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BACTERIAL ENDOTOXINS

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ENDOTOXINS

Endotoxins are part of the outer membrane of the cell wallof Gram-negative bacteria.

The term "endotoxin" is used to refer to any cell-associatedbacterial toxin, but in bacteriology it is properly reserved to

refer to the lipopolysaccharide complex associated with the

outer membrane of Gram-negative bacteria.

Examples:Salmonella, Shigella, Pseudomonas, Neisseria etc.

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CHEMICAL STRUCTURE OF ENDOTOXINS

Lipopolysaccharides are complex amphiphilic

molecules with a molecular weight of about 10kDa.

LPS can be extracted from whole cells by treatment

with 45% phenol at 90oC. Mild hydrolysis ofLPS

yields Lipid A plus polysaccharide.

LPS consists of three components or regions: Lipid A,

an R polysaccharide and an O polysaccharide.

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Region I : Lipid A

Lipid A is the lipid component ofLPS.

It contains the hydrophobic, membrane-anchoring region ofLPS.

It consists of a phosphorylated N-acetylglucosamine (NAG)

dimer with 6 or 7 fatty acids (FA) attached. Usually 6 FA arefound. All FA in Lipid A are saturated.

Some FA are attached directly to the NAG dimer and othersare esterified to the 3-hydroxy fatty acids that are

characteristically present.

The structure ofLipid A is highly conserved among Gram-negative bacteria.

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Region II : Core (R) antigen or R polysaccharide

It is attached to the 6 position of one NAG.T

he R antigenconsists of a short chain of sugars. For example: KDO - Hep

- Hep - Glu - Gal - Glu - GluNAc -Two unusual sugars,

heptose and 2-keto-3-deoxyoctonoic acid (KDO), are

usually present, in the core polysaccharide. KDO is uniqueand invariably present in LPS and so it has been used as an

indicator in assays for LPS.

With minor variations, the core polysaccharide is commonto all members of a bacterial genus (e.g. Salmonella), but it

is structurally distinct in other genera of Gram-negative

bacteria.

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Region III : Somatic (O) antigen or O polysaccharide

It is attached to the core polysaccharide.

It consists of repeating oligosaccharide subunits made up

of 3 - 5 sugars. The individual chains vary in length ranging

up to 40 repeat units.

The O polysaccharide is much longer than the core

polysaccharide, and it maintains the hydrophilic domain ofthe LPS molecule. A major antigenic determinant

(antibody-combining site) of the Gram-negative cell wall

resides in the O polysaccharide.

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LPS & VIRULENCE

Both Lipid A and the polysaccharide side chains act as

determinants of virulence in Gram-negative bacteria.

LPS elicits a variety of inflammatory responses in an animaland it activates the complement system by the properdin

pathway.

Toxicity is associated with the lipid component i.e Lipid A,

whereas immunogenicity is associated with

the polysaccharide component.

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The O polysaccharide and Virulence

Virulence, and the property of smoothness, is associated

with an intact O polysaccharide.

Small changes in the sugar sequences in the side chains ofLPS result in major changes in virulence.

The O polysaccharide helps in virulence in the followingways:

1. Allow organisms to adhere specifically to certain tissues,especially epithelial tissues.

2. Smooth antigens probably allow resistance tophagocytes, since rough mutants are more readily engulfedand destroyed by phagocytes.

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Lipid A and Virulence

The physiological activities of

LPSare mediated mainly bythe Lipid A component of the LPS.

Exerts its toxic effects when released from multiplying cells

in a soluble form, or when the bacteria are lysed.

Injection of living or killed Gram-negative cells or purified

LPS

into experimental animals causes a wide spectrum ofnonspecific pathophysiological reactions, such

as fever, changes in white blood cell counts,disseminated

intravascular coagulation, hypotension, shock and death.

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ENDOTOXINS vs EXOTOXINS

Endotoxins are less potent and less specific in their action,since they do not act enzymatically.

Endotoxins are heat stable (boiling for 30 minutes does not

destabilize endotoxin), but certain powerful oxidizing

agents such as superoxide, peroxide and hypochlorite, have

been reported to neutralize them.

Endotoxins, although antigenic, cannot be converted to

toxoids.

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References

Online Textbook of Bacteriology by Dr. Kenneth

Todar.

www.google.com

en.wikipedia.org

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Presentation by:

Siddharth Rangnekar

IG-MBT VIII

Roll no. 29