what are gram +ve and gram -ve bacterias?????

 


Bacterial Morphology

Bacterial Structure 

:: Cytoplasmic Structures ::

 Gram positive and Gram negative bacteria have similar internal, but very different external structures. The cytoplasm of the bacterial cell contains the DNA chromosome, the mRNA, ribosomes, proteins, and metabolites). Unlike eukaryotes, the bacterial chromosome is a single, double-stranded circle that is contained not in a nucleus but in a discrete area known as the nucleoid. Histones are not required to maintain the conformation of the DNA, and the DNA does not form nucleosomes. Plasmids, which are smaller, circular, extrachromosomal DNAs, may also be present. Plasmids are most commonly found in Gram negative bacteria, and although not usually essential for cellular survival, they often provide a selective advantage: many confer resistance to one or more antibiotics

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Gram-positive and Gram-negative bacteria are classified based on the ability to retain the gram stain. The gram-positive bacteria would retain the gram stain and observed as violet color after the application of iodine (as mordant) and alcohol (Ethanol). On the counterpart, the gram-negative bacteria would be stained by counter stain (safranin), as they are destained due to alcohol wash. Thus, they appear as pink color under a microscope.

Difference between Gram-positive and Gram-negative bacteria:

Gram positive bacteria

In most of the gram-positive bacteria, the cell wall consist of many layers of peptidoglycan which forms a thick and rigid structure. The cell wall of the gram-positive also contains teichoic acids which is made up of alcohol (glycerol or ribitol) and phosphate.

Two types of teichoic acids are found in the gram-positive bacteria; one is the lipoteichoic acid which spans the peptidoglycan layer and is linked to the plasma membrane, and the other is teichoic wall acid, which is connected to the peptidoglycan layer. They have a negative charge on them, and thus they can bind and regulate the movement of cations across the cell membrane.

The cell wall is called murein. The peptidoglycan is made from repeating units of NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid). The repeating units of NAM and NAG make thick interlinked parallel layers to form a cell wall. The linking between the parallel layers of peptidoglycan is due to the formation of a peptide linkage between the short peptide (4 amino acids; commonly L-alanine, D-glutamate, meso-diaminopimelic acid and D-alanine) by the action of transpeptidase.

For the fact file:

The antibiotics such as penicillin do not kill the bacteria. It affects the murein cell wall by preventing its synthesis and also preventing the synthesis of trans-peptide linkages. Thus, the bacteria becomes susceptible to damage from environmental factors and the antibodies.

The antibiotic penicillin irreversibly binds to and inhibits the activity of the transpeptidase enzyme by forming a highly stable penicilloyl-enzyme intermediate. Because of the interaction between penicillin and transpeptidase, this enzyme is also known as penicillin-binding protein[1].”

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Gram negative bacteria

The cell wall is made up of few layers of peptidoglycan and an outer membrane. The outer membrane is made up of LPS (Lipopolysaccharides), lipoproteins, and phospholipids.

The peptidoglycan remains bound to lipoproteins of the outer membrane. It is present in the periplasm which is a gel-like fluid between the outer membrane and the plasma membrane. The periplasm is filled with degrading enzymes and proteins aiding in the transportation of the molecules. The cell wall of gram-negative bacteria lacks the teichoic acid.

The cell is more susceptible to mechanical breakage as compared to the gram-positive bacteria as the cell wall has thin layer of peptidoglycan. However, due to the presence of the outer membrane made up of lipoproteins and other components, the cell is not easily affected by antibodies, enzymes, metals, detergent, salts (bile salts), dyes, etc.

The outer membrane is permeable due to the presence of porins. The membrane is permeable to food, nutrition, H2O, Vitamin B12, Iron, etc.

  • The LPS of the outer membrane is made up of a large complex molecules. They consist of lipids and carbohydrates. There are three major components of LPS: Lipid A, Core polysaccharide, and the O-polysaccharide. Lipid A is the lipid portion of the LPS which is embedded in the top layer of the outer membrane. The core polysaccharide is attached to lipid A and consists of unusual sugars, providing stability. The O-polysaccharide extends outwards from the core polysaccharide and sugar molecules. It acts as an antigen and is useful for the identification of the species.​​​​​​​
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