MICROBIAL TOXINS

Question 1

Define and describe the term “microbial toxins”.

Answer 1

Microbia toxins are macromolecular products of microbes that cause harm to susceptible animals by altering cellular structure or function.

Question 2

Explain how a microbial toxin is implicated in pathogenesis of an infectious disease.

Answer 2

• Purified toxin causes the same sx/S as infection by the toxin-producing microbe.
• Antitoxin prevents disease caused by the toxin-producing microbe.
• Virulence of individual bacterial strains correlates with the amount of toxin that they produce.
• Nontoxinogenic mutants are avirulent; that virulence is restored if the microbe regains the ability to produce toxin.

Question 3

Toxins that facilitate the spread of microbes through tissues

Answer 3

Hyaluronidase, collagenase, elastase, deoxyribonuclease, streptokinase—> toxic enzymes that break down ECM or degrade debris in necrotic tissue, enhancing spread of microbes.

Question 4

Toxins that damage cellular membranes.

Answer 4

Hemolysin and Cytolysins- insert in cell membranes and assemble into multimeric complexes that form pores, thereby causing lysis of target cells.

Lecithinases degrade specific cell membrane components and disrupt integrity of membranes

Question 5

Toxins that stimular cytokine production

Answer 5

Superantigens (include pyrogenic exotoxins) activate T cells , bind to both MHC Class II and Vb chains on T cells, activating a greater number of T cells than any specific antigen does. This stimulates excessive production of cytokines (i.e. IL-2 and INF gamma), causing pathological effects.

Question 6

Diphetheria Toxin/Pseudomonas aeruginosa exotoxin A

Answer 6

Both toxins are ADP ribosyltransferases.

Both inactivate EF-2, which is required for peptide chain elongation.

Question 7

Shiga toxins of Shigella dysteriase, E. coli

Ricin

Answer 7

RNA N-glycosidases—> removed an adenine residue from a ribosomal subunit, thereby inactivating ribosomes.

Question 8

Heat- labile enterotoxins of Vibrio cholerase and E coli

Answer 8

ADP ribosyltransferases that increase cAMP activation through Gs protieins, resulting in active increased Cl- secretion and diarrhea.

Question 9

Pertussis toxin

Answer 9

ADP ribosyltransferase, increases cAMP activity by inhibiting Gi proteins—> causes tissue specific effects.

Question 10

Heat-Stable enterotoxin I (ST-I) of E Coli

Answer 10

Activates cell membrane associated guanylate cyclase—> increases intracellular cGMP in enterocytes—> diarrhea

Question 11

Anthrax edema factor (EF) and adenylate cyclase toxin

Answer 11

Adenylate cyclases, enter cells—> increase cAMP levels—> water efflux–> edema.

Enzyme requires activation by Ca2+ and calmodulin.

Question 12

Anthrax lethal factor (LF)

Answer 12

Endopeptidase that cleaves MAP kinase kinase proteins and inactivates them, eventually causing death. Intermediate steps not known.

Question 13

Clostridium difficile toxins A and B

Answer 13

Glucosyl transferases that alter actin cytoskeleton of target cells by transferring glucose from UDP-glucose to several Rho GTPases, and inactivating them.

Question 14

Botulinum toxin/Teatnus toxin

Answer 14

Zn-dependent endopeptidases that inactivate specific SNARE proteins req’d from neuroexocytosis (i.e. VAMP, SNAP-25, and syntaxin).

Botulinum toxin causes flaccid paralysis of skeletal muscles by inhibiting ACh release at myoneural jct.

Tetanus toxin causes sustained muscular contraction of skeletal muscles by inhibiting release of neurotransmitter from inhibitory interneurons in spinal cord.

Question 15

Compare the properties of microbial toxins that have different mechanisms of action.

Answer 15

Bacterial protein toxins: USUALLLY heat labile, immunogenic, and neutralized by specific antibodies. Originally called exotoxins b/c they were found outside of bacterial cells. Diphetheria toxin is secreted into culture medium, botulinum toxin are released by lysis of the bacteria.

Endotoxins are associated with bacterial cells (eg. LPS of gram negative bacteria). Endotoxins are PAMPs that are recognized by the innate immune system, via TLRs CD14 and LPS binding protein.

Low levels of LPS active macrophages, B cells and alternative complement pathway.

High doses of LPS cause shock and disseminated intravascular coagulation.

Question 16

Explain the principles of immunization against toxin-mediated diseases.

Answer 16

-Antitoxins (Antitoxic antibodies) bind to toxins and neutralize them, thereby preventing their toxicitiy.]

-Toxoids (derivatives of toxins that retain immunogenicity but lack toxicity) are used as vaccines for long-term protection against toxin-mediated diseases.
‘
-Passive immunization= admin of antibodies to provide immediate but temporary protection against a toxin or infectious agent.

• Active immunization= admin of toxoid to a patient in order to elicit production of specific anti-toxic antibodies.
• -> primary series of immunizations and periodic boster doses are req’d to achieve and maintain protective levels of antitoxin.
• –> Active immunity can persist for many yrs b/c of immunologic memory.

Question 17

Explain the principles for developing novel therapeutic agents based on toxins.

Answer 17

Immunotoxins contain active fragment of a toxin chemically conjugated to or expressed as a fusion protein with ligands for specific receptors that differ from the receptors for the native toxins.—> provides a new receptor-binding moiety that will redirect the toxic component to target cells that express the alternative receptor.

Used in cancers, autoimmune diseases, and other disorders.