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.
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Anyway some occasions, grams positive bacteria can be appear  as grams negative …
This can occur due to 4 main reasons ,
- 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.
- 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.
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- 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.
- 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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