Bacteria | Definition, Types & Classification

Bacteria Definition

Bacteria are prokaryotic single-cell microorganisms. Due to its prokaryotic nature, these cells lack organelles or central true nucleus. They are less complex in nature as compare to eukaryotic cells. In the word, capital B represent its domain of Bacteria that is one of the three domains of life of living organisms. Other two domains are Archaea and Eukaryota.

Domain Bacteria examples:

There are three main domains, bacteria is one of them. Examples of this domain are given below in the diagram below:


Bacteria Characteristics

Bacteria Facts

  1. They are basically single-celled organisms.
  2. They lack cellular organelles like chloroplasts and mitochondria.
  3. They did not have the Nucleus as the eukaryotic cells have.
  4. Rather than this its DNA, which is double strand, and circular, present in Nucleoid.
  5. The nucleoid is an irregularly shaped region that does not have a nuclear membrane.
  6. They also have a cell membrane and a cell wall that is often made of peptidoglycan.
  7. The cell membrane and cell wall collectively called cell envelope.
  8. Many cells require a cell wall in order to survive.
  9. They reproduce by the process of binary fission.
  10. It is basically the splitting of the bacterial cell when it comes to a specific size.
  11. By the binary fission two daughter cells are produced which have the exact same amount of DNA as parent cell has.
  12. They exchange their genetic material between the cells by the process called Horizontal gene transfer.

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Bacteria Classification

They are basically classified into five major groups. This classification is done on the basis of its shape. The spherical shape is called Cocci, Rod shape is called bacilli, Spiral shape is called spirilla, comma shape is called vibrios.

Bacteria Shapes

They usually occur in three shapes coccus, spiral, and Bacillus.


  1. Cocci shape:

These are ovoid or spherical in shape. They are found in the chain or in the isolated form. For example, Diplococci are cocci occurs in pairs, streptococci exist in chains, and staphylococci are present in clusters of manifold cocci. Tetrads are which have square arrangements of four cocci. Sarcinae eight cocci cubes.

  1. Spiral Shape:

They are spiral-shaped. Spirillums are basically thick, threatening spirals. Spirochetes are also spirals which have thin and flexible texture. Vibrios are comma shape rods which have a little twist.

  1. Bacilli Shape:

They are basically rod-shaped. They sometimes present in isolate form while occasionally arranged in a chain. Diplobacilli are basically two bacilli which are arranged adjacent to each other.

Bacteria Structure:�


Types of Bacteria

Broadly there are two main types:

  1. Gram-positive bacteria

  2. Gram-negative bacteria



Examples of Bacteria

1. Escherichia coli.

Common species are Escherichia coli. Its shape is Bacillus. Commonly found animals intestine including humans. In the intestine, they produce vitamin B complex and vitamins K. E. coli usually utilize in the laboratory for the research purposes. They reproduce rapidly due to this they are extensively used for research purposes. They are usually harmless but some species of E. coli can cause diseases in humans. They usually cause gastrointestinal problems in humans.


2. Lactobacillus acidophilus

Lactobacillus acidophilus is bacillus shape which commonly finds in vagina and intestine of human beings. Being probiotic, it is present in yogurt and food produces by fermentation.


3.Clostridium botulinum

Clostridium botulinum causes special disease Botulism. C. botulinum bacteria generate the neurotoxin botulinum that causes botulism. Its symptoms include blurred vision, paralysis, nausea, muscles weakness and trouble to breathe.


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