Microbiology Lecture 8 - Pathogenesis and Toxigenicity Part 1

Toxin: microbial poison; any substance that is toxic to a cell

Toxins are proteins made by living organisms (and other organisms) that have a negative or toxic effect on the host

Sometimes the effect of toxins can be fatal to the host

The capacity of microbes to make and secrete toxins.

Microbes can be described as toxigenic.The organism carries genes for these toxic molecules on their chromosome

Some toxins are plasmid-borne and can be shared between related species by horizontal/lateral transfer

Toxinosis : disease caused by a toxin.Requires the pathogen to colonize, grow and produce toxins that are secreted.

Intoxication: disease that results from ingestion of the (pre-formed) toxin does not require the pathogen

Toxemia: the presence of toxins in the blood

Toxicology: the study of toxins, including structure, mechanism of action, regulation, etc.

Toxoid: attenuated (weakened) toxin; used to as vaccines to stimulate a protective immune response

Antitoxins: antibodies produced by the body (animal host) to neutralize toxins

Yes

Yes

• emetic - vomiting 
• Neoplastic - tumor growth

Are produced as proenzymes within the microbe; this form is inactivy.The toxin is then activated once it leaves the cell via Enzymatic cleavage or phosphorylation

Bacterial toxins harmful to humans are listed by the body system they target.

If toxic to other bacteria, they are called bacteriocins.If these toxins are used in the treatment of human bacterial infections, they are called antibiotics

Toxins are antigenic; stimulates the immune system

mycotoxins

phycotoxins 

• Cyanobacteria: Bacteria
• Dinoflagellates: Protozoa

Are the cause of shellfish poisoning in humans

Exotoxins: toxins secreted by live bacteria

Endotoxins: released when the bacteria dies

Exotoxins: toxins secreted by live bacteria

Endotoxins: released when the bacteria dies

Exotoxins: toxins released into the surrounding area

Endotoxins: are critically associated with the bacteria

Exotoxins: work on intracellular targets of the host cell

Endotoxins: work at the surface of the host cell

Yes

Yes

In the cytoplasm - ribosomes

Yes

intracellular

Some toxins are exported across the plasma membrane into the host cell and then folded to the correct conformation.Active once the correct conformation is achieved.

Some toxins are made and folded within the bacterial cell and are only exported if they have the correct conformation.

Neurotoxins: act on the nervous system

Enterotoxins: act on the gastrointestinal tract (also known as the enteric system).Commonly associated with food poisoning

• Nephrotoxins: damage the
• kidneys

Hepatotoxins: damage the liver

Can result in flaccid (limp) or rigid paralysis .E.g. botulinum toxin made by Clostridium botulinum; etiological agent for botulism, a severe form of food poisoning

Can cause rigid paralysisE.g. tetanus toxin made by Clostridium tetani; etiological agent for tetanus (or lockjaw)

Are exotoxins that specifically target the intestinal mucosa

• Affect cAMP levels
• Result in increase elimination of water from intestinal cells

Cause vomiting and diarrhea

because the action of the toxin is within the cytoplasm

The toxin is secreted directly into the cytosol in a contact-dependent manner

Advantages -Efficient: poison enters only the target cell

Selective: intracellular target is specific

gram + make a pore 

gram - inject it

Molecular syringes that resemble flagella (Type III)

Molecular syringes that resemble conjugative pili (Type IV)

A Toxin enters without the aid of the bacteria

B domain/subunit Binds to cell surface receptors of host Helps to translocate the A domain/subunit A domain/subunit.

Is transferred across the plasma membrane into the host cellEnters the cytoplasm and enzymatically acts on host targets

-Diphtheria toxin isAn AB toxin Produced by Corynebacterium diphtheriae

-The A sub-unit inhibits protein synthesis by blocking the transfer of an amino acid from the tRNA to the polypeptide.Specifically inactivates elongation factor 2 (EF-2)

-Diphtheria is a childhood disease of the respiratory system

Botulin (also called Botulinum toxin)

Produced by Clostridium botulinum.Is a group of seven related A-B toxins; 1A + 1B

Are the most potent biological toxins known

Binds to the pre-synaptic membranes of stimulatory motor neurons found at the neuromuscular junction.Blocks the release of acetylcholine (neurotransmitter).This prevents the muscle from receiving an excitatory signal.No muscle contraction

• Produced by Clostridium tetani
• An A-B toxin Targets the motor neuronsIs transported to the spinal column and binds to ganglioside lipids of inhibitory neurons

Inhibitory neurons produce glycine which stops the release of acetylcholine, which causes muscles to contract

Tetanospasmin blocks the release of glycine, resulting in continuous contraction of muscles.

Cholera toxin is called choleragen

Produced by Vibrio cholerae Is an A-B toxin: 1A + 5B Binds to the ganglioside GM1 of intestinal epithelial cells

Activates the enzyme, adenyl cyclase which causes increased production of cAMP

cAMP prevents the movement of Na+ in cells and the net movement of Cl- out of cells

The change in ion concentration results in the secretion of large amounts of water from the small intestine

Water reabsorption by the large intestine is slower than water loss

Patients suffer severe dehydration

Work by enzymatically attacking the cell causing cell lysis.The action of these toxins are most easily observed with erythrocytes, hence they are often called hemolysins

They also work on cells other than erythrocytesAction is observed by streaking the organism on (sheep’s) blood agar plates

Yes

Hemolysins they are Produced by: staphylococci, streptococci, clostridia, Escherichia

Hemolysis: Lyses red blood cells (erythrocytes)

• Results in anemia
• Purpose:Allows pathogen greater access to iron

Action: Enzyme causes pore-formation in the plasma membrane,Death is by necrosis (lysis of the cell where the cellular contents spill out and damage neighboring cells)Also destroys leucocytes (white blood cells

Alpha hemolysis: the partial destruction of erythrocytes (RBCs)

Beta hemolysis: the complete destruction of erythrocytes (RBCs)

Gamma hemolysis: no hemolysis

Hemolysins produced by the streptococci Does not work on the phospholipids of the cell membrane. Instead, works on sterols.Similar effect to hemolysins in staphylococci.

Example: streptolysin A; streptolysin O

Lecithin is a phospholipid that protects cells from oxidative stress

• Action: Enzymes that hydrolyze membrane phospholipids (e.g. lecithin)
• Purpose: disrupt plasma membrane Damages the cell resulting in cell death and release of cellular contents

e.g. alpha toxin of Clostridium perfringens

Produced by: staphylococci, streptococci, and some bacilli

Result:Destroy host white blood cells (leukocytes)causes the disintegration of macrophages and neutrophils

Purpose:Prevents phagocytosisReduces the immune response

Action:Enzyme attaches to the plasma membrane of leukocytes

Causes pores to form Results in the degranulation of lysosomes within leukocytestriggers the release of lysosomal enzymes into the cytoplasm

Leukostatin Produced by several species of bacteria

Action: Interferes with the ability of phagocytes to phagocytize cellsPurpose: anti-phagocytic

Collagenase Produced by: clostridia

• Action: hydrolyzes collagen Collagen: the fibrous protein constituent of bone, cartilage, tendon, and other connective tissue.
• Forms the framework for connective tissuesPurpose: Allows for deeper penetration into tissue

Produced by: staphylococci, streptococci, clostridia•

Also called spreading factor•Action: Hydrolyzes hyaluronic acid (HA)•HA is a glue-like substance (mucopolysaccharide) that helps to hold cells of tissues together; sometimes referred to as “tissue cement”.•

Purpose: tissue penetration•Allows bacteria to pass between epithelial cells and move deeper into tissues

ElastaseProduced by several bacterial species (next slide)

Action: Breaks down elastin and lamininElastin is an elastic protein found in connective tissue, along with collagen.

It allows many tissues in the body to resume their shape after stretching or contracting.Laminin is a major protein of the basal lamina, part of the basement membrane of tissues and organs

Purpose: Allows for deeper penetration into tissue.

Proteases Proteases are also referred to as peptidases or proteinases

Purpose: to destroy or cleave proteins

Several types of microbial proteases are important in establishing an infection

Immunoglobulin a Protease Produced by: Streptococcus pneumoniae IgA plays an important role in protecting the host against respiratory pathogens

Action:Cleaves IgA into the two parts—the Fab portion and the Fc portionPurposeInactivates IgA; decreases the immune response

Cysteine proteaseProduced by Streptococcus pyogenes

• Action:Degrades proteins
• Purpose: destroys antibodies decreases the immune response, especially opsonization and antigen presentation (specific immunity)

Inactivates complement (innate immunity)

Coagulase•Produced by: Staphylococcus aureus and Yersenia pestis•Boils and abscesses are associated pathologies.•Action: Catalyzes blood clotting;•coagulates fibrinogen present in plasma;•accelerates coagulation rates during an infection•Fibrinogen is a glycoprotein produced by the liver and present in blood plasma; is essential for blood clotting.•Purpose:•Facilitates resistance to phagocytosis•Protects the pathogen from phagocytic leukocytes such as macrophages and neutrophils

KinasesAlso called fibrolysinsProduced by:staphylococci (staphylokinase)streptococci (streptokinase)Action: digests blood clots; breaks down fibrinPurpose: Prevents isolation of infectionWhen coagulation occurs, pathogens are trapped in a local area in the blood stream and cannot disseminateDissolving blood clots allows for greater tissue penetration and the spread to other areas of the body

Lipases Present in staphylococciAction: digests lipids

Purpose: Helps bacteria to colonize oily skin

NucleaseRefers to deoxyribonuclease(Dnase)Produced by: staphylococci, streptococciAction: cleaves DNAPurpose:Destroys leukocytes; anti-phagocyticLowers the viscosity of exudate, giving the pathogen greater latitude for spreadingExudate: fluid that has escaped out of tissues or blood vessels due to injury or inflammation

Beta-lactamasesPresent in many different species, especially staphylococci and streptococciAction: cleaves members of the penicillin drug familyPurpose: Confers resistance to beta-lactam antibioticsBeta-lactam drugs work on peptidoglycan, preventing cell wall synthesis, ultimately leading to cell deathAllows pathogen/infection to persist

MucinasePresent in Entamoeba histolytica (protozoa)Enzyme digests the protective coating on mucous membranesIs a virulence factor in amebic dysentery

KeratinaseDigests keratin, the principal component of skin and hairPresent in dermatophytesDermatophytes: fungal species that reside in the epidermal layer of the skin.Organisms cause superficial cutaneous infectione.g. ring worm (also called tinea)

SuperantigensProteins that activate a large number of T-lymphocytes, without stimulation by specific antigensPresent in bacteria and virusesActivates T-cells with different antigen specificitiesEffect: T-cells release a large amount of cytokines, which may lead to organ failure and immunosuppression

Examples:Staphylococcal enterotoxins (specifically SE-B)There are 24 different typesStaphylococcal toxic shock syndrome toxin, 1 and 2