MLAB 2434 – CLINICAL MICROBIOLOGY
SUMMER, 2005
CECILE SANDERS & KERI BROPHY
Chapter 6 – Host-Parasite
Interaction
Chapter 6 – Host-Parasite
Interaction (cont’d)
Pathogen – microbe that can cause
disease in a susceptible host
 Opportunistic Pathogen – microbe
that can cause disease only if a
significant change occurs in host
resistance or within the organism
itself
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Chapter 6 – Host-Parasite
Interaction (cont’d)
Opportunistic Infections infections caused by opportunistic
pathogens
 Iatrogenic Infections – infections
resulting from medical treatment or
procedures
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Chapter 6 – Host-Parasite
Interaction (cont’d)
Virulence – relative ability of a
microorganism to cause disease, or
the degree of pathogenicity
 Virulence factors – factors such as
capsules, toxins, enzymes, cell wall
receptors, pili, etc. that allow
pathogens to evade or overcome
host defenses
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Chapter 6 – Host-Parasite
Interaction (cont’d)
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Host Resistance Factors
Physical barriers – skin
 Cleansing mechanisms
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• Desquamation
• Fluids of the eye (IgA and lysozyme)
• Respiratory, digestive, urinary, and
genital tracts have fluids and
movements to cleanse the surfaces
Chapter 6 – Host-Parasite
Interaction (cont’d)
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Antimicrobial Substances
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Lysozymes
Antibodies
β-lysins
Interferon
Indigenous Microbial Flora
Phagocytosis
Chemotaxis
Inflammation
Immune Responses
Chapter 6 – Host-Parasite
Interaction (cont’d)
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Infectious Agent Factors
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Adherence – most infectious agents must
attach to host cells before infection
occurs
Proliferation – pathogens must be able to
replicate after attachment to host cells
(overcome host resistance factors)
Tissue Damage – makes the infection
visible; results from toxins or from host
inflammatory substances
Chapter 6 – Host-Parasite
Interaction (cont’d)
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Exotoxins
• Most have two units, one for attachment and
the other is the toxin
• Commonly coded by phages or plasmids
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Endotoxins
• G- bacteria
• Toxicity caused by the lipid A portion of the
lipopolysaccharide in the cell wall
• Cause dramatic host responses, including
inflammation, fever, hypotension, septic shock,
death
Chapter 6 – Host-Parasite
Interaction (cont’d)
Invasion – all pathogens have the
ability to penetrate and grow in
tissues
 Dissemination
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• Spread of organisms to distant sites
• Some pathogens stay at site (C.
diphtheriae); others spread (Salmonella
ssp.)
Chapter 6 – Host-Parasite
Interaction (cont’d)
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Routes of Transmission
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Airborne
• Coughing, sneezing, talking
• Droplet nuclei
• Airoborne pathogens must be resistant
to drying and inactivation by ultraviolet
light
• Examples: Strep throat, otitis media,
diphtheria, rhinoviruses (colds)
Chapter 6 – Host-Parasite
Interaction (cont’d)

Transmission by Food and Water
• Ingestion of contaminated food or
water
• Sometimes oral-fecal route
• Pathogens must be able to survive
stomach conditions and compete with
normal flora of the gut
• Preformed toxins (Clostridium
botulinum, S. aureus) vs. toxins
produced after infection (C. difficile,
V. cholerae)
Chapter 6 – Host-Parasite
Interaction (cont’d)
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Close Contact
• Passage of organisms by salivary, skin,
and genital contact
• Examples: Infectious mononucleosis,
STDs
Cuts and Bites
 Arthropods
 Zoonoses – diseases of animals
accidentally transmitted to humans;
examples: plague, rabies, tularemia
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Chapter 6 – Host-Parasite
Interaction (cont’d)
Epidemiology – study of the
occurrence, distribution and causes
of disease and injury
 Definitions
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Carrier – person or animal who
harbors and spreads microorganisms
that cause disease but does not
become ill; examples: N.
meningitidis, S. pyogenes
Chapter 6 – Host-Parasite
Interaction (cont’d)
Endemic – Organism or disease is
constantly present in a population;
examples: Cholera is endemic in
third world countries
 Epidemic – Disease affects a
significantly large number of people
at the same time in a geographic
area; examples: Influenza
 Pandemic – Worldwide epidemic
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Chapter 6 – Host-Parasite
Interaction (cont’d)
Incidence Rate – number of times a
new event occurs in a given period;
usually given as cases per 1000 or
100,000 population
 Incubation Period – time between
exposure to a pathogen and the
onset of symptoms
 Index Case – first case of a disease
which serves as source of infection
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Chapter 6 – Host-Parasite
Interaction (cont’d)
Morbidity Rate – rate at which an
illness occurs
 Mortality Rate – number of deaths
caused by a disease in a population
 Nosocomial Infection – infection
acquired during hospitalization
 Reservoir – source of infection, such
as a person, animal or something in
environment
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Chapter 6 – Host-Parasite
Interaction (cont’d)
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Surveillance – collection of data
pertaining to disease occurrence
Surveillance and Reporting
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Certain diseases are required by law
to be reported to public health
authorities
http://www.tdh.state.tx.us/ideas/report/conditions/
Chapter 6 – Host-Parasite
Interaction (cont’d)
Diseases that have significant
effect on the population (STD) or
have potential for grave
consequences (anthrax, plague)
 Centers for Disease Control and
Prevention (CDC)
 World Health Organization (WHO)
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