Structural components of Bacterial cell wall

Structure and components of Bacteria cell, Pinterest photo

The bacteria cell wall is associated with several structural components among which are,

·       Peptidoglycan (Mureim)

·       Cytoplasmic membrane

·       DNA

·       Storage granules

·       Ribosome

·       Flagella

·       Capsule and

·       Pilli

Peptidoglycan (Murein):- This is the principle structure of bacterial cell wall; it is a large mesh like polymer made up of many identical sub units. The polymer contains two sugar derivatives, N-acetyl Glucosamine (NAG) and N- acetyle Muramic acid (NAM), as well as several amino acids three of which are not found in proteins, D-glutamic acid, Mesodiaminopimelic acid (DAP) and D- alamin.  Peptidoglycan occur in both Gram-positive and Gram - negative bacteria although more abundant in Gram – positive bacteria.  A peptide chain of 4 alternating D-L -amino acid is connected to the carboxyl group of NAM, some bacteria replaces the DAP with L-lysine. Often, the carboxyl group of the terminal D-alamine is directly connected to amino group of DAP, to increase the strength of the structures between the peptide and cross linkages, although sometimes a peptide linkage is used instead.

·       Gram - positive cell wall:-  This group of bacteria has a thick cell wall made up of Peptidoglycan that contains a peptide  chain together with  teichoic acid. Teichoc acids is present in large amount and are found on the surface of peptidoglycan, and because it is negatively charged makes the Gram- positive  cell wall negative during Gram staining procedure.

·       Gram - negative cell wall:- Gram - negative bacteria cell wall is more complex than  that of Gram - positive bacteria because of the presence of additional outer  membrane external to the peptidoglycan layer made up of Lipopolysacharride (LPS).

Cytoplasmic membrane:-  Cytoplasmic membrane also know as cell membrane or plasma membrane is the physical and metabolic barrier between the interior and exterior of bacteria cell, it exhibits a well define selective permeability. The principle energy bacteria system- the bacteria electron transport system is located in the plasma membrane. It has several functions which includes

·       Elimination of waste product from the cell

·       Assist in passive diffusion

·       Contains enzymes important in uptake of nutrient.

Cytoplasmic components of bacteria cell wall

Cytoplasmic membrane of most bacteria contains DNA, ribosome and storage granules. Many of the complex organelles found in eukaryotic cell are absent in prokaryote because the plasma membrane performs many of the functions of organelles present in eukaryotic cell.

      

 DNA: - Nuclear membrane is absent in bacterial cell, thus the DNA is concentrated in the cytoplasm as a nucleoid and consists of a double- stranded, circular and covalently closed  super coiled  DNA  molecule. In some bacteria, a small portion of the DNA molecule persist as extrachromosomal element known as plasmid, Plasmids are also circular but smaller than the bacteria chromosome, they encode different types of genes and are known to be instrumental in the transmissions of special properties such as antibiotics drug resistance, resistance to heavy mental and virulence factor necessary to cause infection.

        Storage granules:-  Storage granules are also known as inclusion granules.  They are not a permanent structure of bacteria cell; they function in the storage of excess metabolite. Their number in a cell differs with the species of bacteria and their metabolic activity.

         RIbosomes:-  Prokaryotic ribosomes  are complex globular structures  made up of several RNA molecules and several proteins. They are important in proteins synthesis. Bacteria ribosomes are approximately 20nm in diameter and their coefficient is 70 Svedberg units(S). They are made up of two subunits with a sedimentation Coefficient of 50s and 30s.

  The cells also contains mesosomes which are complex invagination of   of cytoplasmic membrane that are important in cell division.

       External Structures of the Bacteria Cell Wall

Flagella:- Bacteria possesses  flagella which serve mainly in locomotion and is responsible for their motility. This structure is immunogenic, and must bacteria have developed a system of antigenic variation that enables them switch between types.

 There are three kinds of flagella based on their distribution; their main component is the flagellin.  We have the 

·       Peripericule,  which are distributed on the surface of the bacterium

·       Monotrichous, which are single flagella attached to the bacterium

·       Polar flagella which are bundled at one or both end of the bacterium.

·       Ampietrichous, this is when the bacterium have one flagella  on each end of its pole.

Flagellum are made up of three parts, 

·       Filament: - the longest and most visible part which extends from the cell surface to the tip.

·       Basal body:- This part is embedded in the cell

·       Hook:-Hook is short and curved; it joins the flagella to the basal body and act as a flexible coupling.

Capsule: - Capsule surrounds the bacteria cell wall and is composed of complex polysaccharide which is visible on the microscope using Quelling (swelling) reaction. Virulent bacteria such as the Bacillus Anthrax have a polypeptide capsule composed of D-glutamic acid which are resistant to proteolytic enzymes enabling the capsule to resist phagocytosis. 

Pili:- Pili are also known as fimbriae,  they are hair like appendage that are smaller than flagella found on many Gram- negative  bacteria. They are only seen on electron microscope due to their micro size.

They occur as slender tubes of helically arranged protein. Some pili play an important role in helping the bacterium attach itself to solid surfaces such as rock and host tissues. There are two types of pili, common and sex pili.

·       Common pilli:-   Help bacteria attach to mucosal surface during host infection.

·       Sex pili :-  These pill occur in small number and are usually coded by plasmid.

·       sThey are important in bacteria reproduction (conjugation).