They are different types of bacteria that require different types of antibiotics/anteseptics. The walls have different thicknesses and a few other differences. When you look at them under a microscope you dye them to see their wall types. Why this is important is that gram negative bacteria will not be killed by lets say penicillin. Gram positive bacteria will be killed by penicillin. That is why when you go to the doctor they don't prescribe penicillin for everything. So if you had a cleanser that just worked on gram positive bacteria and your sore happened to have gram negative bacteria in it the bacteria would continue to grow. Hope that made sense.