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Chapter 14
Pathology
Definitions!
• Pathology – study of disease
• Etiology – cause of disease
• Pathogenicity – how a pathogen
overcomes host defenses to produce
disease
• Pathogenesis – development and
progression of disease
• Epidemiology – occurrence and spread of
disease
Symbiosis
• A relationship between two organisms
• Mutualism – both benefit
• Commensalism – one benefits, other is
unaffected
• Parasitism – one benefits, other is harmed
Normal Flora in Humans
• Resident flora –
permanent
microbes in the
body
– Most are
commensals or
mutualistic
• Transient flora –
temporary microbes
in the body
Symbiosis – Special Cases
• Microbial antagonism – competition between
microbes. Normal flora outcompete pathogens in
our bodies. This is a form of mutualism between
human and normal flora.
• Opportunism – disease caused when organism
is in a new environment, the host is
immunocompromised, or normal flora disturbed
– E.g. E. coli in UTI, Pneumocystis in immunodeficient
patients, overuse of antibiotics
Koch’s Postulates – Proves Etiology
Summary of Koch’s Postulates
1. The same pathogen must be present in
every case of disease.
2. The pathogen must be able to be
isolated and cultured in pure media.
3. The cultured pathogen from step 2 must
be able to cause disease again.
4. The same pathogen must be able to be
isolated from the organism given
disease.
Exceptions to Koch’s Postulates
• The pathogen may not be able to be
cultured in pure media (e.g. viruses,
Rickettsia).
• Diseases may have multiple causes (e.g.
nephritis, UTI)
• One pathogen causes multiple
diseases/symptoms (e.g. Mycobacterium
tuberculosis affects lung, brain, kidney,
skin and bones)
Pathogenicity in Bacteria
• Adherence – binding to host cells
• Penetration – entry into host cells
• Toxins – compounds that harm the host
Adherence
• Bacterial structures (pili, fimbriae, capsules,
slime layers) allow attachment to host
• Specific proteins such as adhesins may be used
for specific attachment to receptors on the host
Biofilms
• Mass of pathogens in cooperative adherence.
• First organisms that attach secrete materials that
assist others to attach and colonize
• E.g. dental plaques on teeth.
Penetration
• Endocytosis – adherence can trigger
endocytosis in host cell. Invasins may be
involved that help rearrange cytoskeleton
to facilitate entry and intracellular
movements.
Penetration
• Tissue degradation – enzymes secreted that
dissolve barriers. E.g. hyaluronidase breaks
down hyaluronic acid, a sugar that holds cells
together. This is the cause of gangrene.
Penetration
• Clotting
– Coagulase produced by staph allows clots to form,
protecting the colony.
– Streptokinase can dissolve clots to free pathogen so
they can spread.
Toxins
• Endotoxins
–
–
–
–
Produced by GramPart of the LPS
Nonspecific
Low toxicity (fever,
aches)
– Released when
bacteria die or divide
• Exotoxins
– Produced mostly by
Gram+
– Secreted
– Specific targets
– High toxicity
Exotoxins Types
• Hemolysins – destroy red blood cells.
• Neurotoxins – attacks nervous system.
• Superantigens – provoke intense immune
response.
Pathogenesis
• Types of Infections
– Duration and severity
– Placement of infection
– Sequence
• Disease Progression
– Stages of infection
Severity and Duration of Disease
• Acute – quick (weeks) but severe
– E.g. flu, ebola
• Chronic – long lasting (months-years) and
continuous
– E.g. tuberculosis
• Latent – has an inactive phase
– E.g. HIV
Placement
• Local – confined to one area
• Focal – localized to one area but
toxins/pathogens can affect other areas
• Systemic – affects entire body
Sequence of Infections
• Primary infection – the first infection of a
healthy person
• Secondary infection – the second
pathogen. Usually opportunistic
– Superinfection – a type of opportunistic
infection when the normal flora is destroyed.
Disease Progression
1. Incubation – infection, no symptoms
2. Prodromal – early/mild symptoms
3. Invasive – most acute and dangerous.
Acme is the peak
4. Decline – begin recovery, symptoms
subside
5. Convalescence – fully recovered, body
regains strength
Disease Progression