Q.8 Which one of the following components of bacterial cell acts as endotoxin?
(A) Peptidoglycan of Gram–positive bacteria (B) Lipopolysaccharide
(C) Porins (D) Peptidoglycan of Gram–negative bacteria

Lipopolysaccharide (LPS) from the outer membrane of Gram-negative bacteria is the primary endotoxin, responsible for triggering strong immune responses like fever and inflammation when released.​

Option Analysis

Peptidoglycan of Gram-positive bacteria (A):
Peptidoglycan forms the thick cell wall in Gram-positive bacteria but lacks toxicity characteristic of endotoxins; it may cause mild inflammation via other mechanisms, not as a true endotoxin.​
Gram-positive bacteria do not produce LPS, so their peptidoglycan does not act as endotoxin.​

Lipopolysaccharide (B):
LPS, embedded in the outer membrane of Gram-negative bacteria, consists of lipid A (toxic core), core polysaccharide, and O-antigen; lipid A provokes cytokine release and sepsis-like symptoms upon cell lysis.​
This matches the definition of bacterial endotoxin universally across sources.​

Porins (C):
Porins are β-barrel proteins in Gram-negative outer membranes that facilitate nutrient and antibiotic diffusion but show no endotoxic activity; they may contribute to inflammation indirectly.​

Peptidoglycan of Gram-negative bacteria (D):
Gram-negative peptidoglycan is thin and inner, covered by the outer membrane; it provides structural support but does not function as endotoxin, unlike LPS.​

Lipopolysaccharide (LPS) serves as the key bacterial endotoxin component in Gram-negative bacteria cell walls, triggering potent immune responses vital for CSIR NET Life Sciences understanding.​

Endotoxin Structure

Endotoxins are lipopolysaccharides with lipid A as the toxic anchor, linked to core oligosaccharides and O-antigen chains, released during Gram-negative bacterial lysis.​
This structure enables LPS to bind Toll-like receptor 4, activating inflammation pathways.​

Role in Gram-Negative Bacteria

Gram-negative bacteria feature an outer membrane rich in LPS (up to 75% coverage), distinguishing them from Gram-positive types lacking this endotoxin.​
LPS maintains membrane integrity while posing risks like septic shock in hosts.​

Why Not Other Components?

• Peptidoglycan in both Gram types supports rigidity but elicits weaker responses, not classic endotoxicity.​

• Porins enable passive diffusion without toxic bioactivity akin to LPS.​