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Which type of bacteria have an outer membrane containing lipopolysaccharides (LPS)?

  • A. Gram-positive bacteria
  • B. Gram-negative bacteria
  • C. Both Gram-positive and Gram-negative bacteria
  • D. Neither Gram-positive nor Gram-negative bacteria
Correct Answer: B

Rationale: The correct answer is B: Gram-negative bacteria. Gram-negative bacteria have an outer membrane containing lipopolysaccharides (LPS). In the Gram staining technique, these bacteria appear pink/red due to the thin peptidoglycan layer, which allows the crystal violet stain to be washed out, revealing the safranin counterstain. Gram-positive bacteria (choice A) have a thick peptidoglycan layer but lack an outer membrane with LPS. Choice C is incorrect since only Gram-negative bacteria possess LPS in their outer membrane. Choice D is incorrect as all bacteria can be classified as either Gram-positive or Gram-negative based on their cell wall structure.