What is the Gram stain and what does it indicate?

The Gram stain is a differential staining technique that classifies bacteria based on cell wall structure. The procedure involves: fixing cells, applying crystal violet (primary stain), adding iodine (mordant), decolorizing with alcohol/acetone, and counterstaining with safranin. Gram-positive bacteria (thick peptidoglycan layer) retain crystal violet, appearing purple. Gram-negative bacteria (thin peptidoglycan, outer membrane) lose the primary stain and appear pink/red from safranin. The distinction is important for: antibiotic selection (different susceptibilities), understanding pathogenicity, taxonomic classification, and process development. Gram staining is a fundamental quality control test for culture purity.