Microbiology- a clinical approach by Anthony
Strelkauskas et al. 2010
Chapter 7: Principles of disease
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How diseases are caused (etiology), how they
can be characterized, and the concepts of
sepsis and shock are important for developing
an in-depth understanding of infections.
It is important to understand the differences
between normal microbial flora and abnormal or
infectious microbial organisms.
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A disease is any negative change in a person’s
health.
◦ What is health?
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Etiology is the cause of a disease.
Normal microbiota consists of useful
microorganisms colonizing our external and
internal body surfaces.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2080455/
1. Health is the absence of any disease or impairment.
2. Health is a state that allows the individual to adequately cope with
all demands of daily life .
3. Health is a state of balance, an equilibrium that an individual has
established within himself and between himself and his social and
physical environment.
and causing organ damage
and many anaerobic bacte
and many gram negative anaerobic bacteria
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Can protect us through microbial antagonism
◦ Many bacteria produce bacteriocins which are localized
bacterial antibiotics.
◦ Bacteriocins can kill invading organisms but do not affect
the bacteria that produce them.
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Produces vitamins (Vit K and B)
Stimulates and trains our immune system
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Can become pathogenic
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◦ In an immunocompromised host
◦ When inoculated at a different body site
 E. coli is part of the normal flora of the digestive tract but can
cause infection if it enters the urinary tract.
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Etiology is the cause of disease.
Proof of etiology of infectious diseases can be
found using Koch’s postulates.
◦ 1: The same pathogen must be present in every case
of the disease and absent in the healthy.
◦ 2: The pathogen must be isolated and grown in pure
culture.
◦ 3: When inoculated into a healthy new host the pure
pathogen must cause the same disease.
◦ 4: The pathogen must be re-isolated from these newly
infected hosts.
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In some cases, Koch’s postulates cannot be
used.
Some microorganisms cannot be grown in pure
culture on agar media in the laboratory:
◦ Treponema pallidum (syphilis)
◦ Mycobacterium leprae (leprosy)
◦ Viruses and rickettsial organisms
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Polymicrobial infections
One microorganism can cause multiple diseases.
1.
2.
3.
4.
5.
Viral infection
The results seen
after a disease
occurs.
The cause of the
disease.
The portal of exit.
None of the above.
1.
2.
3.
4.
5.
Acute disease
Latent disease
Chronic disease
Secondary infection
Systemic infection
There are five specific phases
1. Incubation period – the time between the initial
infection and the first symptoms. The more
virulent the pathogen, the shorter the incubation
time.
2. Prodromal period – when the first mild and rather
general symptoms appear.
3.
Period of illness-most severe signs and symptoms
and rather specific
4.
Period of decline- symptoms and signs decline,
secondary infections may arise.
5.
Period convalescence- no signs or symptoms but
patient is not yet fully restored.
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Some diseases are communicable.
◦ They can spread from one person to another.
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Some diseases are not communicable.
◦ They cannot spread from one person to another and
simply remain within the infected host.
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Some communicable diseases are easily spread
from person to person and these are referred
to as being contagious.
◦ They spread very easily through contact with an
infected person.
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Isolation:
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Quarantine:
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Vector control:
◦ It prevents an infected individual from having contact with
the general population
◦ Patients are usually isolated in the hospital
◦ Can be difficult to achieve as it cannot be imposed until firm
diagnosis
◦ Exposed humans or animals are separated from the general
population while they still appear healthy.
◦ Lasts as long as the incubation period for the disease in
question
◦ If there are no longer any symptoms, the quarantine is lifted
◦ Rarely used today because it is difficult to enforce
◦ It is used to control the population of vectors, such as
mosquitoes, that carry pathogens.
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Herd immunity is an important concept in limiting
the spread of infection.
It is conferred to people through vaccination or if
they are naturally exposed to the infection and
prevents re-infection by the same pathogen.
When a majority of a population (herd) is immune
to an infection there are very few potential hosts
and the disease essentially disappears.
Good examples of herd immunity are polio and
smallpox.
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Current herd immunity for polio is high.
◦ The polio vaccine is routinely administered to children
so there are few targets available for infection.
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Current herd immunity for smallpox is low.
◦ Smallpox has been putatively wiped out worldwide.
◦ As a result, no one is vaccinated for this infection
anymore except for the military.
◦ Since vaccinations have ceased, the number of people
immune to small-pox is low and there are many
potential targets available for infection.
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Disease duration can vary depending on the overall
health of the host.
There are four categories of disease duration:
◦ Acute diseases develop quickly and last only a short time
e.g. measles.
◦ Chronic diseases develop slowly but last for a long time e.g.
tuberculosis, hepatitis B.
◦ Sub-acute diseases have an gradual onset (usually 6 to 12
months) and are almost always fatal e.g. sclerosing panencephalitis.
◦ Latent diseases remain in the host after the symptoms
disappear and can become reactivated years later e.g.
chicken pox/shingles.
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Some pathogenic bacteria are capable of
maintaining infections in hosts, even in the
presence of inflammatory and specific
antimicrobial mechanisms as well as a perfectly
good immune response.
Persistent bacterial infections are treated with
specific antimicrobial therapy.
Examples of persistent bacterial infections
include:
◦ Mycobacterium tuberculosis (causes tuberculosis)
◦ Salmonella enterica serovar Typhi (causes typhoid fever)
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Infections can be localized.
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Infections can be systemic.
◦ A local infection is contained (walled off) such as a
boil or an abscess.
◦ Local infections are the easiest to deal with medically.
◦ Systemic infections occur when pathogens move away
from the initial infection location (also known as the
focus of infection).
◦ This movement is usually associated with the blood or
the lymphatic system.
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Bacteremia – bacteria present in the blood
Septicemia – bacteria multiplying in the blood
with organ spread and organs dysfunction
Toxemia – toxins in the blood
Viremia – viruses in the blood
howshealth.com
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Primary – the initial infection which has acute onset
of symptoms.
Subclinical – no symptoms are visible even though
the person is infected
◦ These people are carriers of the disease and can infect
others.
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Secondary – seen in people that are already
weakened from a primary infection. Secondary
infections are caused by another microorganism
and can be more dangerous.
Example: Influenza virus causing flu and
subsequent infection with Haemophilus influenzae
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Normal microbiota helps to protect against
opportunistic pathogens.
Etiology is defined as the cause of a disease.
Koch’s postulates can be used to evaluate and
identify the etiology of a disease.
Disease can be acute, chronic, sub-acute, or
latent/ persistent.
Examples for infections that may become latent
and persistent are tuberculosis (Mycobacterium
tuberculosis) and typhoid fever (Salmonella enterica
serovar Typhi ).
Infection can be local or systemic, primary or
secondary
1.
2.
3.
4.
5.
Quarantine
Isolation
Herd immunity
Treat with
antibiotics.
None of the above.
1.
2.
3.
4.
5.
Mycobacterium
tuberculosis in the lungs
Salmonella enterica serovar
Typhi surviving in the
gallbladder
Saprophytic bacteria
surviving on sloughed cells
of the ear
Chlamydia species surviving
in epithelial cells of the
genital tract
Bacteria in the colon
providing vitamins K and B
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11:45pm – 1:20pm
Chapters 1 thru 6: Lecture, Reading, Chapter
End Self Study Questions
Twenty-five Multiple Choice Questions = 50
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