Gram Stain: Principle, Steps, Results and Common Mistakes

MLT Globe · 2026-10-02

The Gram stain, developed by Hans Christian Gram in 1884, is still the most widely used stain in microbiology. It sorts most bacteria into two groups in a few minutes and often guides the first choice of antibiotic.

Principle

The difference lies in the cell wall. Gram-positive bacteria have a thick peptidoglycan layer. After crystal violet and iodine form a large dye complex inside the cell, the decolouriser dehydrates this thick wall and traps the complex, so the cell stays purple.

Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane rich in lipids. The decolouriser dissolves the outer membrane and washes the dye complex out. The cells are then stained pink-red by the counterstain.

The four steps

1. Crystal violet (primary stain) – about 1 minute, then rinse.
2. Gram's iodine (mordant) – about 1 minute, then rinse.
3. Decolouriser (acetone–alcohol or alcohol) – a few seconds, until the run-off is almost clear, then rinse at once.
4. Safranin (counterstain) – about 30–60 seconds, rinse, blot dry and examine under oil immersion.

Timings vary with the reagents, so follow your laboratory's procedure.

Reading the smear

• Gram-positive cocci in clusters – suggests staphylococci.
• Gram-positive cocci in chains or pairs – suggests streptococci or enterococci.
• Gram-negative diplococci – suggests Neisseria or Moraxella.
• Gram-negative rods – for example Enterobacterales or Pseudomonas.

Also report pus cells and epithelial cells: many squamous epithelial cells in sputum suggest a poor, saliva-contaminated sample.

Common mistakes

• Over-decolourising: Gram-positive cells look Gram-negative. This is the most common error.
• Under-decolourising or thick smears: Gram-negative cells look Gram-positive.
• Old cultures: Gram-positive bacteria with damaged walls may stain Gram-variable.
• Overheating during heat fixing: distorts the cells.

Use known Gram-positive and Gram-negative control organisms to check your reagents and technique. Read more in the lab techniques section and practise bacteriology MCQs.

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