Chapter 12
Mechanisms of
Infectious Disease
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Infection or Colonization With
Microorganisms
• “Infection” or “colonization” means that microorganisms are
multiplying in or on the host
Discussion:
• Do you have any infections or colonizations at this moment?
– List as many as you can identify
• Are they normal, or are they making you ill?
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Infection or Colonization With
Microorganisms (cont.)
• Over 300 different species of bacteria live in the
large intestine
• Bacteria and fungi live on our skin
• The mouth and pharynx contain many species
of bacteria
• The vagina contains acid-producing bacteria
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Microflora
• The microorganisms normally living in or on
your body
• Some are useful
• Many have no effect
• Pathogens cause disease
• All are capable of causing disease if your health
and immunity are weakened
• Opportunistic pathogens
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Question
Tell whether the following statement is true or false.
All interactions between humans and microorganisms are
detrimental.
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Answer
False
Rationale: Some microorganisms perform important
functions for their human hosts, like producing vitamins,
assisting digestion, or preventing harmful pathogens
from entering the host.
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Kinds of Infectious Agents
• Prions
– Small modified infectious host proteins
– Abnormally shaped versions of your own proteins
– Cause normal proteins to change their shape and
become new prions
– Can clump together and damage cells
– Cause degenerative disease in the central nervous
system, e.g., mad cow disease
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Kinds of Infectious Agents (cont.)
• Viruses
– Protein coat
surrounding nucleic
acid core
– Have no metabolic
enzymes of their own
– Insert their genome
into a host cell’s DNA
– Use that cell’s
metabolic machinery
to make new viruses
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Kinds of Infectious Agents (cont.)
• Bacteria
– Cells without
membrane-bound
organelles
(prokaryotes)
– Can live independently
– Use infected organism
for food and shelter
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Kinds of Infectious Agents (cont.)
• Bacteria (cont.)
– Can produce toxins
– Exotoxins are proteins
released by bacteria
– They damage or kill
host cells
– Endotoxins are parts of
the bacterial cell wall
– They cause host
immune reactions
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Kinds of Infectious Agents (cont.)
• Mycoplasmas, rickettsiae, chlamydiae
– Smaller than bacteria
– Mycoplasmas lack cell walls
– Rickettsiae and chlamydiae have to live inside
cells to metabolize, like viruses
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Kinds of Infectious Agents (cont.)
• Fungi
– Most require a
cooler
temperature
than human core
body
temperature
– Most infections
are on the
surface of the
body
Tinea is a fungal infection of the skin
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Kinds of Infectious Agents (cont.)
• Parasites
– Protozoa: malaria,
amoebic dysentery,
giardiasis
– Helminths: roundworms,
tapeworms, flukes
– Arthropods: ticks,
mosquitoes, mites,
lice, fleas
Trichomonas vaginalis is a
protozoan that infects the vagina
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Question
Which pathogen is an intracellular parasite consisting of a
protein coat surrounding a nucleic acid?
a. Prion
b. Virus
c. Bacteria
d. Protozoa
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Answer
b. Virus
Rationale: Viruses have no organized cellular structure
like bacteria and protozoa. Viruses can only replicate
inside another cell. Prions cannot reproduce at all.
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Discussion
• How many ways could you have become
infected today? How could you have
experienced:
– Direct contact with a pathogen?
– Ingestion of a pathogen?
– Inhalation of a pathogen?
– Contact with a zoonosis?
– Contact with a nosocomial infection?
– Contact with a fomite?
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Symptomatology
infection
inflammatory and
immune
responses attack
infective agent
SPECIFIC: signs
and symptoms of
local damage and
inflammation
NONSPECIFIC:
signs and
symptoms of
systemic
inflammation
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Scenario
A 5-year-old boy has an ear infection.
• He complains of pain in his ear and cannot hear on that
side
• When you look into his ear, you see a red, bulging
eardrum with pus behind it
• He has a fever, sweats, and complains of joint aches
• Blood tests show an elevated white blood cell count
Question:
• Use the model of symptomatology to classify these signs
and symptoms
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Discussion
What stage of an infection are you in?
• How many people in the class are in:
– The incubation stage?
– The prodromal stage?
– The acute stage?
– The convalescent stage?
– The resolution stage?
How can you tell?
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Terms for Infection and Damage
• -itis means inflammation
– May or may not be due to infection
• -emia means in the blood
• Sepsis or septicemia means bacterial toxins
in the blood
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Virulence Factors
• Make an infection more likely to cause disease
– Toxins: exotoxins and endotoxins
– Adhesion factors help infective organism
stick to the body
– Evasive factors help keep immune system
from killing infective agent
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Question
Tell whether the following statement is true or false.
Certain bacterial cells release proteins called endotoxins
during growth.
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Answer
False
Rationale: Exotoxins are proteins; endotoxins contain no
protein (they are composed of lipids and
polysaccharides). Endotoxins are not released during
bacterial cell growth.
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Scenario
A woman’s stomach contained the bacterium
Helicobacter Pylori.
• For many years, the woman was healthy
• Then she took on a new, stressful job; moved; and
began to care for her elderly parents
• A few months later, she began to suffer stomach pains
and vomited blood
• She was diagnosed with a bleeding ulcer
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Scenario (cont.)
Question:
• How does each of these terms relate to her case?
– Portal of entry
– Site-specific pathogen
– Opportunistic pathogen
– Evasive factors
– Invasive factors
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Serology
• After exposure to an
infectious agent, the
body produces
antibodies
• Antibody titer rises
• IgM: rises during the
acute phase, then falls
• IgG: remains elevated
after the acute phase
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Scenario
A month-old baby is ill:
• Serum analysis shows that she has IgG against
HIV and IgM against Pneumocystis
Question:
• What inferences can you make?
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Antibacterial and Antiviral Drugs
• We have more drugs to kill bacteria than to kill
viruses, and more drugs to kill viruses than to
eradicate prions
Question:
• Why has it been easier to develop antibacterial
drugs than antiviral drugs?
• Why not use antibacterial or antiviral drugs to
destroy prions?
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Antibiotics Kill Bacteria By Targeting:
• Cell wall synthesis
• Protein synthesis
• Nucleic acid synthesis
• Bacterial metabolism
Bacteria Fight Back By:
• Inactivating antibiotics
• Changing antibiotic binding sites
• Using different metabolic pathways
• Changing their walls to keep antibiotics out
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Antiviral Agents Kill Viruses By:
• Blocking viral
RNA or DNA
synthesis
• Blocking viral
binding to cells
• Blocking
production of the
protein coats
(capsids) of new
viruses
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Question
What type of infections are treated based upon the results
of a Gram stain?
a. Fungal
b. Viral
c. Bacterial
d. Parasitic
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Answer
c. Bacterial
Rationale: Bacteria are commonly classified according to
Gram stain. Gram-positive and gram-negative
organisms are treated with specific antibiotics that
target that type of infection. For example, penicillin
targets gram-positive organisms. If the cause of
bacterial infection is unknown, broad-spectrum
antibiotics may be prescribed, targeting both grampositive and gram-negative organisms.
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