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 an outer lipid membrane.

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  • 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 the 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 of 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-colored stain.
  • In the gram staining procedure, gram-negative cells do not retain the purple-colored stain.
  • Gram-positive bacteria produce exotoxins.
  • Gram-negative bacteria produce endotoxins.

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