The typical bacterial cell wall consists of peptidoglycan polymers, polypeptide chains cross-linked with short peptides. The cell wall prevents swelling due to the osmotic imbalance of the bacteria being hypertonic to the surrounding fluid.

It is important for the MCAT to know that differences in the peptidoglycan cell wall are the basis for the differences in bacteria observed in Gram staining. Gram-positive bacteria have a thick mesh-like cell wall made of peptidoglycan while Gram-negative bacteria have a thinner cell wall as well as a lipopolysaccharide outer membrane. A bacterium might additionally possess a thick outer capsule or have pili present.