Bacteria are classified based on a chemical stain that can be seen through the microscope. Bacteria that turn purple under the microscope are called “gram-positive” and those that turn pinkish or red under the microscope are called “gram-negative.”
The Gram stain test was developed by Hans Christian Gram in the late 1800’s. He found that when he stained some bacteria turned purple under the microscope. These were called “gram-positive.” Other bacteria didn’t turn purple and appeared pinkish or red under the microscope. These were called “gram-negative.”
Whether and how the stain attaches to the bacteria or not, is related its structure. Gram positive bacteria, like those that cause strep throat or many skin infections, have a thick wall made of a protein that retains the chemical in the purple dye. Gram negative bacteria, like those that cause cholera or urinary tract infections, has two protective walls, making it harder to penetrate, and will look red or pink under a microscope.