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What are the structural aspects of bacterial organisms?

Bacteria are the most simplest unicellular living organisms in nature. Their structural features are quite different from other single celled organisms. In this resource, I gave importance to explain their various structures present in these single celled organisms and their functions. I also gave an account of how these bacteria differs from other Eukaryotic unicellular organisms and what is their position in classification.


Bacteria are unicellular Prokaryotic organisms present everywhere in nature. So they are known as omnipresent organisms. Literally there is no place in nature where bacteria cannot live including hot lava's, snow caps of Himalaya's, hot spring water, deeper depths of Seas and Oceans, acidic or alkaline soils, radioactive wastes, on the surface of your body and inside your body etc. It is roughly estimated a 70 Kg. human include 30 trillion body cells but such body roughly hosts 39 trillion Bacterial cells.Bacteria are the smallest of all unicellular organisms which cannot be seen with our naked and can be seen only under microscopes.

Sizes and Shapes of Bacteria

Scientists roughly estimated that over 10 to the power of 30 varieties of species of bacteria may exist in nature at present. The size of bacteria ranges from 0.3 micrometers to 5 micrometers. The smallest of bacteria are Mycoplasma which roughly measures 0.3 micrometers. The largest species of bacteria which are roughly visible to human eye measures 0.7 mm is Epulopiscium fishelsoni and another bacterium which measures 0.5 mm is Thiomargarita namibiensis.

Bacteria are usually found in four different shapes. Bacteria are usually live as single individual cells but in a variety of species of bacteria they tend to live in specific groups.

Basing on the shapes, bacteria are four kinds:-

  1. Spherical Bacteria:-

    Bacteria which are spherical in shape are known as Coccus bacteria.

    If spherical bacteria exists in single individual cells are known as Monococcus bacteria. If two bacteria exist in a pair are known as Diplococcus bacteria. Pneumonia disease causing bacteria are diplococcus in nature.

    If spherical bacteria exists in the form of a chain are known as Streptococcus bacteria. Meningitis disease causing bacteria are Streptococcus variety.

    If spherical bacteria found in the form of grape bunch are known as Staphylococcus bacteria. This type of bacteria cause various skin infections.

  2. Rod shaped bacteria:-

    Bacteria which are rod shaped are known as Bacillus bacteria. If rod shaped bacteria exists in single cells are known as Monobacillus bacteria. If two rod shaped bacteria exists together are known as Diplobacillus bacteria. If rod shaped bacteria are present in the form of a chain are known as Streptobacillus bacteria. If rod shaped bacteria exist in the form of picket fence are known as Palisades.

  3. Comma shaped bacteria:-

    Comma shaped bacteria are known as Vibrio. Vibrio cholerae is a Cholera disease causing species of bacteria.

  4. Spiral bacteria:-

    Bacteria which are spiral or Corkscrew shaped are known as Spirillum

Differernt Shapes of Bacteria

Different Shapes of Bacteria (Courtesy:-www.wikispace.com)

Gram Positive and Gram Negative Bacteria

In bacteria two major groups of bacteria are
present -Gram positive bacteria and Gram negative bacteria. Because of cell wall nature of certain bacteria they absorb stain are known as Gram positive and certain bacteria which cannot absorb stain are known as Gram negative. A Danish scientist Christian Gram developed a stain to distinguish the two groups of bacteria in 1884.
Examples for Gram Positive Bacteria:- Staphylococcus, Lactobacillus, Bacillus
Examples for Gram Negative Bacteria:- Salmonella, Escherichia coli, Azatobacter

Structural organization of a generalized Bacterial cell

The following is the description of a generalized bacterial cell as seen under Electron Microscope. A Bacterium is a unicellular Prokaryotic organism. A Prokaryotic cell is characterized by an ill defined nucleus without nuclear membrane, lack of double membrane d cell organelles and ribosomes present are of smaller type.
A Generalized Structure of a Bacterial Cell

Generalized Structure of a Bacterial Cell (Courtesy:-www.wikieducator.com)
  1. Bacterial Cell Wall

    A Bacterial cell on its outer surface is covered by a tough rigid Cell Wall made of Peptidoglycon or Muramic acid. Cell wall material include Polysaccharides with amino acids. Cell wall of bacterial cell is protective in nature. Presence of Cell wall is considered to be a plant characteristic feature even though bacterial cell wall is not made with the chemical cellulose.

  2. Cell Membrane

    Next to the Cell Wall, a living Cell membrane or Plasma membrane is present which is identical to Plasma membrane of Eukaryotic cell. It is semi-permeable in nature and allow materials through it preferentially. The Cell membrane of a bacterial cell get folded into specialized structures called Mesosomes. These Mesosomes increase the surface area of Cell membrane and also secrete respiratory enzymes which help in cellular respiration of bacterial cell. So Mesosomes compensate the lack of mitochondrial cell organelles as found in a eukaryotic cell. In some bacterial species like Purple bacteria, photosynthetic membranes are cut off from the Cell membrane of bacteria. These photosynthetic membranes include Bacteriochlorophyll pigment in them which help to carry photosynthesis like in green plants. Mesosomes closely associate with DNA of bacterial cell and helps in binary fission.

  3. Cytoplasm

    After Cell membrane, a fluid granular living material present in a bacterial cell is known as Cytoplasm. But all the double membrane d cell organelles like Mitochondria, Endoplasmic reticulum, Golgi complex, Plastids etc. are not found in the cytoplasm of bacterial cell. But in Cytoplasm a smaller 70S type of ribosomes can be found which can be helpful for their protein synthesis.

  4. Plasmids

    In some species of bacteria we can observe small bits of extra circular DNA molecules present in their cytoplasm called Plasmids. These Plasmids include very few genes which provide extra survival advantage for them like resistance.

  5. Cell Inclusions

    In the Cytoplasm of bacterial cell we can find glycogen granules and oil droplets which can serve as reserve food material in them. Presence of glycogen and oil droplets as a reserve food material is a characteristic feature of animal cells.

  6. Nucleoid of Bacterial Cell

    A bacterial cell lacks a well organized double membrane d nucleus like as we can find in a eukaryotic cell. Instead a well defined nucleus at the center of bacterial cell we can observe a single circular DNA molecule or a Chromosome with few thousands of genes. Such a primitive or ill defined nucleus present at the center of a bacterial cell is commonly known as nucleoid.

  7. Capsule

    In certain resistant varieties of bacteria the outer surface of the bacterial cell wall is covered by a slimy mucilaginous covering called Capsule. This mucilaginous covering is formed by the dissolution of the chemical material present in the cell wall of resistant varieties of bacteria. This Capsule layer protect the bacteria from the acidic and enzymatic actions of the host body in which they live.

  8. Flagella

    Certain species of bacteria possess long flagella on the outer surface of bacterial cell which help in their locomotion in liquid medium. If a single flagellum is present at one end of the bacterium are Monotrichous bacteria, one flagellum at two ends of the bacterium are Amphitrichous bacteria and flagella present around the body are Peritrichous bacteria.

  9. Sex Pili

    In certain species of bacteria short hair like projections are present all around the surface of bacteria are known as Pili or sex pili. The hair like projections of these bacteria help in adhering two bacteria together in their sexual reproduction for exchanging their genetic material.

Position of Bacteria in Classification

There is a lot of dispute with regard to the position of bacteria in the classification of organisms. Some scientists believe that bacteria belongs to plants because they possess a rigid cell wall, some species of bacteria can prepare their own food, can synthesize vitamins like plants. Some scientist they believe that bacteria are animals as some species of bacteria possess flagella for their locomotion, they store reserve food material like glycogen and oil droplets as in animal cells, as bacteria cannot prepare their own food.
As these organisms cannot fit properly either into plants or animals, Whittaker created a special kingdom for them in his modern Five Kingdom System of classification. In his classification he kept Bacteria under a Kingdom Monera where all organism which are unicellular, prokaryotic and heterotrophic in nature were placed.


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