One of the first things you learn about in biology classes is something called a Gram stain. You have probably heard of it, and most likely knew what it was at some point in your life, but have long since forgotten.
A gram stain is a test to determine whether a bacterium or particular fungi has a thick layer of something called peptidoglycan (a structural agent) in its cell wall. Those bacteria or fungi who have this thick layer of peptidoglycan in their cell walls hold on to a violet colored dye used in the Gram stain test and are therefore called “Gram positive”. Those that do not are called “Gram negative”.
Simple? Actually, yes. When the test is all said and done, Gram positive bacteria and fungi show up under the microscope as dark violet in color, while Gram negative bacteria and fungi take on a pinkish color because they do not hold the dye. It’s not hard to distinguish what type you’re looking at, as long as you are able to tell the difference between pink and purple.