Generally, vancomycin is active against gram-positive bacteria. However, which of the following is more likely to exhibit resistance?

• A. Staphylococcus epidermidis
• B. Streptococcus pyogenes
• C. Enterococcus gallinarum
• D. Bacillus subtilis

Answer: (C)

Vancomycin works by binding the D-Ala-D-Ala terminus of peptidoglycan precursors, blocking cell wall synthesis. While many Gram‑positive bacteria are susceptible, some Enterococcus species have inherent resistance due to the vanC gene, which leads to a different terminal on the peptidoglycan, D-Ala-D-Ser, that vancomycin binds poorly. This makes Enterococcus gallinarum intrinsically less susceptible to vancomycin compared with the other listed organisms, where resistance is not inherent and is less predictable. So, Enterococcus gallinarum is the organism most likely to exhibit vancomycin resistance.