microbiology lec3

Medical MicrobiologyMedical Microbiology

Lecture 3Lecture 3

Dr. Saleh M Y OTHDr. Saleh M Y OTH

PhDPhDMedical Molecular Biotechnology and Infectious DiseasesMedical Molecular Biotechnology and Infectious Diseases

02/10/201002/10/2010IMS - MSUIMS - MSU

(1.1) Extracellular Toxins:

Exotoxins are toxins excreted by pathogenic microorganisms, including bacteria, fungi, algae, and protozoa.

An exotoxin can cause damage to the host by destroying cells or disrupting normal cellular metabolism.

They are highly potent and can cause major damage to the host.

Bacterial Toxins

(1) Exotoxins and (2) Endotoxins

(1.1) Extracellular Toxins…..

Most exotoxins can be destroyed by heating.

They may exert their effect locally or produce systemic effects.

Well-known exotoxins include the:

- botulinum toxin produced by Clostridium botulinum and the

- Corynebacterium diphtheriae exotoxin, which is produced during life-threatening symptoms of diphtheria.


Exotoxins are susceptible to antibodies produced by the immune system, but many exotoxins are so toxic that they may be fatal to the host before the immune system has a chance to mount defenses against it.


Exotoins causing Mon-Membrane Damaging

1. Hyaluronidase - This is also called the spreading factor because it catalyzes the breakdown of hyaluronic acid, the substance that cements the human cells together.

This allows the bacterial cells to spread through tissue causing a condition known as cellulitis.

2. Coagulase - This enzyme catalyzes the conversion of fibrinogen to fibrin with resultant clot formation.

3. Lipase - Production of excessive amounts of lipase allow bacteria to penetrate fatty tissue with the consequent formation of abscesses.

(1.1) Extracellular Toxins …..


4. Fibrinolysin - This catalyzes the conversion of plasminogen to the fibrinolytic enzyme plasmin. Thus it acts opposite of coagulase to facilitate the invasion of the bacteria to colonize anoher cells or tissue.

In Staphylococcus aureus, the gene for fibrinolysin is on a bacteriophage and is expressed during lysogeny.

5. IgA protease - Many bacteria which colonize the mucous membranes produce an IgA protease which degrades secretory IgA.



6. Collagenase - This enzyme catalyzes the degradation of collagen or break down the peptide bond in the collagen , a scleroprotein found in tendons, nails and hair.

This toxin facilitates the spread of gas gangrene. They normally target the connective tissue in muscle cells and other body organs.


Exotoins causing Membrane Damaging

1. Hemolysins - There are many different types of hemolysins but, in each case, the end result is lysis of the red blood cell (RBCs) with resultant anemia.

2. Leukocidins - Also, there are many different types of leukocidins, and some are specific for only one type of leukocyte. However, the ends result in lysis of leukocytes with resultant leucopenia.


Exotoins causing Membrane Damaging

3. Phospholipase - This enzyme attacks any cell with phospholipid in its membrane. The result is widespread cell lysis.

Lecithinase - (phospholipase C) is an enzyme which breaks down the lecithin in the human cell plasma membrane, resulting in cell lysis. It is especially active on RBCs. It is also called a toxin.

(1.2) Intracellular Toxins

ADP-ribosyl transferases

1. Diphtheria toxin: produced by Corynebacterium diphtheriae causing inhibition of protein synthesis and cell death.

2. Pseudomonas aeruginosa exotoxin A. This works in the same manner as diphtheria toxin, leading to Human epithelial cell death.

3. Pseudomonas aeruginosa exoenzyme S. It also causes human epithelial cell death.

4. Cholera toxin. Vibrio cholerae growing in the intestine secretes an exotoxin composed of many subunits, causes a sustained increase in adenylate cyclase activity which results in excess secretion of isotonic fluid into the intestine with resulting diarrhea.

(1.2) Intracellular Toxins …

ADP-ribosyl transferases

5. Bordetella pertussis toxin. Causing whooping cough (pertussis) and leads to death if no treatment.


Non-Ribosylating Toxins

1. Shiga toxin. Produced by Shigella sp. it is neurotoxic, cytotoxic and enterotoxic.

2. Anthrax toxin. Bacillus anthracis produces this exotoxin composed of three distinct proteins:

a- protective antigen, b- edema factor and

c- lethal factor.

Dermal necrosis is the result of the toxin action.


Non-Ribosylating Toxins

3. Tetanus toxin. Clostridium tetani produces an endopeptidase that cleaves synaptobrevins: this interferes

with vesicle formation at the myoneural junction and the neural-neural junction in the spinal cord. The result is

muscle spasm.

The tetanus toxin serologically cross-reacts with the botulinum toxin.

2. Botulinum toxin. Clostridium botulinum produces an endopeptidase that blocks the release of acetylcholine at

the myoneural junction. Muscle paralysis is the result.


Toxins with Undefined Mechanism of Action

1- Trachea toxin. Bordetella pertussis tracheal cytotoxin kills cilia-bearing

cells.

Exfoliating toxin. Staphylococcus aureus produces an exfoliating toxin which causes a sloughing of skin (scalded skin syndrome).

Toxic shock syndrome toxin. Staphylococcus aureus produces this toxin which has an undefined mode of action but is mediated through induction of interleukin-1. It causes hypotension, rash, fever and desquamation of skin.

Erythrogenic toxin. Streptococcus pyogenes produces this toxin which is similar to toxic shock syndrome toxin.

Mycolactone. Mycobacterium ulcerans produces this toxin which causes skin and muscle necrosis.

Bacteial Endotoxins

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

Gram-negative bacteria are those bacteria that do not retain crystal violet dye in the Gram staining protocol.

Endotoxin is invariably associated with Gram-negative bacteria whether the organisms are pathogenic or not.

Although the term "Although the term "endotoxinendotoxin" is occasionally " is occasionally used to refer to any cell-associated bacterial used to refer to any cell-associated bacterial toxin.toxin.

In bacteriology it is properly reserved to refer to In bacteriology it is properly reserved to refer to the the lipopolysaccharidelipopolysaccharide complex ( complex (LPSLPS) ) associated with the associated with the outer membraneouter membrane of Gram- of Gram-negative pathogens such as:negative pathogens such as:

Escherichia coliEscherichia coli,,

SalmonellaSalmonella,,

ShigellaShigella,,

PseudomonasPseudomonas,,

NeisseriaNeisseria,,

Haemophilus influenzaeHaemophilus influenzae,,

Bordetella pertussisBordetella pertussis

Vibrio choleraeVibrio cholerae

The The biologicalbiological activity of activity of endotoxinendotoxin is associated with is associated with the the LPSLPS. .

ToxicityToxicity is associated with the lipid component ( is associated with the lipid component (Lipid Lipid AA) and ) and immunogenicityimmunogenicity is associated with the is associated with the polysaccharidepolysaccharide components. components.

The cell wall antigens (The cell wall antigens (O - somatic antigensO - somatic antigens) of Gram-) of Gram-negative bacteria are components of negative bacteria are components of LPSLPS. .

LPSLPS elicits a variety of inflammatory responses in an animal and it activates elicits a variety of inflammatory responses in an animal and it activates complement by the alternative (properdin) pathway, so it may be a part of complement by the alternative (properdin) pathway, so it may be a part of the pathology of Gram-negative bacterial infections.the pathology of Gram-negative bacterial infections.

In vivoIn vivo, Gram-negative bacteria probably release minute , Gram-negative bacteria probably release minute amounts of endotoxin while growing. amounts of endotoxin while growing.

This may be important in the stimulation of natural immunity. This may be important in the stimulation of natural immunity.

It is known that small amounts of endotoxin may be released in It is known that small amounts of endotoxin may be released in a soluble form by young cultures grown in the laboratory. a soluble form by young cultures grown in the laboratory.

But for the most part, endotoxins remain associated with the But for the most part, endotoxins remain associated with the cell wall until disintegration of the organisms.cell wall until disintegration of the organisms.

In vivoIn vivo,, this results from autolysis, external lysis mediated by this results from autolysis, external lysis mediated by complement and lysozyme, and phagocytic digestion of complement and lysozyme, and phagocytic digestion of bacterial cells. bacterial cells.

Compared to the classic exotoxins of bacteria, 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.

Property Endotoxin Exotoxin

Chemical NatureLipopolysaccharide (mw = 10kDa)

Protein (mw = 50-1000kDa)

Relationship To Cell Part of outer membraneExtracellular, diffusible

Denatured By Boiling No Usually

Antigenic Yes Yes

Form Toxoid No Yes

PotencyRelatively low (>100ug)

Relatively high (1 ug)

Specificity Low degree High degree

Enzymatic Activity No Often

Pyrogenicity Yes Occasionally

microbiology lec3

Documents

intracellular toxins

extracellular toxins

botulinum toxin

toxin action

exfoliating toxin

tetanus toxin

trachea toxin

cholera toxin