Wednesday, 6 July 2011

Difference between gram positive and gram negative bacteria

Thick and Thin (peptido_glycan)layers
The distinguishing characteristic of Gram positive bacterium is the composition of its cell wall – several peptidoglycan layers joined together forming a thick and rigid structure. By contrast, gram-negative bacteria have only a thin peptidoglycan layer.

The cell wall of the gram positive bacterium has (teichoic acids)
  • Teichoic acids have two types: lipoteichoic acid and wall teichoic acid.

  • Lipoteichoic acid traverses the peptidoglycan layer and is physically connected to the plasma membrane.

  • wall teichoic acid does not traverse the peptidoglycan layer and is physically connected to this layer alone, never touching the plasma membrane. 

  • Both types of teichoic acids are negatively charged because they contain the phosphate group in their molecular structure.

          teichoic acid role :
  1. Because of the nature of their charges, they play a role in binding and regulating the movement of positive ions (cations) into and out of the cell.

  2. They also help in promoting cell growth as well as preventing extensive wall damage and possible cell lysis

  3. teichoic acids contribute to the bacterial cell wall’s antigenic specificity, making it very possible to identify gram positive bacteria in various laboratory tests.
see image of teichoic acid 

characteristic of gram negative bacterial cell wall
  1. outer membrane and few peptido_glycan layers
The presence of an outer membrane and the possession of only few peptidoglycan layers in the cell wall distinguish Gram-negative bacteria from Gram-positive ones.
  • lipoproteinare the molecules that bind the peptidoglycan to the outer membrane.

  • peptido_glycan found in the preiplasm
  • preiplasm , a space filled with fluid located between the plasma membrane and the outer membrane. A high amount of degradative enzymes and transport proteins are found in the periplasm.

  • no teichoic acid in the gram negative bacterium cell wall.
  • N.B.the cell walls of Gram-negative bacteria are more prone to mechanical breakage because of the low amount of peptidoglycan
  • LPS (lipopolysaccharides):The outer membrane of a Gram-negative bacterium is composed of lipopolysaccharides (LPS), phospholipids, and lipoproteins. 

    The polysaccharide components of outer membrane’s LPS serves as bacterial antigens and are very helpful in identifying species of Gram-negative bacteria in the laboratory.

    The polysaccharide components of outer membrane’s LPS serves as bacterial antigens and are very helpful in identifying species of Gram-negative bacteria in the laboratory. There are certain laboratory tests that detect antigens specific for a single species. The LPS is therefore significant in medical diagnosis of pathogenic infections.

  • porinsThe outer membrane is actually permeable to nutrients due to the presence of porins, proteins that form channels toward the cytoplasm. Porins allow the entry of valuable molecules like disaccharides, nucleotides, peptides, amino acids, iron, and vitamin B12 but it prevents the entry of other molecules, especially the bigger ones.

    see image to illustrate what mentioned before


Gram positive
appear- blue to purple
Structure: Have thick layer of peptidoglycan over inner cytoplasmic membrane. 

lack LPS-lipopolysaccharides

Gram negative

appear: pink to red
Structure: In gram negative bacteria the peptidoglycan layer is thinner and is located between space of the outer and inner cytoplasmic membrane.

cell wall contains LPS which make them virulent



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