1. Primary stain: apply crystal violet to stain all cells. 2. Mordant: add iodine to form a crystal-violet–iodine complex. 3. Decolorization: wash briefly with alcohol or acetone to remove stain from Gram-negative cells. 4. Counterstain: apply safranin (or fuchsine) so decolorized Gram-negative cells appear red/pink while Gram-positive remain purple. This differential stain distinguishes bacterial cell wall types.

Gram staining is a differential bacterial staining method that classifies organisms as Gram-positive or Gram-negative. It uses crystal violet, iodine (mordant), alcohol decolorization, and a counterstain (safranin). Gram-positive bacteria retain crystal violet due to a thick peptidoglycan cell wall and appear purple; Gram-negative have a thin peptidoglycan layer plus an outer membrane, appear pink. It aids identification and treatment decisions.

A normal Gram stain shows no pathogenic bacteria and only expected resident flora for the sampled site. Results are often reported semi‑quantitatively (none, rare, few, moderate, many or 0 to 4+). Quality criteria vary by specimen — for example, good sputum has fewer than ten squamous epithelial cells per low‑power field; cerebrospinal fluid should show no organisms.

Steps: make and air-dry a bacterial smear on a slide, heat-fix; apply crystal violet (primary stain) ~1 min, rinse; add Gram’s iodine (mordant) ~1 min, rinse; decolorize briefly with ethanol/acetone until runoff is clear, rinse immediately; counterstain with safranin ~30–60 sec, rinse; blot dry and examine under oil immersion (100×), using proper controls.

Gram-positive bacteria appear purple or violet after Gram staining because their thick peptidoglycan cell wall retains the crystal violet–iodine complex during the decolorization step. In contrast, Gram-negative bacteria lose that complex and take up the counterstain, appearing pink or red. Gram staining helps differentiate bacterial groups for diagnosis and guides initial antibiotic choices.

Staining principles: fixation to preserve and immobilize tissue/cells; selection of dyes by chemical affinity and charge (acidic dyes bind basic structures, basic dyes bind acidic ones); mordants to enhance binding; differentiation/decolorization to create contrast; counterstaining for background visualization; control of pH and solvent affecting dye behavior; thorough rinsing and proper mounting to preserve the stained preparation.