Principle of Gram Staining:
Gram staining differentiates bacteria based on the structure of their cell walls.
Gram-positive bacteria retain the primary stain, while Gram-negative bacteria do not, due to differences in their cell wall composition.
Procedure:
Preparation of Smear and Fixation: As in simple staining.
Primary Stain: Crystal violet is applied to the smear and left for about 1 minute.
Mordant: Iodine solution is added, which forms a complex with the crystal violet, and left for another minute.
Decolorization: The slide is washed with alcohol or acetone for a few seconds. This step differentiates Gram-positive from Gram-negative bacteria.
Counterstain: Safranin is applied for 1-2 minutes to stain the decolorized Gram-negative bacteria.
Observation:
Gram-Positive Bacteria: Appear purple or blue because they retain the crystal violet-iodine complex.
Gram-Negative Bacteria: Appear pink or red due to the uptake of the counterstain safranin after the decolorization step.
Advantages:
Differentiates a broad range of bacteria.
Guides antibiotic therapy as Gram-positive and Gram-negative bacteria often respond differently to treatments.
Limitations:
Some bacteria (e.g., Mycobacterium, Listeria) may not fit neatly into Gram categories.
Over-decolorization or under-decolorization can lead to misclassification.