Gram-positive and Gram-negative Bacteria"Bacteria are classified by gram staining. This is a process that determines many things - resistance to antibiotics, for one thing. It is also one of the first things scientists will do when trying to identify an unknown bacteria. It involves staining a group of bacteria with four different liquids. First, crystal violet is added. Then it's stained with iodine, and finally with safranin. Then it goes through an alcohol wash. The bacteria that retain the purple stain from the crystal violet are gram-positive, and those that take on the pink stain from the safranin are gram-negative."
"This has to do with the outer layer of the cell - gram positive bacteria have a thick layer of peptidoglycan, which absorbs the gram stain. Gram-negative bacteria have a thick lipid bilayer on the outside, which is selectively permeable - not everything can pass through it, and one of those things is the gram stain. The gram-positive bacteria (since things can pass through it easily) are much more susceptible to antibiotics than gram-negative bacteria."
Enger & Ross