English: Diagram depicting the action of the antibiotic vancomycin and one way bacteria have evolved resistance to it.

Background: The bacterial cell wall consists of strands of repeating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) subunits. The NAM subunits have short peptide chains attached to them. (The exact composition of these can vary. The proximal alanine is usually L-ala and the distal two are usually D-ala.) These peptide chains are involved in forming cross-links between the strands of the cell wall. These cross-links are essential to a functioning cell wall.

1. Vancomycin is added to the bacterial environment while it is trying to synthesize new cell wall. Here, the cell wall strands have been synthesized, but not yet cross-linked.
2. Vancomycin recognizes and binds to the two D-ala residues on the end of the peptide chains. However, in resistant bacteria, the last D-ala residue has been replaced by a D-lactate, so vancomycin cannot bind.
3. In resistant bacteria, cross-links are successfully formed. However, in the non-resistant bacteria, the vancomycin bound to the peptide chains prevents them from interacting properly with the cell wall cross-linking enzyme.
4. In the resistant bacteria, stable cross links are formed. In the sensitive bacteria, cross-links cannot be formed and the cell wall falls apart.