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    Salmonella appears as pink rods under the microscope. It is gram-negative rod-shaped bacilli.

    • Cell wall

    Gram-negative bacteria have different cell wall than gram-positive bacteria. Because of the change in cell wall stricture, the bacteria are classified gram-positive bacteria and gram-negative bacteria. The gram-negative bacteria cell wall has some differences from the gram-positive cell wall. They have single layer of peptidoglycan. The cell wall thickness is 70-120 Å. It is two layered cell wall that has 20-30% lipid content and is 10-20% murein content. There are three components of the gram-negative bacterial cell wall.

    1. Peptidoglycan
    2. Lipopolysaccharides
    3. Teichoic acid
    4. Peptidoglycan:

    Peptidoglycan is the monomeric form of carbohydrate that is only found in bacteria only. They are the complex polysaccharides that synthesize the cell wall of bacteria. This peptidoglycan is the polymers of sugar and amino acids. The carbohydrates that are N-acetylglucosamine and N- acetylmuramic acid are linked with the amino acids. Three to five peptide chains linked the sugar and protein together. These amino acids are homopeptide and are similar to each other. These polymers combine to give the structure of the bacterial cell wall. Gram-negative bacteria have low content of peptidoglycan. They have a thin layer of peptidoglycan that is present in the cell wall. Gram-positive bacteria have high content of peptidoglycan that is present in their cell walls.

    1. Lipopolysaccharides:

    The outer membrane is present in gram-negative bacteria that surround the cell wall of bacteria.  This membrane is composed of lipo-protein, lipopolysaccharides, and phospholipids. The lipopolysaccharide is composed of two portions i.e. polysaccharides and lipid A portion. Lipid A portion of this membrane is an endotoxin in nature. This endotoxin is the distinguishing feature of gram-negative bacteria. The lipid A portion induces fever and shock by lowering the blood pressure of whole body. Vasodilation of blood vessels occurs when the whole BP of body becomes low. This can cause death of the organism.


    This membrane has many pores that are called as porins. Porins are permeable lipoproteins and allow the transportation of substrate and nutrients to move in the cell. These porins can either be selective or non-selective. Selective porins allow the transportation of selective substances while non-selective porins allow any substance to move in the cell. The outer membrane has many structures that help the bacteria to attach to the surfaces.

    Periplasmic space;

    Periplasmic space is the area between the outer membrane and cell wall that is present in gram-negative bacteria. This space plays a vital role in the bacteria as it stores macromolecules. Macromolecules help in the metabolism of cell and environmental responses. It contains many binding proteins that bind amino acids, vitamins, iron, and enzymes. These substances are essential for the nutrition of bacteria.

    1. Teichoic acid:

    Teichoic acid is present in the gram-negative bacterial cell wall. It is present in minute quantity and present as lipo-teichoic acid. These are polymers of polyglycerol that are supplemented with phosphates and some sugar residues. Their function is unknown up till now. Some theories revealed that they are essential for the assembly of peptidoglycans monomers in the cell wall.

    Staining of bacteria:

    Gram-negative bacterial cell wall retains the secondary dye color. As they have thin layer of peptidoglycan, primary stain and mordant can’t bind with the peptidoglycan and makes a weak CV-PG-I complex. This complex and lipids of the cell wall are washed by using alcohol. Saffarine gets space in the cell wall and binds with them. These bacteria appear as pink color under the microscope.

    • Bacterial morphology:

    Bacterial morphology deals with the shape of bacteria.

    1. Bacilli:

    These are rod-shaped bacteria under the microscope. Their width and length vary from bacteria to bacteria. They resemble as pilli of bacteria. They may have square ends or round ends.

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