What is the "defining molecule" in the cell walls of bacteria?

The major molecule on the bacterial cell wall is Murein or peptidoglycan.

Its main function is protection of the inner components of the cell including cytoplasmic membrane. It also gives the cell its typical shape allowing it to function normally. It has both the sugar and protein components. the sugar component is made up of alternating N-acetylglocaseamine (NAG) and N-acetylmuramic acid (NAM). The protein component constitutes of a peptide chain of three to five amino acids.