Bacteria: Definition, Characteristics, Examples

Bacteria are a type of simple, microscopic, unicellular organism. In 1675, Dutch scientist A. van Leeuwenhoek was first discovered bacteria. He proved the existence of bacteria in his own made microscope and named them animalcules.

Later, French scientist Louis Pasteur and German scientist Robert Koch proved through various experiments that bacteria are the cause of various diseases in plants and animals.

Ehrenberg first introduced the term bacteria in 1828. The French surgeon Sedillot (1878) called them the microbes.


Bacteria are the simplest microscopic organisms in the living world. They do not have a well-organized nucleus and membrane surrounding cellular organelles. Also, 70s type ribosomes are present in the cell. 

Previously, bacteria were considered animals due to the presence of cilia in some of them. Later, they were included in the plant kingdom because of the cell wall present in the bacterial cell, like plant cell.

In modern times, bacteria are considered to belong to a separate kingdom called the Protista. They are called Protista because they are similar to unicellular fungal yeast and unicellular protozoa.


Bacteria are unicellular, simplest, microscope organisms containing the primitive nucleus and cell wall.

Characteristics of Bacteria

The characteristics of bacteria are as follows:

  • These are the smallest and simplest microbes in cellular organisms.
  • They are mostly unicellular, very few are mycelial or colonial, or filamentous.
  • They are motile or non-motile. In motile bacteria, flagella are present.
  • The protoplasm consists primitive type of nucleus. The nucleus does not have a nuclear membrane, nucleolus, or nuclear reticulum and also, contains only a spiral double-stranded DNA fiber. 
  • Bacterial DAN is called nucleoid or genophore.
  • The protoplasm contains 70s type of ribosomes and mesosomes. Also, there are no membrane-bound cell organelles (like plastids, mitochondria, ER, etc) are present in the bacterial protoplasm.
  • Bacterial cells have a well-formed cell wall outside the cell membrane. The cell wall is made up of peptidoglycan or mucopeptide.
  • Most of them are heterotrophic, and some of them are autotrophic.
Figure: A bacterial cell

Others Characteristics

  • In autotrophs, various types of photosynthetic pigments (like bacteriochlorophyll, bacterioviridin, and carotenoids) are present in their bodies.
  • They are both anaerobes and aerobes.
  • They can survive at temperatures of 70°C to 80°C.
  • In the absence of chromosomes, bacterial mitosis and meiosis cell division do not occur.
  • They can be reproduced by vegetative, asexual, and sexual methods.
  • Asexual reproduction is mainly binary fission.
  • Sexual reproduction is of a special type – gene recombination takes place by conjunction, transformation, and transduction.
  • They can synthesize vitamins.
  • A few of them can fix atmospheric nitrogen.
  • They accumulate energy by oxidizing salt.
  • They play an important role in maintaining the fertility of the soil.
  • A few bacteria are important for their production of antibiotics and alcohol.
  • They cause various diseases in plants, animals, and man.


Some common bacterial species are Diplococcus, Bacillus, Salmonella sp, Mycobacterium sp, Spirillum, Nitrosomonas, Rhizobium, Chlorobium, etc.

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