TOXIGENESIS

Two types of bacterial toxins

• lipopolysaccharides, which are associated

with the cell walls of Gram-negative bacteria.

• proteins, which may be released into the

extracellular environment of pathogenic

bacteria.

Endotoxins

Endotoxins are:

Toxic to most mammals.

They are strong antigens but they seldom elicit immune responses which give full protection to the host against secondary challenge with the endotoxin.

They cannot be toxoided.

Endotoxins released from multiplying or disintegrating bacteria significantly contribute to the symptoms of Gram-negative bacteremia and septicemia, and therefore represent important pathogenic factors in Gram-negative infections.

All endotoxins produce the same range of biological effects in the host, causes a wide spectrum of nonspecific pathophysiological reactions related to inflammation such as:

Fever ,changes in white blood cell counts ,disseminated intravascular coagulation ,hypotension ,shock ,lethality

Bacterial Endotoxins

• Endotoxins

– Host cell receptors (TLR) bind to

components of pathogen

– Pathogen associated molecular

patterns PAMPS

• LPS – gram - cell walls

• Flagella

• Lipoteichoic acid – gram + cell walls

– Signal transduction pathways begin to

illicit a cellular response

• Production of cytokines

Inflammation

Opsonization

MAC

Exotoxins

-several types and specificities.

-They usually have two general functions, which can often be uncoupled: the ability to bind to a host cell receptor, and enzymatic activity. For example, cholera toxin ,labile toxin, and many others have five binding (B) subunits coupled to the active enzymatic subunit (A). Several vaccines are made from the B subunits without the catalytic subunit, ensuring no toxin activity. The B subunits bind to host cell molecules such as carbohydrates (for example, cholera binds the GM1 ganglioside, a glycolipid on the intestinal surfaces). Following binding, the A subunit then enters the cell, which is facilitated by the B subunits.

• Exotoxins

– Initial location outside cells

– Transported into host cells

– Alter host cell physiology

and metabolism

– Typical A – B toxins

AB toxin enters cells via:

1) Receptor mediated endocytosis

2) Fusion of vesicle with lysosome

3) Acid environment of lysosome

reduces disulfide bonds and

releases A into cell

4) A has various cellular activities

Also see page

399 of textbook Bacterial Exotoxins

Mechanism of Action

of cholerae toxin

• Normally – Adenylate cyclase (AC) enzyme

which makes cAMP

– Epithelial cells secrete digestive fluid (HCO3

-) in response to small increases in cAMP levels

• cholerae exotoxin – Over activation of AC

– ADP ribosylation of AC

– Causes 100X increase in cAMP

– Causes huge amounts of water and Cl- to leave via channels

Pyrogenic exotoxins

They " produced by Staphylococcus aureus and

Streptococcus pyogenes have been designated as

superantigens. They represent a family of

molecules with the ability to elicit massive

activation of the immune system. These proteins

share the ability to to elicit massive activation of

the immune system. These proteins share the

ability to stimulate T cell proliferation by

interaction with Class II MHC molecules on

APCs and specific V beta chains of the T cell

receptor. The important feature of this

interaction is the resultant production of IL-1,

TNF, and other lymphokines which appear to be

the principal mediators of disease processes

associated with these toxins.

CHARACTERISTICS OF BACTERIAL

ENDOTOXINS AND EXOTOXINS

Properities Endotoxin Exotoxin

Chemical nature Lipopolysaccharide Protein

Relationship to cell Part of outer membrane Extracellular, diffusible

Denaturated by boiling No Usually

Antigenic Yes Yes

Form toxoid No Yes

Potency Relatively low (>100ug) Relatively high (1 ug)

Specifity Low degree High degree

Enzymatic activity No Usually

Pyrogenicity Yes Occasionally

Pathogenicity Islands Pathogenicity Islands (PAI) are a distinct class of genomic

islands which are acquired by gene transfer. They are incorporated in the genome of pathogenic bacteria but are usually absent from non-pathogenic organisms of the same or closely related species. They usually occupy genes which contribute to virulence of the pathogen. Typical examples are adhesins, toxins, iron uptake systems, invasins, etc.

One species of bacteria may have more than one pathogenicity island. For example, in Salmonella, five pathogenicity islands have been identified. They are found mainly in Gram-negative bacteria, but have been shown in a few Gram-positives. Pathogenicity islands may be located on the bacterial chromosome or may be a part of a plasmid. Pathogenicity islands play a pivotal role in the virulence of bacterial pathogens of human .

Transmission of infection

• Specific bacterial species (or strains within a

species) initiate infection after being

transmitted by different routes to specific sites

in the human body. For example:-

• Bacteria are transmitted in airborne droplets to

the respiratory tract

• By ingestion of food or water

• By sexual contact.

The role of bacterial biofilms

• A biofilm is an aggregate of microorganisms in which cells are stuck to each other and/or to a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS). Biofilm EPS, which is also referred to as "slime," is a polymeric of DNA, proteins and polysaccharides. Biofilms may form on living or non-living surfaces, and represent a prevalent mode of microbial life in natural, industrial and hospital settings.

• Biofilms have been found to be involved in a wide variety of microbial infections in the body, by one estimate 80% of all infections. Infectious processes in which biofilms have been implicated include common problems such as urinary tract infections, catheter infections, middle-ear infections, formation of dental plaque, gingivitis, coating contact lenses.

Polymicrobic biofilm grown on a stainless steel surface in a

laboratory