The cell wall of a bacterium is composed of

Explanation

Peptidoglycan, also called murein, is a mesh of sugar chains cross linked by short peptides, and it is unique to bacteria, which is why penicillin can block its synthesis and kill a growing bacterium without harming human cells. The Gram stain works by distinguishing a thick peptidoglycan layer from a thin one. A phospholipid bilayer is a membrane, not a wall.