Question

18. Some pathogenic bacteria release toxins that can harm the host. Exotoxins are produced by members of both Gram-positive and Gram-negative bacteria. However, only Gram-negative bacteria produce endotoxins. How do the two toxins differ? Exactly what molecule/structure is the endotoxin?

Answer 1

ANSWER 1: DIFFERENCE BETWEEN ENDOTOXIN AND EXOTOXIN.

PROPERTY	ENDOTOXIN	EXOTOXIN
1. Definition	It is a toxin which is released only after the lysis of cell.	It is a toxin which is released by a living bacterial cell into its surrounding.
2. Source	Gram negative bacteria	Gram negative and Gram positive bacteria, both
3. Location	Found in the cell wall of bacteria	Found in the cytoplasm of bacteria
4. Chemical nature	Lipopolysaccharide	Protein
5. Controlling gene	Present on chromosomal genes	Present on extra- chromosomal genes(plasmid)
6. Toxicity	Moderately toxic	Highly toxic(can be fatal)
7. Potency	Less potency	High potency
8. Effect of heat	Heat stable	Heat labile (Destroyed at 60°C)
9. Denaturation	Does not denaturate on boiling	Denaturates on boiling
10. Filtration	Cannot be filtered	Can be filtered
11. Deterioration	Does not deteriorate at room temperature	Deteriorates at room temperature
12. Toxoids	Cannot form toxoids	Can form toxoids
13. Vaccine	No vaccine available	Vaccine available
14. Mode of action	General (produces general symptoms and does not act on specific cells)	Specific(acts specifically on target cells)
15. Immunogenicity	Weakly immunogenic	Highly immunogenic
16. Pyrogenicity	Pyrogenic	Non pyrogenic
17. Organism	Neisseria spp., Escherichia coli etc.	Vibrio cholera, Staphylococcus aureus, Clostridium botulinum etc.
18. Disease	Meningitis, Urinary tract infections etc.	Tetanus, Diphtheria, Botulism etc.

ANSWER 2: STRUCTURE OF ENDOTOXIN

Endotoxins are present in the outer membrane of the cell wall. They are mainly LIPOPOLYSACCHARIDE (LPS).

Lipopolysaccharide is a major component of the cell wall of gram negative bacteria. This lipopolysaccharide is made up of following three parts:-

1. Lipid A- It is the lipid component of LPS. It is hydrophobic in nature. Lipid A is mainly made up of phosphorylated saturated fatty acid chain. This constituent of Gram negative bacterial cell walls is mainly responsible for its toxicity.

2. Core- It attaches to the Lipid A molecule. It is hydrophilic in nature. Its main component is sugar, that is, it may be an oligosaccharide or a polysaccharide. Mostly the sugar which are present is Heptose or 2-keto-3-deoxyoctonoic acid. Non carbohydrate components are also present in the core of LPS.

3. O antigen- It is the outermost part of LPS. It is attached to the core. It is the hydrophilic part of LPS. The antibody binding site is present here which is responsible for the immume reactions. It is mainly composed of 2 to 6 sugars. This is a main distinguishing features between strains of bacteria because its composition differs in different strains.