Introduction & Structure Of Bacteria | BACTERIOLOGY NOTES



Bacteria are the large group of unicellular microorganisms that are ubiquitous in nature. They are extremely small organisms and cannot be seen individually by the unaided eyes, requires a special instrument called Microscope for the enlarged view of these microbes. On the solid media plates, they grow as the colonies that contain the large no. of individual cells of the bacteria.

Bacteria are the Prokaryotic organisms characterized by the lack of the well-defined nucleus, cell-organelles like Mitochondria, Golgi apparatus, Endoplasmic Reticulum etc., contains 70S Ribosome (30S & 50S Subunits), surrounded by a protective cell wall containing peptidoglycan, have small rings of DNA called Plasmids.

The largest bacteria of the world is Thiomargarita namibiensis measures 100 – 300 μm (Micrometer) in diameter approximately, the smallest known bacteria on the earth is Mycoplasma genitalium which measure 200 – 300 nm (Nanometer) and the Longest known bacteria on this planet is Epulopiscium fishelsoni 


A typical structure of bacteria is as follows: 

A bacterial cell consists of the following components –

  • An outer layer or cell envelope consists of two components:

         1.) A rigid cell wall

        2.) A cytoplasmic or plasma membrane (below the cell wall)

  • Components of the cell interior – Inside the cell envelope, there is protoplasm consists of the cytoplasm and cytoplasmic inclusions such as Ribosomes, mesosomes, granules, vacuoles and the nuclear body.

Besides these essential components, some bacteria may possess additional structures like capsule flagella and fimbriae.


It is a tough and rigid structure surrounds the bacteria like a shell and gives a particular shape to a bacterium.

It protects the internal structures of the bacterial cell.

It is about 10 – 25nm in thickness.

It weighs about 20 – 30% of the dry weight of the cell.

Chemically the cell wall is composed of peptidoglycan (mucopeptide or murein) which is composed of two amino acids – N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) alternatively arranged and cross-linked by a peptide bond.

Gram-negative bacteria contain lipopolysaccharide, gram-positive bacteria contains teichoic acid in cell wall whereas acid-fast bacilli contain mycolic acid in the cell wall.

Due to these differences in the chemical nature of the bacterial cell wall, bacteria differ in staining property.

The cell walls of gram-positive bacteria have simpler chemical nature than those of gram-negative bacteria.


It acts as a semi-permeable membrane – controls the inflow and outflow of metabolites to and from the cytoplasm

It is 5-10nm thin and contains enzymes necessary for cell wall synthesis.

Chemically, the membrane consists of lipoproteins with small amounts of carbohydrates.


It is a viscous watery solution containing a variety of organic and inorganic solutes.

It differs from the eukaryotic cytoplasm in not exhibiting internal mobility (protoplasmic streaming) and in the absence of endoplasmic reticulum or mitochondria.

It contains Ribosomes, Mesosomes, Intracytoplasmic inclusions, and vacuoles.

  • Ribosomes – These are the sites of protein synthesis having sedimentation constant 70S; made up of 50S and 30S subunits of Ribosomes.
  • Mesosomes – Also called Chondroid, are vesicular, multi-laminated or convoluted structures formed as invaginations of the plasma membrane into the cytoplasm. Principal sites of respiratory enzymes in bacteria – analogous to mitochondria in eukaryotes. More prominent in gram-positive bacteria.
  • Intracytoplasmic inclusions – these are the storage form of food material in bacteria which may be of various types, the chief of which are volutin, polysaccharide, lipid and sulfur granules. These are characteristic of different species and depend on the age and condition of the culture.

Volutin granulesPolyphosphate/ metachromatic/ babes-Ernst granulesCorynebacterium, spirillum, rhizobium and bacillus.
PolysaccharidesPolyglucan or Iodophillic granulesClostridium, Citrobector, Escherichia, klebsiella
Lipid granulesSudanophillic or poly-beta-hydroxy butyrate granulesRhizobium, bacillus, alcaligenes and soil bacteria
Sulfur granulesSulfur GlobulesThiomagrita, Thiobacillus.

  • Vacuoles – These are fluid-filled cavity present in the bacteria. It is separated from the cytoplasm by a membrane. Gas vacuoles are gas-filled cavities present in bacteria and provide buoyancy to organisms in the aquatic habitat.


Bacteria do not possess a true nucleus.

It has no nuclear membrane or nucleolus

It is a single large molecule of dsDNA which is coiled and resides freely inside the cytoplasm.

Some bacteria possess extrachromosomal genetic material consist of DNA, called as plasmids or episomes.


It is an amorphous viscid secretion of bacteria which surrounds them as the outermost layer.

When it remains as loose un-demarcated secretion, it is known as the slime layer.

When this secretion is organized into a sharply defined structure, it is known as the capsule.


Motile bacteria possess one or more unbranched, long sinuous filaments called flagella, are cytoplasmic appendages protruding from the cell wall.

They are the organ of locomotion in the bacterium.

Composed of a protein – flagellin, 5-20mm in length and 0.02mm in diameter.

Arrangement of flagella varies species to species, which is mainly of types as follows –

  • Monotrichous – Presence of one flagellum at one pole, e.g.
  • Amphitrichous – Presence of tufts of flagella at both ends, e.g. Spirillum.
  • Lophotrichous – Presence of tuft of flagella at one pol, e.g. Pseudomonas.
  • Peritrichous – Presence of many flagella surrounding the outer surface of the body, e.g. Bacillus.


These are very fine hair like appendages projecting from the surface as straight filaments.

They are shorter and thinner than flagella, i.e. 0.1 – 1.0mm long and less than 10nm thick.

Fimbriae are found in some gram-negative bacteria.

There are 3 types of fimbriae have been identified – common, ‘sex’ Pili and col I (colicin) pili.


Some bacteria have the ability to form the highly resistant resting stage during unfavorable environmental conditions called as Spores.

Bacterial spores are formed inside the bacterial cell, are called as endospores.

Sporulation is the process by which bacterial cell forms spore under unfavorable conditions.

Spore develops from a part of protoplasm is called as forespore near one end of the cell and the remaining part of the cell is called sporangium.

Shape and position of spores –

»Shapes – Oval or Spherical

»Position –

  1. Non- bulging spores – central/ sub-terminal/ terminal
  2.  Bulging spores – central/ sub-terminal/ terminal

Bacterial spores are highly resistant to ordinary boiling, disinfectants, and beating.

Germination is the process of conversion of bacterial spores into a vegetative cell under suitable conditions.

 For e.g. – Bacillus stearothermophillus, Clostridium perfringes, Clostridium tetani.

That’s all about Introduction & Structure of Bacteria


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