Who developed the staining technique that divides bacteria into two major categories?

• A. Christian Gram
• B. Louis Pasteur
• C. Robert Koch
• D. Antonie van Leeuwenhoek

Answer: (A)

Gram staining is the differential method that sorts bacteria into two main groups based on cell wall structure. It was developed by Hans Christian Gram. The test uses crystal violet to stain all cells, then a mordant with iodine to fix the dye. After a brief alcohol wash, the thick-walled Gram-positive bacteria retain the violet color, while the thinner-walled Gram-negative bacteria lose the dye and take up a red counterstain, usually safranin. This simple distinction reflects fundamental differences in cell envelope chemistry and provides an immediate, practical framework for identification and subsequent testing. While Pasteur, Koch, and van Leeuwenhoek made pivotal contributions to microbiology, none authored this staining technique.