What is the most dramatic effect of a microbial infection on a host cell?
What are some of the effects of the coxsackievirus?
myocarditis, kills insulin producing B cells (infantile diabetes)
What are the two mechanisms that can lead to cell death?
lysis and apoptosis
How do microbial infections cause lysis of host cells?
toxin breaks down cell membrane, organism multiplies in host cell causing lysis, killer lymphocytes cause host lysis because of intracellular microbe
What happens in patients with gas gangrene?
membrane damaging toxins lyse red blood cells
What happens in patients with Rocky Mountain Spotted Fever?
rickettsiae grow intraceullularly in endothelial cells of small blood vessels leading to lysis and a skin rash
What is apoptosis?
programmed cell death, normal part of cell cycle to ensure maintenance of healthy cells
What does shigella virus due to apoptosis?
takes over the system of apoptosis
What do HIV and herpes simplex do to apoptosis?
stimulate premature apoptosis
What does the Epstein-Barr virus do to apoptosis?
Why are toxins deadly to host cells?
they alter a key aspect of metabolism (like a hormone or pharmacological agent)
Why are tetanus, botulism, cholera, and whooping cough so dangerous?
because of the toxins they have
T/F. If present in enough numbers, agents can obstruct vital passages, example?
True, Ascarsis is a round worm infection that can obstruct intestines and/or bile duct
Mechanical obstruction due to a microbial infection is usually caused by what?
the inflammatory response to the infection
What happens in elephantiasis?
filariae worms get lodged in the lymphatics causing swelling of limbs and scrotum
T/F. Any duct or tube-like organ can be obstructed during an infection?
The symptoms of infectious disease are usually due to what?
T/F. Host response are present in acute and chronic disease and can be local or systematic?
If the host response is harmful, is it still beneficial? Example?
Yes because without it microbes would take over, TB is chronic and can be lived with for many years but if imunnocompromised it can become lethal very quickly
T/F. Toxins are important disease components in fungi, protozoa, and worms.
What are the four categories of bacterial toxins?
(1) exotoxins and type III cytotoxins (2) endotoxin and membrane damaging endotoxins (3) superantigens (4) exoenzymes
What is the site of action of exotoxins and class III cytotoxins?
What is the difference between exotoxins and class III cytotoxins?
exotoxins must bind to host cell receptor to be transported into host, class III cytotoxins are injected directly into host cells
What is the type I secretion system?
an autotransport system built into the bacterium that transports toxins out of the cell
What is the type II secretion system?
an apparatus within the bacterial cell membrane that transports toxins out of the cell
How are exotoxins secreted?
usually via type I or II secretion systems, but sometimes they're located on bacterial cell surface or within cytoplasm and are released upon lysis
T/F. Bacterial exotoxins differ in their specificity
True, some toxins are specific for one cell type, others are more broad spectrum
T/F. All bacteria make toxins.
False, example is pnuemococci
T/F. Bacteria may make one or multiple toxins
Are toxins dispensable to bacteria?
Yes because they aren't needed for growth, but they may be needed for survival and spread
Where are the genes for toxins located and why?
on dispensable genetic elements like plasmids and temperate bacteriophages because they are dispensable
The location of the genes for toxins allows for what benefits?
genes to make toxins can easily pass to non-toxin producing bacteria
Are toxins continuously made by bacteria?
sometimes, but other bacteria may only synthesize toxins when in the stationary phase
Why do some bacteria only secrete toxins in the stationary phase?
toxin helps bacterium obtain nutrients that have become sparse
When is diphtheria toxin produced?
when the diphtheria are starved of iron, the toxin breaks down host cells to get iron
How do bacteria release toxins during spore formation?
the bacterial cells in which the spores are formed eventually lyse leading to release of toxins in the cytoplasm
In a heterogeneous population of spores and live bacteria what is the result in terms of toxin production?
toxin production will be continuous
T/F. Toxins work at extremely low concentrations and are among the strongest poisons for humans
What is the structural organization of exotoxins?
A and B domain
What is the function of the exotoxin B domain?
binds to the cell membrane allowing the toxin to be endocytosed, once endosome becomes acidified the B domain goes into a conformational change and binds with endosome membrane to create a port for the translocation of the A domain into the cytoplasm
What is the function of the exotoxin A domain?
is specific for each toxin and is the enzymatic part of the toxin that modifies host components
How can the AB domains of exotoxins be arranged?
single AB protein with the domains covalently bound, A:5B noncovalently bound, independent proteins that associate at the cell membrane
Are exotoxins synthesized in their active form?
no, they're made as protoxins that are activated upon binding to the cell membrane - proteolytic cleavage and a disulfide bond reduction at the junction of the A and B domains
How are exotoxins internalized?
bind to host receptor, then receptor mediated endocytosis
How does the A domain of exotoxin get into the cytoplasm after being internalized?
the B domain creates a pore in the endosome that allows the A domain to exit endosome and enter cytoplasm
What are the typical targets of the A domain exotoxin?
proteins, but may also be RNA (shiga toxin)
Which toxins use ADP ribosyltransferases?
diphtheria toxin, cholera toxin, toxin A of p. aeruginosa
What is the ADP ribosyltransferases?
the toxin serves as a catalyst for the reaction that transfers ADP ribose from NAD to target proteins
Which toxins are neurotoxins?
tetanus and botulinum
What are neurotoxins?
What type of toxin is diphtheria toxin?
How does diphtheria toxin work in the cell to cause cell death?
the A domain ADP ribosylates EF-2 (elongation factor 2), which catalyzes the hydrolysis of GTP required for movement of ribosomes on mRNA, without this factor protein synthesis can't occur
How many A domains of diphtheria toxin are needed to kill one cell?
one A domain
Why is the diphtheria toxin so potent and powerful?
the A domain stays active for a long time
What does the A domain of the diphtheria toxin bind to on EF-2?
a modification in one of its histidine residues
What cells are resistant to diphtheria toxin?
those that lack the modified histidine residues on the EF-2
What type of toxin is cholera toxin?
a AB5 exotoxin
What is cholera toxin's effect on cells?
elevates intracellular cAMP levels, but doesn't kill the cells
What cells do cholera toxin specifically work on, how do they select only these cells?
intestinal epithelial cells, the B pentamer specifically binds intestinal mucosa cells
What does the A domain of the cholera toxin do?
ADP ribosylates one of the arginine residues of a G protein which up regulates cAMP production - the ribosylation causes the G protein to be in the active GDP form. The increased cAMP causes massive movement of fluids across the intestinal membrane and into the intestinal lumen
ADP ribosylation of a G protein is a common method of attack for what type of endotoxin bacteria?
pathogens that cause diarrhea
Why doesn't ADP ribosylation cause the same effects with different exotoxins?
because the exotoxins are ribosylating different proteins leading to different effects
What is the most potent protein toxin to humans?
What are the effects of tetanus toxin?
irreversible muscle contraction leading to spastic paralysis
What are the effects of botulin toxin?
blocks muscle contraction, yielding flaccid paralysis
What species is responsible for producing botulinum and tetanus toxin?
spore forming bacteria of genus clostridium
What cells do the B domains of botulinum and tetanus toxin bind?
receptors on neurons
What does the A domain of the botulinum toxin do?
migrates to peripheral neurons and enters via endocytosis, enters cytoplasm and inhibits vesicle fusion at the peripheral nerve ending and inhibits the release of stimulatory neurotransmitters
What does the A domain of the tetanus toxin do?
migrates to peripheral nerves and enters via endocytosis, migrates up the axon across the interneuronal junction and into the internal neuron where it inhibits the release of inhibitory neurotransmitters
What usually causes death in botulism and tetanus?
unbalanced motor neurons leading to death from decreased ventilation
How does tetanus enter the body?
usually by entrance of the tetanus bacilli via penetration of a foreign object (rusty nail)
What happens when tetanus bacilli enters the body?
it usually doesn't move from its site of entry, but the exotoxin it releases travels to the central nervous system
How does botulism occur?
production of toxin on the infection of the intestine of an infant by clostridium botulinum (infant botulism), or by ingestion of the toxin made in food contaminated by the bacteria and store under anaerobic conditions (canning)
Does botulism require the presence of the bacteria to cause pathology?
no only the toxin is needed
Does tetanus require the presence of the bacteria to cause pathology?
How do type III cytotoxins enter host cells?
via a contact-dependent mechanism where the bacteria directly contacts the host cell, on contact the bacteria uses a type III secretion apparatus to generate a pore in the host cell membrane and deliver the toxin to the host cytoplasm
What is the stimulus provided by the host cell that initiates type III cytotoxin entry?
thought to be temperature, oxygen tension, and salt concentration.
Do type III cytotoxins have contact with Ab?
no they're delivery prevents that
What is the MOA of type III cytotoxins?
interfere with ability of host to respond to infection by direct cell killing or through modulation of the actin cytoskeleton (needed for phagocytosis)
What type of toxins bind to the cell membrane and modulate cell physiology without entering?
endotoxins and membrane damaging toxins
What is endotoxin?
the LPS of the outer membrane of gram negative bacteria,( lipoteichoic acid from gram positive, lipoglycans from myobacteria - considered pattern recognition receptors)
What is the difference between high and low concentrations of endotoxin?
low concentrations lead to alarm reactions (host response), large concentration leads to shock and possible death
T/F. Endotoxin is only toxic in high concentrations?
Why are endotoxins also called pattern recognition receptors?
because the endotoxins activate the host innate immune system for response to infection (ultimately the adaptive immune system is activated)
What are the 3 parts of bacterial endotoxin?
a glycolipid- lipid A, a core of sugars, long side chain of sugars - O antigen
What is the biologically active portion of endotoxin?
What is needed for lipid A to be biologically active?
complexed with large molecular weight carriers such as the core and o antigen
What is lipid A composed of?
short chain fatty acids (12, 14, 16)
What are the primary targets of endotoxin?
mononuclear phagocytes, neutrophils, platelets, and b lymphocytes
How do endotoxins bind to their primary targets?
via TLR's located on the host cells
What happens when endotoxins bind to host cells?
a signal transduction cascade that leads to cytokine production
What are the host's alarm reactions?
fever, complement activation, macrophage activation, b lymphocyte activation
T/F. Low titers of An to endotoxin are found in most healthy people
True, endotoxins serve as a constant low-level stimulation of the immune response in healthy people
What cytokines are responsible for fever?
IL-1 and TNF-alpha
What pattern recognition receptors can elicit fever?
LPS of gram negative and lipoteichoic acids of gram positive
What pathway of complement does endotoxin activate?
the alternative pathway
In low concentrations what is the result of endotoxin induced complement?
phagocytosis, macrophage chemotaxis, MAC, opsinization
What are the anaphylatoxins and what do they do?
C3a and C5a, they increase capillary permeability and release of lysosomal enzymes from neutrophils (degranulation)
What effect does endotoxin have on the activation of macrophages?
increase lysosomal production, enhance rate of phagocytosis, secrete hydrolases
The high activation state of macrophages caused by endotoxin also allows the macrophages to do what?
kill cancer cells by direct attachment and by releasing TNF
What are biological response modifiers?
endotoxin derivatives that are being evaluated for use as anti-cancer drugs
How does endotoxin elicit the growth of B lymphocytes?
causes the release of IL-1 which stimulates b lymphocyte growth
Does endotoxin stimulate the growth of T lymphocytes?
T/F. Endotoxin is an immunological adjunct, why/why not?
True, because it stimulates the formation of B lymphocytes that lead to Ab protection for the future
What does fully activated endotoxin produce?
bacterial sepsis - body overwhelmed by bacteria
What are the symptoms of endotoxic shock?
serious hypotension, disseminated intravascular coagulation
What bacteria are the ones that cause bacterial sepsis?
gram negative bacteria like e. coli p. aeurginosa, and meningococci
What is the cause of endotoxin induced hypotension?
IL-1 and TNF-alpha
What is DIC?
small thrombi in small vessels with consequent damage to the areas supplied by the blood vessels
Which organs are particularly vulnerable to DIC?
kidneys, brain, lungs, adrenals
What is waterhouse-fridrichsen syndrome?
in meningococcal infection there is adrenal insufficiency due to infarction that leads to rapid death
In what 3 ways does endotoxin contribute to coagulation?
activates clotting factor VII (hageman), platelets release granules, neutrophils release proteins that stabilize fibrin clots
How do membrane damaging toxins act to hurt cells?
act as lipases or by inserting themselves in the membrane to make pores
What is lecithinase?
a lipase toxin used by clostridium of gas gangrene, lyses cells indiscriminately because its main substrate is lecithin and is found in all mammalian membranes
What are hemolysins ?
membrane damaging toxins that lyse RBC's and WBC's
What are pore-forming toxins?
membrane damaging toxins that create pores in membranes allowing water to flow into the cytoplasm and lyse cells or decrease cell viability and protein synthesis
What is an advantage of the membrane damaging toxins?
they are able to avoid the host's first line of defense
What are the pores of membrane damaging toxins (and pores caused by MAC) composed of?
fortified protein structures that are unusually resistant to proteases and detergents
What is the alpha-toxin of staph aureus?
a pore forming toxin - produces homogenous pores (each pore has same number of proteins)
What are heterogeneous pore forming toxins?
the form pores that vary in size and number of monomer toxin molecules
What is streptolysin O?
a heterogeneous pore forming toxin produced by certain streptococci, binds to cholesterol in cell membrane. Only lyses RBC's, but indirectly causes WBC cell death because it acts on the membrane of lysosomes and releases hydrolytic enzymes. The hydrolytic enzymes also cause tissue damage
What are superantigens?
toxins stimulate the production of large amounts of antibodies (not against superantigen), by complexing MHC complexes of APC with the t cell receptor on lymphocytes to stimulate antigen-independent activation of the lymphocyte
What are exoenzymes?
toxins that act in the extracellular matrix of the cell
What are some examples of exoenzyme toxins?
spreading factors that facilitate the dispersal of infecting organisms, gram positive secrete hyluronidase, Dnase
What is streptokinase?
enzyme made by streptococcus pyogenes that activates plasminogen and coverts it to plasmin that in turn attacks fibrin clots - eliminates fibrin barriers that might interfere with its spread, collagenases, and elastases
Do exotoxins tend to confer immunity for subsequent infections?
no because usually the encounter with the endotoxin is such a small amount that its not an effective immunogen
Does injecting an endotoxin into a person lead to immunization?
no because at the levels needed it would be too toxic
How can endotoxins be immunized against?
endotoxins can be chemically modified so that they aren't as toxic but still keep their immunogenicity - toxoids
What are toxoids?
chemically inactivated toxins that are used for vaccination
When do you receive the diphtheria and tetanus vaccinations?
series through first 5 years of life and boosters every 10 years (DTP)
What is passive immunization?
administration of an antitoxin (usually made in horses or other animals) to someone who has been exposed to toxin-producing pathogens
What is the disadvantage of anti-toxin administration?
serum sickness - immune reaction against foreign proteins
When must anti-toxin be administered?
to the non-immune person it must be given rapidly before the exotoxin enters cells and is unavailable to Ab
Which domain of endotoxin is being studied for use as a vaccination using recombinant DNA?
the B domain, because B domain can be added to serum without being toxic
T/F. Not all toxin-mediated diseases respond to vaccinations.
What is the difficulty with creating a successful cholera vaccine?
to be effective the vaccine must elicit an IgA response in the intestines, cholera injections give poor protection, but oral vaccines made of the disarmed pathogen stimulate intestinal IgA production
How are exotoxins being used in drug targeting?
coupling an Ab to the A domain of an exotoxin to target specific cell populations