E. coli Gram Stain: Introduction, Principle, Procedure and Result Interpretation

E. coli in gram stain

Introduction of E. coli 

E. coli in Gram stain as shown above picture. They are bacteria that come under the family Enterobacteriaceae. They are normally found in the environment, foods, and intestines of people and animals. They are a large and diverse group of bacteria. They are normal flora when they are in our intestines. But if they spread to another part of your body, they can cause severe infections like urinary tract infections (UTIs), diarrhea, pyogenic infections, septicemia, respiratory illness and pneumonia, and other illnesses. Gram stain is a differential stain and therefore it uses to differentiate Gram-positive and Gram-negative bacteria. It was devised originally by a Danish bacteriologist, Hans Christian Joachim Gram (1884) as a method of staining bacteria in his laboratory. The morphology of these bacteria is Gram-negative rods measuring  1-3 µm × 0.4-0.7 µm. Most strains are motile by peritrichate flagella. They are non-sporing and non-encapsulated.

Principle of Gram Stain

The reaction is dependent on the permeability of the bacterial cell wall and cytoplasmic membrane, to the dye–iodine complex. In Gram-positive bacteria, the crystal violet dye iodine complex combines to form a larger molecule which precipitates within the cell. The alcohol /acetone mixture which acts as a decolorizing agent causes dehydration of the multi-layered peptidoglycan of the cell wall. This causes a decrease in the space between the molecules causing the cell wall to trap the crystal violet iodine complex within the cell. Hence the Gram-positive bacteria do not get decolorized and retain primary dye appearing violet.

Also, Gram-positive bacteria have more acidic protoplasm and hence bind to the basic dye more firmly. In the case of Gram-negative bacteria, the alcohol, being a lipid solvent, dissolves the outer lipopolysaccharide membrane of the cell wall and also damages the cytoplasmic membrane to which the peptidoglycan attaches. As a result, the dye-iodine complex does not retain within the cell and permeates out of it during the process of decolonization. Hence, when a counterstain uses, they take up the color of the stain and appear pink.

Requirements for Gram Stain of 

a) Compound light microscope

b) Reagents and glasswares

  • Bunsen flame
  • Wire loop
  • Clean grease-free slides
  • Marker pen
  • Crystal violet (Basic dye)
  • Gram’s iodine(mordant)
  • 95% ethanol (decolorizing agent)
  • 1% safranin or dilute carbol fuchsin or neutral red

c) Quality control strains

Positive Control (PC) : Staphylococcus aureus (ATCC 25923)

Negative Control (NC): Escherichia coli (ATCC 25922)

d) Specimen ( Overnight culture of E. coli  on MacConkey agar)

Preparation of Bacterial Smear: from the Solid Medium

  • Take a clean, and grease-free slide for making a smear.
  • Take a loopful of 0.85% saline i. e. physiological saline and place it on the center of the slide.
  • With a straight wire touch the surface of a well-isolated colony from the solid media and emulsify in the saline drop forming a thin film.
  • Allow the smear to air dry.
  • Heat fix the smear while holding the slide at one end, and by quickly passing the smear over the flame of the Bunsen burner two to three times.

Procedure of Gram Stain for E. coli 

  1. Cover the smear with crystal violet and allow it to stand for one minute.
  2. Rinse the smear gently under tap water.
  3. Cover the smear with Gram’s iodine and allow it to stand for one minute.
  4. Rinse smear again gently under tap water.
  5. Decolorize the smear with 95% alcohol.
  6. Rinse the smear again gently under tap water.
  7. Cover the smear again gently with safranin for one minute.
  8. Rinse the smear again gently under tap water and air dry it.
  9. Observe the smear first under the low power (10X) objective, and then under oil immersion (100X) objective.


Positive Control:   violet color, round in shape in single, pairs and cluster

Test: red color and rod in shape

Negative Control: red in color and rod in shape

Result and Interpretation

Gram-positive: purple or violet color

Gram-negative: Pink or red in color

Cocci: round in shape

Bacilli: rod in shape

Positive Control(PC): Gram-positive cocci in single, pairs and cluster

Test: Gram-negative bacilli

Negative Control(NC): Gram-negative bacilli as shown above image.

Further Readings

  1. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
  2.  Manual of Clinical Microbiology. Editors: P.R. Murray, E. J. Baron, M. A. Pfaller, F. C. Tenover and R. H. Yolken, 7th ed 2005, Publisher ASM, USA
  3.  Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.
  4. Clinical Microbiology Procedure Handbook Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
  5. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
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