Materials and Reagents
| Item | Typical Quantity | Notes |
|---|---|---|
| Microscope slides (clean, frosted) | 1015 per batch | Presterilized glass is preferred. |
| Inoculating loop or sterile swab | 1 per smear | Heatsterilize before use. |
| Crystal violet solution (0.1% w/v) | 10mL per 10 slides | Store in dark bottle. |
| Iodine solution (Grams iodine, 1% w/v) | 10mL per 10 slides | Acts as a mordant. |
| Decolorizer (95% ethanol or acetone) | 150mL per 10 slides | Prepared fresh; must be 100% alcohol. |
| Safranin counterstain (0.5% w/v) | 10mL per 10 slides | Provides pink/red colour to Gramnegative cells. |
| Distilled water | As required | For rinsing steps. |
| Heat source (Bunsen burner, incubator) | For fixing slides. | |
| Microscope (oilimmersion objective 100) | Final observation. |
Safety Precautions
- Wear lab coat, gloves, and eye protection at all times.
- Ethanol and acetone are highly flammable; keep away from open flames.
- Dispose of used slides and reagents according to institutional biohazard guidelines.
- Handle bacterial cultures in a biosafety cabinet when possible.
StepbyStep Procedure
1. Preparation of the Smear
- Label each slide with a unique identifier.
- Using a sterile loop, transfer a small amount of bacterial culture (fresh colony from < 24h plate) onto the centre of the slide.
- If the organism is a liquid culture, place a 1L drop on the slide.
- Spread the inoculum into a thin, even film using a second sterile loop or the edge of the loop. Aim for a monolayer of cells.
- Allow the smear to airdry completely ( 23min).
- Fix the dried smear by passing the slide briefly (23s) through a Bunsen flame, or by heating at 60C for 10min in an incubator. Do not overheat, which can distort cell morphology.
2. Primary Stain Crystal Violet
- Flood the fixed smear with crystal violet solution, ensuring complete coverage.
- Let the stain remain for 1minute. For thicker smears, increase time to 1.5minutes.
- Rinse gently with distilled water until the runoff is clear ( 10s). Avoid vigorous streaming that could detach cells.
3. Mordant Grams Iodine
- Cover the slide with iodine solution and let stand for 1minute. The iodine complexes with crystal violet, forming a larger molecule that is less soluble.
- Rinse again with distilled water for 10s.
4. Decolorization
The critical step that distinguishes Grampositive from Gramnegative organisms. Timing and reagent concentration must be strictly controlled.
- Tilt the slide and pour the decolorizer (95% ethanol or acetone) over the smear, using enough to completely flood the surface.
- Observe the flow; the stain will begin to wash off. Allow the reagent to act for 1020seconds. For most organisms, 15seconds is sufficient; for heavily Grampositive species, a shorter time may be needed.
- Immediately rinse the slide with a copious stream of distilled water for at least 30seconds to halt the decolorization.
5. CounterStain Safranin
- Apply safranin to the slide, covering the smear completely.
- Let stand for 45seconds to 1minute. This stains any cells that have lost the crystal violetiodine complex.
- Rinse gently with distilled water for 10seconds, then blot the slide edge with bibulous paper to remove excess liquid. Do not wipe the smear directly.
6. Drying and Examination
- Airdry the slide in a dustfree area (23minutes). Do not use heat at this stage.
- Observe under a light microscope using oil immersion (100 objective). Adjust illumination to clearly distinguish dark purple (Grampositive) from pink/red (Gramnegative) cells.
Interpretation of Results
After microscopic examination, record the following observations:
- Grampositive: Cells retain the crystal violetiodine complex and appear deep violetpurple. Cell walls are thick ( 2080nm) and contain teichoic acids.
- Gramnegative: Cells lose the primary stain during decolorization and take up the safranin, appearing pink/red. Their wall consists of a thin peptidoglycan layer ( 710nm) and an outer membrane rich in lipopolysaccharide.
- Variable/Gramindeterminate: Some organisms (e.g., mycobacteria, Nocardia) may show uneven staining. In such cases, consider alternative stains (ZiehlNeelsen, acidfast).
Note any morphological characteristics (cocci versus bacilli, chains, clusters) that may aid in identification.
Common Problems and Solutions
Problem: All cells appear pink (Gramnegative) even though the organism is known to be Grampositive.
Possible causes:
Possible causes:
- Overdecolorization reduce exposure time or use a less aggressive reagent (e.g., ethanol instead of acetone).
- Thin smear a dense film can prevent proper penetration of crystal violet and iodine.
- Insufficient primary stain extend crystal violet exposure to 1.52minutes.
Problem: Cells appear purple but the smear is difficult to visualize due to clumping.
Solutions:
Solutions:
- Prepare a more diluted inoculum or use a loop to create a thinner film.
- Ensure the slide is completely dry before fixing.
Problem: Background staining (bluepurple haze) obscures cells.
Solutions:
Solutions:
- Rinse slides thoroughly after each staining step.
- Replace staining solutions regularly; contaminants can accumulate.
References & Further Reading
- Beveridge, T. J. Use of the Gram Stain in Microbiology. J. Clin. Pathol. 2013;66:847852.
- Harold, J. & Kline, K. Laboratory Techniques in Microbiology. 5th ed. Academic Press, 2020.
- American Society for Microbiology. Gram Staining Protocol. ASM Lab Manual, 2022.
