Bacteria: Structure and Pathogenesis · USMLE Step 1

What major difference distinguishes the cell envelope of Gram-negative bacteria from Gram-positive bacteria?

  1. Presence of surface teichoic acids
  2. A very thick peptidoglycan layer
  3. Outer membrane with LPS layer
  4. Lack of a cytoplasmic membrane
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Correct answer: Outer membrane with LPS layer

Gram-negative bacteria have an outer membrane containing lipopolysaccharide (LPS), in addition to the inner cytoplasmic membrane and thin peptidoglycan. Gram-positive bacteria lack this outer membrane.

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