Penicillin targets exposed peptidoglycan in Gram-positive bacteria

Steps:

• Penicillin inhibits peptidoglycan cross-linking in bacterial cell walls, preventing synthesis.
• Gram-positive bacteria have a thick peptidoglycan layer directly outside the plasma membrane, with no outer membrane barrier.
• Gram-negative bacteria have a thin peptidoglycan layer surrounded by an outer membrane, which blocks antibiotic entry.
• Thus, penicillin accesses and disrupts the target more easily in Gram-positive bacteria.

Why A is correct:

• Gram-positive cell walls lack an outer membrane, allowing direct exposure of the peptidoglycan layer to penicillin, as defined by Gram staining based on cell wall structure.

Why the others are wrong:

• B: Gram-negative bacteria's thinner peptidoglycan is protected by the outer membrane, reducing penicillin's effectiveness.
• C: Option is incomplete and provides no valid reasoning.
• D: More periplasm in Gram-negative bacteria does not weaken them; the outer membrane actually shields the peptidoglycan from penicillin.

Final answer: A