7313515 normal flora microbial pa tho genesis and host parasite relationship
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Microbial Pathogenesisand Host-Parasite Relationships
Medical Microbiology
Normal Flora
For : www.esnips.com/web/m4mn
By: Mohammed M.M..Manaa
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Normal Flora
General aspects
Remember definition: organisms frequently found
on or within body of healthy individuals Most are bacteria, but some are viruses, fungi,
and protozoa
We do not carry all of them all of the time
Each person has individualized normal flora
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Normal Flora
Some are found only on body; others also
found in environment
Problem: some people have transient normalflora (pathogens)
Example: about 10% of population havemeningococcus or pneumococcus as normal
flora
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Importance
Depends on pathogen andon defenses ofhost:
Candida(yeast) causes pneumonia in peopleundergoing cancer chemotherapy
Pneumocystis carinii(common inhabitant of lung)causes pneumonia and death in AIDS patients
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Immune Stimulation
Antigenic stimulation by normal flora do
not have high antibody titers
Serve as defense mechanism even in lowconcentration
Bacterial stimulation leads to production of IgAthat is secreted through mucus membranes
Probably interfere with colonization of deeper tissues
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Immune Stimulation
Sometimes antibodies elicited by normal
flora cross-react with normal tissue:
Antibodies against ABO blood group substances: A - make B antibodies
B - make A antibodies
O - make antibodies against both
Why? Bacteria from intestinal flora contain Ag that cross-react with both A & B blood substances
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Immune Stimulation
Cross-reactivity does not normally cause
disease
Possible for antibodies cross-reactive to microbial Agto cause problem
Lupus erythematosusproduction of Ab against host DNA
Some evidence that Ag may be cross-reacting bacterial LPS
May cross-react with pathogen (meningococcus)
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Physical & Chemical Aspects
Keeps out invaders
Mechanisms:
Physical advantage of previous occupancy
Some produce bacteriocins or antibiotics
Relevance to lab work: E. coliK-12 cannot
compete with intestinal flora
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Physical & Chemical Aspects
Antibiotic effects: wipes out normal flora
Both endogenous and exogenous organisms can
cause disease Infecting dose of Salmonelladecreases one million-fold
when mice given streptomycin
Patients treated with some potent antibiotics:
Suffer from diarrhea due to overgrowth of yeasts, andstaphylococci
Administration of clindmycin-Clostridium difficile(minormember of normal flora) causes pseudomembranous colitis
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Physical & Chemical Aspects
Role in human nutrition and metabolism
E. coliand Bacteriodessynthesize vitamin K
Metabolism of key compounds involves excretion
from liver into intestine and their return to the liver
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Physical & Chemical Aspects
Source of carcinogens
Large intestinal flora
Many potential carcinogens are only active after beingmodified
Some modifications are carried out by enzymes ofintestinal bacteria; example: cyclamate converted tobladder carcinogen (cyclohexamine) by bacterial
sulfatases Importance of carcinogen production not clear
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Ecology of Normal Flora
Use of germ-free animals
Immune systems not well developed
Have to be fed vitamins
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Ecology of Normal Flora
Parts of body colonized
Contain large numbers:
Skin
Respiratory tract (nose and oropharynx)
Digestive tract (mouth and large intestine)
Urinary tract (anterior parts of urethra)
Genital system (vagina) Most are strict anaerobes
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Strategies for StudyingMicrobial Pathogenesis
Medical Microbiology
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Identification of Pathogens
Traditional:associate disease withorganism
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Kochs Postulates
3 Pure culture inoculated into susceptibleanimal should produce disease
4 Same bacterium re-isolated in pure culture
from experimental animal
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Kochs Postulates
Some assumptions questioned in light of
more modern approaches and newinformation about host-parasite interaction
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Challenge to Postulate #2
Places considerable emphasis on culturing
organisms in pure culture
Some organisms have notbeen cultured inlaboratory media
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Challenge to Postulate #2
For example, Treponema pallidium,Mycobacterium lepraeclearly cause disease:
Antibiotics cause both symptoms and organisms
from tissues to disappear
Immune response in infected patients to surface
Ag of bacteria from infected tissue
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Challenge to Postulate #3
Implies all members of a bacterial species are
equally virulent and only a single species
causes disease
Different strains of species vary in virulence
Different strains can cause different diseases
Same symptoms caused by numerous organisms
Disease caused by multiple organisms
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Challenge to Postulate #3
Well known fact that cultivation of some
pathogens can lead to loss of virulencefactors
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Challenge to Postulate #4
Requires pathogen be reinoculated into an
animal and produces symptoms of disease
Some diseases dont affect animals, or cause
different symptoms from human form
Therefore, to be practical, Kochs Postulatesrequire animal models
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Identification of Pathogens
Molecular version
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Molecular Version
Emphasis shifted from identification of
pathogens to identification of virulence factors
Not completeagreement on requirements toprove a particular gene or product plays a role
in disease, but criteria widely accepted
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Molecular Version
1 Gene or product found in strains that cause
disease and not in avirulent bacteria
If gene found in organisms not known to
cause disease, gene should be mutated to lessactive or inactive form, or not expressed
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Molecular Version
2 Disrupting gene in virulent strain reduces or
eliminates its virulence
Introduction of cloned gene into avirulent strain
should make it virulent
Systems with multiple genes:
These other genes would also have to be modified
or introduced
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Molecular Version
3 Gene is expressed in bacteria inside host
sometime during disease process
4 Ab to gene product should be protective or incases where cell-mediated immunity
involved, gene product should elicitprotective immunity
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Identification without Culturing
Combine PCR:
Polymerase Chain Reaction with 16S r-RNA phylogeny
16S r-RNA found in all bacteria
Conserved (domain) and variable (particularorganism) sequences
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Identification without Culturing
Sizable database and similarities in sequence
correspond well to evolutionary relationships
Sequence will either identify it as member of
known or unknown species
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Identification without Culturing
PCR primers that recognize two conserved
regions of 16S rRNA flanking a variableregion are used to amplify and clone a DNA
segment from a clinical speciman
If amplified segment is obtained, indicatesbacteria present in speciman
It can be sequenced to identify bacterium
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Identification without Culturing
Fluorescently labeled probe of sequence can
then visualize bacterium in clinical speciman
Rules out PCR amplification of contaminatingDNA from other sources