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Difference between Gram Positive and Gram Negative Bacteria

Gram positive bacteria have a thick peptidoglycan layer and no outer lipid membrane whilst Gram negative bacteria have a thin peptidoglycan layer and have an outer lipid membrane.


  • The diagram above illustrates the differences in the structure of Gram positive and Gram negative bacteria. The two key features that lead to the differing visualization properties of Gram positive and Gram negative species are the thickness of the peptidoglycan layer and presence or absence of the outer lipid membrane. 
  • This is because the wall structure affects the cell’s ability to retain the crystal violet stain used in the Gram staining procedure which can then be visualized under a light microscope.
  • As Gram positive bacteria lack an outer lipid membrane, when correctly referring to their structure rather than staining properties, are termed monoderms. The outer lipid membrane possessed by Gram negative bacteria means that, when referring to their physical structure, they are termed diderms.
  • The Gram staining technique was developed in 1884 by Danish bacteriologist Hans Christian Gram1. Whilst a Gram stain will not tell you the specific species you are looking at, it can be a quick way to narrow down greatly the list of potential candidates and direct follow-up testing where necessary.

What are the major differences in cellular structure between Gram positive and Gram negative bacteria?
  • Most bacteria can be broadly classified as Gram positive or Gram negative. Gram positive bacteria have cell walls composed of thick layers of peptidoglycan. Gram positive cells stain purple when subjected to a Gram stain procedure. Gram negative bacteria have cell walls with a thin layer of peptidoglycan.


Difference between Gram-Positive and Gram-Negative Bacteria 
  • The cell wall of gram-positive bacteria is composed of thick layers peptidoglycan.
  • The cell wall of gram-negative bacteria is composed of thin layers of peptidoglycan.
  • In the gram staining procedure, gram-positive cells retain the purple coloured stain.
  • In the gram staining procedure, gram-negative cells do not retain the purple coloured stain.
  • Gram-positive bacteria produce exotoxins.
  • Gram-negative bacteria produce endotoxins.

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