Describe briefly the structure of bacteria?

In this post we are discussed briefly the structure of bacteria.

Introduction to bacteria: 

The agents of human infectious disease belong to five major groups of organisms ,and these are fungi, bacteria, protozoa, helminths,and viruses.

Bacteria

Bacteria is present in the prokaryote kingdom.

Depending upon the shape, bacteria are classified into three basic groups, these are cocci, bacilli, spirochetes.

The size of bacteria varies from about 0.2 to 5 μm.

The size of the smallest bacteria (Mycoplasma) and the largest viruses (poxviruses) are almost same.The longest bacteria rods are the size of some yeasts and human RBC.

Structure of bacteria:

File:Simple diagram of bacterium (en).svg , Image by-domdomegg , Source, License-CC BY-4.0

Cell wall of the bacteria: 

This the outermost component which is present in all the bacteria (except Mycoplasma species).

The significance of this complex bacterial cell wall are to protect the protoblasts from external noxae, which then help to withstand and to maintain the osmotic pressure gradient between the cell interior and the extracellular environment, to give the cell it's outer form and to facilitate communication with its surrounding.

Some bacteria have surface features external to the cell wall, such as capsule, flagella and pili.

The cell wall is located external to the cytoplasmic membrane and is made up of peptidoglycan.

Cytoplasmic membrane of bacteria:

The cytoplasmic membrane is present just inside the peptidoglycan layer of the cell wall.It is composed of phospholipid bilayer similar in microscopic appearance to that in eukaryotic cells.

Cytoplasm of bacteria:

When observed through the electron microscope it is seen that the cytoplasm of the bacteria has two distinct areas , these are,

1. An amorphous matrix, in which ribosomes, nutrient granules, metabolites and plasmids are present.
2. An inner, nucleoid region which is composed of DNA.

Ribosomes of bacteria:

The ribosomes of the bacteria are the place, where protein synthesis takes place, like the eukaryotic cells. But the chemical composition and the size of the bacterial are different from that of eukaryotic ribosomes.

The size of bacterial ribosomes is 70S , and contains 50S and 30S subunits, but the size of the eukaryotic ribosomes are 80S , and it contains 60S and 40S subunits.

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Granules present in the bacteria:

There are several different types of granules present in the cytoplasm of bacteria.It helps in the storage of nutrients.

Nucleoid:

This is the area of the cytoplasm in the bacteria in which DNA is located.The DNA which is present in the prokaryotes ,are circular molecule,and which has a molecular weight of about 2 × 10^9 and also contains about 2000 genes.

There is resemblance to the eukaryotic nucleus becuse in the nucleoid nuclear membrane, nucleolus ,mitotic spindle, histone are not present.

There is no introns present in the bacterial DNA.

Plasmids:

Plasmids are extra chromosomal, double-stranded , circular DNA molecules that are capable of replicating independently of the bacterial chromosome.

Although plasmids are usually extra chromosomal, they can be integrated into the bacterial chromosome.In both gram positive and gram negative bacteria plasmids occur, and several different types of plasmid can exist in one cell, these are 

      1.Transmissible plasmids

        2. Neurotransmissible plasmids.

Transposons:

Transposons are prices of DNA , that move fastly from one site to another either within or between the DNAs of bacteria, plasmids and bacteriophages.They are nicknamed as ''jumping genes'' because of their unusual ability to move.

Transposons can code for drug resistance enzymes, toxins or a variety of metabolic enzymes and can either cause mutations in the gene into which they insert or alert the expression of nearby genes.

There is four identifiable domains present in the transposons.

Capsule:

It is a gelatinous layer present surrounding the entire bacterium.The capsule is composed of polysaccharide (except in the anthrax bacillus).The capsule is important because of it's four important role

  1.It is determinant of virulence of many bacteria since it limits the ability of phagocytes to engulf the bacteria.

  2.By using antiserum against the capsular polysaccharides , specific identification of an organisms can be made.

   3.In certain vaccines capsular polysaccharides are used as antigens , because of their role in eliciting protective antibodies.

    4.In the adherence of bacteria to the human tissues, capsule has an important role, which is an important primary step in causing infections.

Flagella :

Flagella are long, whip-like appendages that help in the movement of bacteria toward nutrient and other attractants,a process which is known as chemotaxis.The long filament, which acts as a propeller,is composed of many subunits of a single protein,flagellin.

Flagellated bacteria have a characteristic number and location of flagella.

Flagella are medially important for two reasons

  1.Some species of motile bacteria are common causes of urinary tract infections.

  2.Some species of bacteria are identified in the clinical laboratory by the use of specific antibodies against flagellar proteins.

Pili of bacteria:

Pili are hair like filaments that extend from the cell surface.They are shorter and straighter than flagella,and are made up of pilin.

Pili have two main role:

   1.They regulate the attachment of bacteria with the specific receptor present on the human cell surface.

   2.A specialized kind of pilus,the sex pilus, forms the attachment between the male and the female bacteria during conjugation.

Glycocalyx(Slime layer) of bacteria:

  It is a polysaccharide coating that is secreted by many bacteria. The glycocalyx is an important component of biofilms.

The glycocalyx mediates adherence of certain bacteria.

 Bacterial spores: 

These highly resistance structures are formed in response to adverse conditions by two genera of medially important gram positive rods: the genus Bacillus and the genus Clostridium.

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