A 45-year-old Indian male with prosthetic heart valve develops fever and new cardiac murmur. Blood cultures grow Enterococcus faecalis. The structure marked **A** in the diagram (thick peptidoglycan layer of Gram-positive bacteria) is the target site for vancomycin therapy. Which of the following best explains why vancomycin is effective against this organism but would be ineffective if the same pathogen were Gram-negative?