Gram-positive bacteria retain the purple stain in Gram staining due to their thick peptidoglycan layer, which has low lipid content. When treated with alcohol:
• The alcohol dehydrates the peptidoglycan layer, causing it to shrink and trap the crystal violet-iodine complex.
• Gram-negative bacteria, with higher lipid content, lose the outer membrane and the stain during alcohol treatment.
This property differentiates Gram-positive and Gram-negative bacteria in microbiological analysis.