Which scientist developed the Gram stain, producing purple Gram-positive and pink Gram-negative results?

• A. Christian Gram
• B. Louis Pasteur
• C. Robert Koch
• D. John Snow

Answer: (A)

The fundamental idea is that bacteria differ in their cell wall structure, and that difference can be revealed by a differential stain. The Gram stain uses crystal violet and iodine to form a complex that binds strongly to thick peptidoglycan in certain cell walls. When alcohol is applied, these thick-walled, Gram-positive cells retain the purple complex, while Gram-negative cells—with thinner peptidoglycan and an outer membrane—lose the purple and take up a counterstain, usually pink or red, making them appear pink. This creates the characteristic purple Gram-positive and pink Gram-negative appearance under the microscope and is the basis for classifying bacteria by this Gram reaction. The person who developed this staining method is Christian Gram. He introduced this differential approach in 1884, which transformed bacterial classification. Other early microbiologists like Louis Pasteur, Robert Koch, and John Snow made essential contributions to the field—germ theory, postulates, epidemiology—but they did not develop the Gram stain.