Final answer:
D-alanine is the unique amino acid found in the peptidoglycan layer of bacterial cell walls.
Step-by-step explanation:
The amino acid that is unique to the peptidoglycan layer of bacterial cell walls is D-alanine. Peptidoglycan is a major component of bacterial cell walls and is composed of long chains of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) subunits. These subunits are cross-linked by peptide bridges that connect NAG and NAM within each peptidoglycan layer. The D-alanine amino acid is part of these peptide bridges and is not typically found in proteins in other organisms.