Bacteria: Definition, Structure, Types, Growth, and Reproduction


Bacteria are single-celled microorganisms with prokaryotic cells. They are single cells with no organelles or true nuclei and are more complex than eukaryotic cells. A bacterium with a capital letter B refers to the bacterial domain, which is one of the three domains of life. The other two areas of life are Archean, members of which are also unicellular organisms with prokaryotic cells and eukaryotes. There are many total biomass of bacteria on Earth is greater than the sum of all animals and plants.

Structure of Bacteria

All of them have a cell membrane, cytoplasm, ribosome, and chromatin body. Most of all have a cell wall. It gives shape to bacteria cells. Some of these have a specific structure, such as capsules, mucus, flagella, tablets, pile, and rash.


Single size sizes range from 0.1 to 600 sizes.

Some members of the smallest bacteria, such as mycoplasma, are only 200 nm in diameter. Its size is equal to the size of the largest virus (poxvirus).

The average size of Escherichia coli (a type of bacterium) is 1.110 1.5 in the afternoon to 2.0 to 6.0 in the afternoon. Some propellers sometimes reach a length of 500 hrs. Staphylococcus and Streptococcus have a diameter of 0.75 to 1.25 pm. Recently, a giant bacterium was discovered in the intestine of fish (Acanthurus nigrophcus). This bacterium Anabaena grows at 600/80 hrs.

Types of Bacteria

Types of Bacteria

The cell wall also makes Gram staining possible. Gram staining is a method of staining bacteria consisting of crystal violet dye, iodine, and counterstain crocin. These are divided into two types: gram-positive, which shows spots under the microscope and appear purple; And gram-negative, which shows only rebound and appears red. Gram-positive appear violet because their cell walls are thick and can occupy the crystalline violet iodine complex.

The thin cell walls of gram-negative cannot capture the violet iodine complex but can hold crocins. This causes gram-negative bacteria to appear red under the gram stain. A gram stain is used for general identification to detect the presence of certain bacteriƫn. It cannot be used to identify in any specific way (eg, species level). Examples of Gram-positive bacteria include Listeria, Streptococcus, and Bacillus, while Gram-negative include proteins, green sulfur, and cyanobacteria.

Shapes of Bacteria

Bacteria are divided into three categories according to their size. These shapes are cocci, bacilli, and spiral. Most of them have a continuous shape. Some bacterial cells are polymorphic and present in different sizes.



Cocci are spherical or ellipsoidal bacteria. They have different arrangements. This arrangement is based on their division aircraft.

  1. Diplodocus: In this case, it splits on a plane and appears in cocci pairs.
  2. Streptococcus: In this case, division also occurs on a flat surface, but forms a long chain of cocci cells.
  3. Quadruple: When cell division occurs in two planes, a quadruple arrangement will be produced. A tetrad is a square of 4 balls.
  4. Sarsina: When splitting into three planes, a satirical system will be produced. Sarcina is a cube of 8 cocci.
  5. Staphylococcus: When the division is on a random plane, a staphylococcal arrangement will be produced. In this case, the cocci are arranged irregularly, like grape clusters.

Examples of cocci are Pneumococcus and Staphylococcus aureus.



Bacillus is a coryneform bacteria. Bacillus is a single cell.

The system of bacteria is as follows.

  • Streptococcus: Streptococcus is a series of bacteria.
  • Diplobacilli: When Diplobacilli appears in pairs, this arrangement is called Diplobacilli.
  • Examples of Bacilli are Escherichia coli, Bacillus subtilis, Pseudomonas.


Spiral bacteria coalesce into a spiral. There are the following forms of spiral:

  • Vibrio: It is a curved or comma-shaped spiral.
  • Spiral: It is a thick rigid spiral.
  • Spirochete: The spiral is thin and soft.
  • An example of a spiral bacterium is Vibrio, Hyphomicrobium.

Respiration in Bacteria

Bacteria are divided into different groups according to their breathing patterns:

  • Aerobic bacteria: These require free oxygen to breathe. They can only develop under aerobic conditions, such as Pseudomonas.
  • Anaerobic bacteria: They do not require free oxygen. They can grow without oxygen, such as spirulina
  • Resultant bacteria: They can grow under aerobic or anaerobic conditions, such as Escherichia coli.
  • Aerobic Mycobacteria: These require low concentrations of oxygen to grow, such as Campylobacter.

Growth and Reproduction

The increase in the number of bacterial cells is called bacterial growth. The number of bacteria increases through asexual reproduction called binary fission. During binary fission, the parent cell expands. Its chromosomal replicas and nuclear material are evenly distributed throughout the cell. Its plasma membrane lies in the center of the cell inwards and divides the cell into two parts. These partitions are repeated after a certain interval, and new subunits are formed. The time interval until the next division is completed is called generation time. After the completion of each division, the daughter’s bacteria will grow and develop their own unique functions.

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