Why Grams Positive bacteria appear as Grams Negative ?

Gram stain is one basic & simple microbiology procedure done in all microbiology laboratories.

Yes , it is simple but gram stain result really important to select other advance microbiology procedures . Also this test gives most significant clue about organism. So gram stain result is really  important thing you should consider.



Anyway some occasions, grams positive bacteria can be appear  as grams negative …

This can occur due to 4 main  reasons ,

  1. Most commonest reason behind it is , In Gram staining , we use the decolourizing agent Iodine Acetone. If we use Acetone alone or if we decolourize for prolong period of time the gram positive cells can be appeared as gram negative.
  2. Another reason is the use of old cultures to prepare gram stain smear. In old cultures, the cell wall of bacteria get weakened and degenerated due to time , therefore logose iodine fix not happen & grams positive organism appear as , Grams negative.

Therefore , you should always select fresh culture colonies to gram stain test.


  1. If the micro organisms are exposed to lysozyme or cell wall acting antibiotics such as penicillin, their cell wall get damaged. in such cases also gives gram negativity , even if it is gram positive bacteria.
  1. The lysozymal content of phagocytic cells causes the cell wall of the engulfed bacteria to be damaged. Therefore this occasion can be a reason for gram potive to appear as gram Negative.

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