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6.1: Introduction

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    The Gram stain is the most widely used staining procedure in bacteriology. It is called a differential stain since it differentiates between Gram-positive and Gram-negative bacteria. Bacteria that stain purple with the Gram staining procedure are termed Gram-positive; those that stain pink are said to be Gram-negative. The terms positive and negative have nothing to do with electrical charge, but simply designate two distinct morphological groups of bacteria.

    Gram-positive and Gram-negative bacteria stain differently because of fundamental differences in the structure of their cell walls. The bacterial cell wall serves to give the organism its size and shape as well as to prevent osmotic lysis. The material in the bacterial cell wall which confers rigidity is peptidoglycan.

    In electron micrographs, the Gram-positive cell wall appears as a broad, dense wall 20-80 nm thick and consisting of numerous interconnecting layers of peptidoglycan (Figures \(\PageIndex{1A}\) and \(\PageIndex{1B}\)). Chemically, 60 to 90% of the Gram-positive cell wall is peptidoglycan. Interwoven in the cell wall of Gram-positive are teichoic acids. Teichoic acids, which extend through and beyond the rest of the cell wall, are composed of polymers of glycerol, phosphates, and the sugar alcohol ribitol. Some have a lipid attached (lipoteichoic acid). The outer surface of the peptidoglycan is studded with proteins that differ with the strain and species of the bacterium.

    Fig. \(\PageIndex{1A}\) : Electron Micrograph of a Gram-Positive Cell Wall

    Fig. \(\PageIndex{1B}\): Structure of a Gram-Positive Cell Wall

    Electron micrograph of a Gram-positive cell wall. Illustration showing the structure of a Gram-positive cell wall.
      The Gram-positive cell wall appears as dense layer typically composed of numerous rows of peptidoglycan, and molecules of lipoteichoic acid, wall teichoic acid and surface proteins.
    (Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0)

    The Gram-negative cell wall, on the other hand, contains only 2-3 layers of peptidoglycan and is surrounded by an outer membrane composed of phospholipids, lipopolysaccharide, lipoprotein, and proteins (Figures \(\PageIndex{2A}\) and \(\PageIndex{2A}\)). Only 10% - 20% of the Gram-negative cell wall is peptidoglycan. The phospholipids are located mainly in the inner layer of the outer membrane, as are the lipoproteins that connect the outer membrane to the peptidoglycan. The lipopolysaccharides, located in the outer layer of the outer membrane, consist of a lipid portion called lipid A embedded in the membrane and a polysaccharide portion extending outward from the bacterial surface. The outer membrane also contains a number of proteins that differ with the strain and species of the bacterium.

    Fig. \(\PageIndex{2A}\) : Electron Micrograph of a Gram-Negative Cell Wall

    Fig. \(\PageIndex{2B}\): Structure of a Gram-Negative Cell Wall

    Electron micrograph of a Gram-negative cell wall. Illustration showing the structure of a Gram-negative cell wall.
      The Gram-negative cell wall is composed of a thin, inner layer of peptidoglycan and an outer membrane consisting of molecules of phospholipids, lipopolysaccharides (LPS), lipoproteins and sutface proteins. The lipopolysaccharide consists of lipid A and O polysaccharide.
    (Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0)

    For further information on the Gram-negative and Gram-positive cell wall, see the following Learning Objects in the Lecture Guide:


    6.1: Introduction is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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