So we see that when we discuss bacterial cells, there are two main categories of bacterial cells. We have gram positive and gram negative. And the way we differentiate these two different types of bacterial cells is by the cell envelope. The cell envelope contains our cell wall, which is made of a material known as a peptidoglycan, and it also contains the inner plasma membrane. In some cases, we also have an outer plasma membrane. Basically, our grampositive bacteria contain a very thick peptidoglycan cell wall, and that allows the purple dye to remain inside that cell.