioned below that show the structure of the typical bacterial cell with different parts. The cytoplasm, plasmid, cell wall, and flagella are clearly marked in the diagram.

Biology article Structure of bacteria

Ultrastructure of a Bacteria Cell

The structure of bacteria is simple body design. Bacteria are unicellular microorganisms with the absence of a nucleus and other cell organelles; therefore, they are classified as prokaryotic organisms. They are very versatile organisms, surviving in extremely inhospitable conditions. Such organisms are called extremophiles. Extremophiles are further categorized into different types based on the types of environments they inhabit:

  1. Thermophiles
  2. Acidophiles
  3. Alkaliphiles
  4. Osmophiles
  5. Barophiles
  6. Cryophiles

Another fascinating feature of bacteria is protective cell wall, which is made up of a special protein called peptidoglycan. The components of the bacterial cell wall form an important basis by which bacteria can be divided. This particular protein is found nowhere else in nature than in the cell walls of bacteria.

But few are without this cell wall and others have a third protective layer called a capsule. Attached to the outer layer is one or more flagella or pili, which function as a locomotor organ. Pili can also help certain bacteria attach to host cells. They do not contain any cell organelle as in an animal or plant cell except ribosomes. Ribosomes are the sites of protein synthesis. In addition to this DNA, they also have circular DNA called a plasmid. These plasmids make certain strains of bacteria resistant to antibiotics.

Classification of Bacteria

Bacteria can be classified into different categories based on their features and characteristics. The classification of bacteria is mainly based on the following:

  • Shape
  • Composition of the cell wall
  • Mode of respiration
  • Mode of nutrition

Classification of bacteria based on Shape

Type of Classification Examples
Bacillus (Rod-shaped) Escherichia coli (E. coli)
Spirilla or spirochete (Spiral) Spirillum volutans
Coccus (Sphere) Streptococcus pneumoniae
Vibrio (Comma-shaped) Vibrio cholerae

Classification of bacteria based on the Composition of the Cell Wall

Type of Classification Examples
Peptidoglycan cell wall Gram-positive bacteria
Lipopolysaccharide cell wall Gram-negative bacteria

Classification of bacteria based on the Mode of Nutrition

Type of Classification Examples
Autotrophic Bacteria Cyanobacteria
Heterotrophic Bacteria All disease-causing bacteria

Classification of bacteria based on the Mode of Respiration

Type of Classification Examples
Anaerobic Bacteria Actinomyces
Aerobic Bacteria Mycobacterium

Reproduction in Bacteria

Biology article reproduction in bacteria

Bacteria follow an asexual mode of reproduction known as binary fission. One bacterium divides into two daughter cells. These are identical to the parent cell and to each other. DNA replication in the parent bacterium marks the beginning of cleavage. Eventually, the cell elongates to form two daughter cells.

The rate and timing of reproduction depends on conditions such as temperature and nutrient availability. Under favorable conditions, Escherichia coli or E. coli produces about two million bacteria every 7 hours. Bacterial reproduction is asexual, but in very rare cases it can undergo sexual reproduction. A genetic recombination in bacteria has the potential to form through transformation, conjugation or transduction. In such cases, bacteria can become resistant to antibiotics because there are differences in the genetic material.

Useful Bacteria

Not all the bacteria are harmful to the humans. There are few bacteria which are beneficial in various ways. Mentioned below are the few benefits of a bacteria:

  • The production of the antibiotics, which is used in the prevention and treatment of bacterial infections – Soil bacteria
  • Ferment food products – Streptococcus and Bacillus
  • Convert the milk into curd – Lactobacillus or lactic acid bacteria
  • It help in digestion and improving the body’s immunity system – Actinobacteria, Bacteroidetes, Firmicutes, Proteobacteria

Harmful Bacteria

Some bacteria can cause a number of diseases. They are responsible for many infectious diseases, such as pneumonia, tuberculosis, diphtheria, syphilis, and tooth decay. Their effects can be corrected by taking antibiotics and prescribed medications. However, prevention is much more effective. Most of these disease-causing bacteria can be removed by sterilizing or disinfecting exposed surfaces, tools, instruments, and other tools. These methods include the application of heat, disinfectants, UV radiation, pasteurization, cooking, etc.

Frequently Asked Question (FAQs)

Q1. How fast can bacteria move?

Ans. Bacteria can reach a speed from 2 microns per second (a gliding bacteria, Beggiatoa) to 200 microns per second (polar bacteria, Vibrio comma). Speed varies with the type of bacteria, but flagellates are undoubtedly faster than gliders.

Q2. What are the different types of bacteria?

Ans. Bacteria can be divided into various types based on characteristics such as cell wall composition, shape, mode of respiration, and mode of nutrition.

Q3. State four examples of bacteria.

Ans. The four examples of bacteria are-

  1. Streptococcus
  2. Bacillus
  3. Actinobacteria
  4. Proteobacteria

Q4. What are the examples of acidophilic bacteria?

Ans. Acetobacter aceti and Alicyclobacillus acidiphilus are two examples of acidophilic bacteria.

Q5. What is harmful bacteria called?

Ans. Harmful bacteria are known as pathogenic bacteria because they cause diseases and illnesses, such as: strep throat, staph infection and cholera.

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