Outer membrane composition: In which organisms is lipopolysaccharide (LPS) a major component of the outer membrane?

Introduction / Context:
Lipopolysaccharide (LPS) is a hallmark molecule of Gram-negative bacteria. It plays structural roles, affects membrane permeability, and acts as endotoxin (particularly the lipid A portion), driving potent immune responses in hosts.

Given Data / Assumptions:

• The question asks where LPS is a major outer membrane component.
• Gram-positive bacteria lack an outer membrane.
• Fungi are eukaryotes and do not have LPS.

Concept / Approach:

Gram-negative envelopes have an inner membrane, a thin peptidoglycan layer in the periplasm, and an outer membrane. The outer leaflet of the outer membrane predominantly contains LPS, contributing to barrier function and immune recognition. Gram-positives possess a thick peptidoglycan wall without an outer membrane and therefore no LPS.

Step-by-Step Solution:

Verification / Alternative check:

Endotoxin tests in clinical labs detect LPS; antibiotics targeting outer-membrane function often have selective effects on Gram-negatives, reflecting the presence of LPS and porins.

Why Other Options Are Wrong:

Gram-positive bacteria do not have an outer membrane; LPS is absent.

Fungi have chitinous cell walls and sterol-containing membranes, not LPS.

None of these is false because Gram-negatives clearly have LPS.

Common Pitfalls:

Confusing LPS with lipoteichoic acid (Gram-positive), or assuming all microbes share similar envelope chemistry. Envelope differences are pivotal in antibiotic susceptibility and immune detection.

Final Answer:

Gram-negative bacteria