Download doc Pathogenesis

Pathogenesis
What is pathogenesis? It comes from Greek.
Patho  disease
Genesis  causing
Bacterial pathogens are therefore bacteria that cause disease.
Infectious diseases are a major cause of death worldwide (more than cancer, cardiovascular
diseases, etc. Even for premature deaths). Secondary bacterial infections are also important
causes of death in viral illnesses.
25-40% of AIDS fatalities are caused by bacteria.
Another reason to study pathogenesis is the prevalence of antibiotics-resistant “superbugs”.
There is therefore a need to develop new drugs.
Infection and disease
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When a pathogen is established in a body : infection
Infection when producing symptoms : disease
Infection without disease : asymptomatic carriage
When a non-pathogen persists in the body : colonization (e.g. normal flora)
o Normal flora is very important for health
o Human body = 1013 cells + 1014 microbes
 Are we organisms or ecosystems?
 Example of commensalism
Host-pathogen relationship
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Parasitism : one partner (pathogen) benefits and the other (host) is harmed.
Pathogens :
o Primary: causes disease by direct interaction with host.
o Opportunistic : causes diseases under certain conditions.
 Pseudomonas aeruginosa is an opportunistic pathogen. Found in soil
and water, it can colonize skin and intestinal tract. Our defenses are
strong enough to prevent infection and disease. However, it can cause
fatal systemic diseases in immunocompromised people and cystic
fibrosis patients (thick mucus cannot clear bacteria). It is the cause of
death of many CF patients.
Host-pathogen relationship
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“Normal” host : susceptible host, e.g. humans
Reservoir host : nonhuman organism infected with pathogen that can also infect
humans (e.g. cows in pathogenic E. coli infections, chickens for Salmonella)
Not all infections lead to disease. What factors determine whether infection leads to disease or
not?
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Number of organisms present (disease)
Virulence of pathogen (disease)
o Products or structural components that contribute to virulence or pathogenicity
Host’s defences (health)
How to describe infections
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Acute , if short/severe VS chronic if persistent
Latent – persistent with few to no symptoms
Nosocomial – develops during hospital stay
Zoonosis – Associated with animals.
Virulence
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Degree or intensity of pathogen
Determined by 3 characteristics of pathogen
o Invasiveness : ability to spread to adjacent tissues
o Infectivity : ability to establish focal point of infection
o Pathogenic potential : degree to which pathogen can cause damage to host
 Toxigenicity : ability to produce toxins
 Immunopathology : ability to trigger exaggerated immune response
Measuring virulence
o LD50 : number of pathogens that’ll kill 50% of an experimental group of hosts
o ID50 : number of pathogens that’ll infect 50% of ...
Pathogenesis for bacterial disease
There are steps for infection by pathogenic bacteria.
1. Maintain a reservoir (place to live b4/after infection)
a. For humans, most reservoirs are :
i. Other humans (Mycobacterium tuberculosis)
ii. Animals
1. Cow  E.coli
2. Poultry  Salmonella, Campylobacter
3. Rats  Yersinia pestis
iii. Environment
1. Soil  Clostridium tetani
2. Be transported to host
a. Direct-contact : host-to-host (coughing, touching, sex, kiss)
b. Indirect contact : vehicles (soil, water), vectors, fomites – inanimate objects
that harbor and transmit pathogens (utensils, doorhandles).
3. Adhere to, colonize and invade host.
a. Adherence : mediated by special molecules or structures called adhesins
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Fimbriae : filamentous structures that help attach
Capsule : inhibits phagocytosis and aids in adherence.
Pili : Bind prokaryotes together for transfer of genetic material.
S layer : outermost regularly structured layer of cell envelopes.
Slime layer : bacterial film, less compact than a capsule, easily removed.
Teichoic and lipoteichoic acids : cell wall components in G+ that aid in
adhesion.
b. Colonization/Invasion : establishment of a site of microbial reproduction on
or within host.
i. Can be active penetration of cell.
1. Production of substances that disrupt cell surface or cell-cell
junctions.
2. Salmonella, for instance, produce proteins that induce massive
rearrangements of the cell surface and cause the cell to engulf
bacteria.
3. Many intestinal pathogenic bacteria attack and dismantle cell-cell
junctions in the intestinal epithelium and gain access between
cells.
ii. Can be passive penetration.
1. Not related to pathogen itself (skin lesion, bite)
2. Once below mucous membrane, bacterium can spread to deeper
tissue (involves production of virulence factors – enzymes or
specific products).
3. If bacteria gains access to circulatory system, there is an access to
all organs and systems.
4. Multiply or complete life cycles in host.
a. Occurs when pathogen finds appropriate environment within hosts
b. Important factors include access to nutrients, pH, temperature, redox potential,
protection from host “attack”
c. Different bacterial pathogens have evolved to survive and multiply within different
environments in host
d. Suitable environments vary greatly depending on species
e. Intracellular pathogens/Extracellular pathogens
f. Some bacteria invade specific cells and live inside
g. Some actively grow in plasma
i. Bacteremia : presence of viable bacteria in blood
ii. Septicemia : presence of bacteria or toxins in bloodstream
5. Initially evade host defences
6. Damage host
7. Leave host, return to reservoir or get to a new host.
The few last steps must occur if the microbe is to be perpetuated. Most bacteria leave
by passive mechanisms (feces, urine, droplets, saliva, desquamated cells). Many
symptoms of disease aid in this purpose (coughing, sneezing, runny nose and diarrhea).
Koch’s postulates (1890): They establish a casual relationship between a disease and a microbe.
++ Corrections from researches.
1. Microorganisms found in abundance in all organisms suffering from the disease and not
in healthy animals.
a. No. Opportunistic pathogens may be isolated from healthy individuals without
diseases or diseases with multiple causes.
2. Microorganisms should be isolated from diseased organism and grown in pure culture.
a. No. Some microorganisms are hard or nearly impossible to culture.
3. Cultured microorganisms should cause the disease when introduced in a healthy
organism.
a. No. Disease can depend on the health status of the host.
4. Microorganisms must be reisolated from step3 host and identified as identical.
Helicobacter pylori and ulcers
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G- bacterium that live in stomach
Cause of gastric ulcers
Strong link to gastric cancer
50% of the world has it
80% are asymptomatic
Until 1980, people didn’t think bacteria could live in the stomach because of its acidity.
Ulcers were thought to be caused by stress or spicy food
This bacterium is hard to culture
Barry Marshall drank a culture of Helicobacter, got ill and showed the organism and the
pathology on biopsy (then took antibiotics). He won a Nobel prize in 2005.
Virulence factors : Molecular Koch’s Postulates, by Falkow (1988)
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Gene encoding factor present in strains of bacteria that cause disease
Not present in avirulent strains
Disrupting the gene reduces virulence
Re-introduction of the gene reintroduces virulence.
Gene is expressedduring infection.
Specific immune response to gene protects the host.
That’s it for lecture 1 from Gruenheid...Lecture 2 right below
Ignaz Semmelweis :
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In the 1800s, the rate of death in childbirth was very high.
Physicians did not scrub up/wash hands between patients.
They commonly went from dissecting patients to delivering babies.
Women who had babies in hospitals were 4x more likely to contact childbed fever.
With handwashing, Semmelweis reduced the mortality to 1%.
Practice wasn’t widely used until his death (ppl realized diseases were caused by
microbes)
Koch’s postulates demonstrated causative agents of anthrax and tuberculosis. Falkow followed
the postulates with a genetic approach on what he called virulence factors.
How are virulence factors acquired?
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Transfer of extrachromosomal genetic elements, plasmids and phages
Many genes that encode bacterial virulence factors are encoded on plasmids and
phages.
These mobile genetic elements can transfer virulence factors between members of the
same species or different species.
Recombination can occur between the extrachromosomal elements and the bacterial
chromosome, making the virulence gene chromosomal.
Pathogenicity Islands :
o Large segments of DNA that carry virulence genes.
o Acquired during evolution of pathogen by horizontal gene transfer.
Regulation of Bacterial Virulence Factors
o Environmental factors control expression of virulence genes
o Signals can turn the genes on. Others are specific to the bacterial niche within
the host.
 Corynebacterium diphteriae
 Gene for diphtheria toxin regulated by iron.
 Higher expression in low iron.
 Low iron is characteristic of the host.
 Bordetella pertussis (whooping cough)
 Expression of virulent genes increased at body T compared to Rt
o Type III Secretion Factors (works like a needle, somewhat...)
 Encoded by approximately 20 genes
 Present in G- negative pathogens (Salmnella, Shigella, Yersinia, E.Coli)
 Absent from non-pathogenic bacteria
 Secretion triggered by contact with host
 The arsenal of proteins varies greatly even though the set of genes no.
o Type IV secretion systems look very much like conjugation apparatus
Virulence Factors : Toxins
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Exotoxins – Secreted. Four types
o AB exotoxins
 A subunit has enzyme activity (responsible for toxic effort).
 B mediates cell entry (binds to specific receptors on target cell).
o Specific host site exotoxins (can be AB)
 Neurotoxins
 Target nerve tissue
 Botulinum toxin causes paralysis
 Enterotoxins
 Target intestinal mucosa
 Cholera toxin causes secretory diarrhea
 Cytotoxins
 Target general tissues
 Nephrotoxin (kidney)
o Membrane-disrupting exotoxins
 Not AB
 Attack host cell membranes
 Lyse host cells by disrupting plasma membrane integrity
 Two types:
 Pore-forming exotoxins
o Leukocidins : kill phagocytic leucocytes
o Hemolysins : kill erythrocytes, leucocytes
 Phospholipases
o Remove the charged polar heads from the phospholipid
part of the host cell membrane.
o Destabilizes the membrane, causing the host cell to lyse.
o Superantigens
 Stimulates T cells to release cytokines
Endotoxins – Part of bacterial cell
o Heat labile proteins inactivated at 60-80 degrees
o Among the most lethal substances known to man
 Botulinum toxin is the most poisonous substance naturally occurring
 A gram of crystalline toxin evenly dispersed and inhaled would kill 1mln
ppl.
o Associated with specific diseases, have specific mechanisms of action
o Highly immunogenic (antitoxins = neutralize antibodies)
o Easily inactivated to form toxoids – inactivated toxins used to elicit immune
response
o Unable to produce a fever in the host directly
Toxigenicity
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Intoxications
o Diseases that result from an entry of a specific preformed toxin into host.
 Botulinum
 Staphylococcus aureus (1/3 are carriers)
o Doesn’t require entry of bacteria
o Usually very fast onset
o Different from infections in that, during an intoxication, the toxin is produced by
a bacteria IN THE FOOD, before it is ingested. On the other hand, an infection
involves the entry of the bacteria inside the patient. Spores are ingested, growth
in GI tract, toxin produced inside the body. Both lead to paralysis though.
Endotoxins LPS of G- bacteria
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Bound to the bacteria, released when cell lyses
Some released during bacterial replication
Toxic component is lipid A
Heat stable
Toxic only at high doses
Weakly immunogenic
Generally similar, despite source
Produce general systematic effects
Usually capable of producing general systematic effects (fever, shock, blood coagulation,
weakness, diarrhea, inflammation, intestinal hemorrhage, fibrinolysis)
Brings about these effects indirectly
o Endotoxins interacts with host molecules and cells
o Activating host systems
o Interaction with macrophages and monocytes through binding of LPS-BS 
release cytokines which produce fever (IL-1, IL-6, TNF-alpha)
Evasion of host defences by bacteria
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Have mechanisms to resist complement system, phagocytosis, and specific immune
response.
EHEC : an extracellular bacterial pathogens
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Cattle are the major reservoir
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o Asymptomatic carriers of EHEC in their intestine
o EHEC is shed in manure and can contaminate meat during slaughter
o Usually associated with ground beef, manure contaminated produce and water.
o Many outbreaks occurred (Mexico, Canada,)
o They have their pathogenicity islands as well
Upon ingestion of contamination of food or water, bacteria colonize the intestinal tract
of humans
Adherence is mediated by Type III Secretion System (attaching/effacing lesions) 
implicated in colitis and diarrhea
Shiga-like Toxins  AB Toxins which acts on the lining of blood vessels (vascular
endothelium)
Inactivated protein synthesis by modifying RNA component of ribosomes  leads to cell
death
This causes breakdown of the lining  haemorrhages
Implicated in more symptoms : bloody diarrhea, kidney failure
Cattle lack the specific receptors for the toxin, which is why they are asymptomatic
carriers.
How does it work?
Looks like Type II Secretion System. Throws in “translocated effector protein” through a needle
look-alike.
Mechanism of pedestal formation :
EHEC has intimins that bind to “TiREHEC” bound to IRSp53, itself bound to EspFU, bound to NWASP that leads to actine polymerization... There is a tight junction disruption during EHEC
infection. Depends on Type III-secreted effector proteins EspF, Map, NIeA
Shiga Toxins (EHEC) :
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Shiga toxin (Stx)
AB toxin : B subunit binds to lipid on the cell surface, whereas A subunit cleaves rRNA,
causing an arrest to protein synthesis.
Receptors on kidney, intestinal cells
Leads to kidney damage and hemorrhagic colitis
EHEC illness :
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Hemorrhagic colitis : watery diarrhea leading to bloody diarrhea
Infectious dose may be as few as 10 organisms.
0-20% of patients (children, elders), develop haemolytic uremic syndrome:
o Haemolytic anemia
o Renal failure (most common cause of acute renal failure in kids)
o Thrombocytopenia (decrease in platelets, tissue, haemorrhages)
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o Can lead to permanent loss of kidney function
Can be fatal, may lead to neurological symptoms, will probably cause chronic renal
problems.
Prevention and treatment
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Contaminated drinking water
o Leakage of sewage into pipes of fresh water
o Boil water
o Replace aging water supply systems
Contaminated ground meat
o Thorough cooking
Contaminated juices
o Pasteurization