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Gram staining

Duration: 10:05

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Jan 13 2023, 8:16 pm

Very nice and informative



Feb 14 2020, 1:22 am

Which method is used for staining mycobacteria?



May 09 2020, 8:11 pm

We warm the plate, add gram stain. Let sit for 20-30 seconds, add iodine to fix the dye then alcohol and finally saffronine. 

Gram positive bacterias turn up purple and gram negative are red. 



May 10 2020, 8:28 pm

I think to diagnose mycobacterium (TB) you have to do an acid fast test. Mycobacterium doesn't stain well because of it thick lipid wall. Therefore, if the gram stain DOESN'T stain, follow up with acid fast r/o mycobacterium. If that isn't getting you anywhere, a wet mount prep may lead you to fungal infection. Please correct me if I'm not completely accurate here.

Mechanism of Gram staining - Dr. Mobeen Syed

Put bacteria on plate (if from another plate then dilute agar first);

  1. Warm the plate to fix bacteria;

  2. Add gram stain; 20-30 seconds; wash it;

  3. Add iodine (mordant)

  4. Add drops of  alcohol, to wash out the gram negative bacteria’s color - 20 - 40 seconds;

  5. Put safranin (counter stain) 20-40 seconds

  • What Happens?

    • * Gram positive (+ive) bacteria turns purple
    • * Gram negative(-ive) bacteria turns red

Go to TC 2:57 in the video for illustration of how it happens:

  • * Covering the Gram +ive plasma membrane (phospholipid bi-layer), there are up to 60 peptidoglycan (PG) layers.
  • * The gram -ive plasma layer is covered by up to three PG layers and an outer membrane containing lipopolysaccharides.
  • * Beta-Lactamase is the most important factor in the periplasmic space, enzymes that can break down penicillins. 

What happens to the gram stain?

  • *The small molecules of dye get trapped in the PG layer of gram +ive bacteria and in both the outer membrane and the PG layers of the gram -ive bacteria.
  • * At this stage both gram +ive and gram -ive look purple under the microscope.
  • * When iodine (+) is added, it attaches to the molecules of dye (-) and creates a bigger molecule, which gets trapped in the PG layers.  Color does not change.
  • *When alcohol is added it dissolves the lipid portion and dehydrates the cells.
  • * In the gram +ive cells the water is pulled out by the alcohol and the 60 layers are compressed, trapping the molecules of dye, keeping the purple color
  • * In the gram -ive cells, the alcohol dissolves the more abundant lipids, and the three PG layers are too weak to trap the dye molecules so they escape, making the cells colorless.
    NB: Using too much alcohol on the gram +ive cells will eventually break down the PG layers allowing the dye to escape so you have to be careful. 
  • *The final step is to add safranin, a counterstain, that will turn the colorless gram -ive cells red, but will not be absorbed by the compressed gram +ive cells which stay purple.

And that is how gram staining works!

Learning objectives of this video are the following:

1. Review of the procedure of gram staining.

2. Results of gram staining in case of gram positive and gram negative bacteria. 

3. How gram staining works? 

Presented by Dr. Mobeen Syed

Following answers are created by ChatGPT. Occasionally the answer may be harmful, incorrect, false, misleading, incomplete, or limited in knowledge of world. Please contact your doctor for all healthcare decisions. Also, double check the answer provided by the AI below.


In addition to the presenter, following authors may have helped with the content writing, review, or approval:

  • Dr. Mobeen Syed

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The DrBeen Corp designates this enduring material for a maximum of 0.25 AMA PRA Category 1 Credits™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

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Faculty Disclosures
Dr. Mobeen Syed Author declares no conflict of interest.

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Dr. Mobeen Syed

Dr. Mobeen Syed

MD., MSc., MSc., BSc

Mobeen Syed is the CEO of DrBeen Corp, a modern online medical education marketplace. Mobeen is a medical doctor and a software engineer. He graduated from the prestigious King Edward Medical University Lahore. He has been teaching medicine since 1994. Mobeen is also a software engineer and engineering leader. In this role, Mobeen has run teams consisting of hundreds of engineers and millions of dollars of budgets. Mobeen loves music, teaching, and doing business. He lives in Cupertino CA.


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