Bacteria are divided into two groups based on how their absorb stain.

Cell walls. This is a rigid layer made of cross-linked protein and sugars that surrounds the bacteria, holding it together and stopping it from exploding. The stain is called Gram's stain and is either purple (+ve) or pink (-ve). This distinction is used because it allows you to quickly cut the bacteria in half, and more importantly decide which antibiotic to give without actually having to grow the thing.

The most prominent bacterial structural characteristic is the cell wall. Bacteria can be divided into two groups (gram-positive and gram-negative) based on differences in cell wall structure as revealed by Gram staining. In a Gram stain, a series of dyes are applied to stain cell walls. Gram-negative bacteria have an outer, lipopolysaccharide-containing membrane and stain pink. They also have a thin peptidoglycan layer located in the periplasm (the region between the outer and cytoplasmic membranes). Gram-positive bacteria lack this covering, but possess a cell wall containing a thick peptidoglycan (called Murein in older sources) layer and teichoic acids; they stain purple. Gram-positive bacteria are more susceptible to antibiotics, while gram-negative bacteria are difficult to treat with antibiotics.