Bacteria: Structure and Pathogenesis · USMLE Step 1

Why are β-lactam antibiotics generally more effective against Gram-positive bacteria than Gram-negative bacteria?

  1. Gram-positive bacteria completely lack ribosomes
  2. Gram-negative bacteria have a thick outer capsule
  3. Outer membrane blocks drug access to peptidoglycan
  4. Gram-positive peptidoglycan wholly lacks D-alanine
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Correct answer: Outer membrane blocks drug access to peptidoglycan

The outer membrane of Gram-negative bacteria limits penetration of β-lactam antibiotics, reducing their access to the peptidoglycan layer and decreasing antibiotic efficacy.

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