Why don't gram negative bacteria usually react to penicillin?

• A. The structure of their cell wall
• B. Their fast growth
• C. They lack cell walls
• D. They are viruses

Answer: (A)

Penicillin works by blocking enzymes that cross-link peptidoglycan in the bacterial cell wall. In Gram-negative bacteria, the cell envelope includes an outer membrane that sits outside a thin peptidoglycan layer. That outer membrane acts as a barrier, limiting penicillin’s access to the wall-synthesis machinery inside. The periplasmic space can also contain enzymes that break down the drug, further reducing its effect. Because this outer membrane structure keeps the drug from reaching its target, Gram-negative bacteria are usually less susceptible to penicillin. In contrast, Gram-positive bacteria lack this outer membrane and have exposed peptidoglycan, making them more vulnerable to penicillin.