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Bacterial Physiology (Micr430)
Lecture 18
Bacterial Pathogenesis
(Based on other textbooks such as Madigan’s)
Microbe-Host Relationships
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If a microbe is not lost or shed from the
host after entry into the host (animal or
plant), there are a few types of
relationships that could develop:
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Commensalism: a coexistance that is
harmless to the microbe and host;
Symbiosis: a mutual benefit to both the
microbe and host;
Parasitism: a benefit to the microbe at the
expense of the host.
Definitions
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Pathogen: An organism, usually a
microorganism, that causes disease to another
organism (animal or plant)
Infection: Growth of an organism within a host
Disease: Injury to the host that impairs host
function
Infectious Diseases: Diseases caused by the
growth of pathogenic microorganisms in the
host body
Overview of Human-Microbial
Interactions
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Animal bodies are favorable environments
for the growth of microorganisms, most
of which do no harm.
The ability of a pathogen to cause
disease is called pathogenicity. An
opportunistic pathogen causes disease
only in the absence of normal host
resistance.
Mucous Membranes
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Pathogen growth on the surface of a
host, often on the mucous membranes,
may result in infection and disease.
Mucous membranes are often coated with
a protective layer of viscous soluble
glycoproteins called mucus.
The ability of a microorganism to cause or
prevent disease is influenced by complex
host-parasite interactions.
Normal Microbial Flora of Skin
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The skin is a generally dry, acidic
environment that does not support the
growth of most microorganisms.
However, moist areas, especially around
sweat glands, are colonized by grampositive Bacteria and other members of
the skin normal flora. Environmental and
host factors influence the quantity and
quality of the normal skin microflora.
Normal Microbial Flora of Skin
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The normal flora of the skin consists of
transient or resident populations of
microorganisms.
The skin is continually being inoculated
with transient microorganisms, virtually
all of which are unable to multiply and
usually die.
Resident microorganism are able to
multiple, not merely survive, on the skin.
Normal Flora of the Oral Cavity
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Bacteria can grow on tooth surfaces in
thick layers called dental plaque.
Plaque microorganisms produce adherent
substances. Acid produced by
microorganisms in plaque damages tooth
surfaces, and dental caries result. A
variety of microorganisms contribute to
caries and periodontal disease.
Normal Flora of the GI Tract
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The stomach is very acidic and is a
barrier to most microbial growth.
The intestinal tract (Figure 21.8) is
slightly acidic to neutral and supports a
diverse population of microorganisms in a
variety of nutritional and environmental
conditions.
Normal Flora of Respiratory Tract
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In the upper respiratory tract
(nasopharynx, oral cavity, and throat),
microorganisms live in areas bathed with
the secretions of the mucous membranes.
The normal lower respiratory tract
(trachea, bronchi, and lungs) has no
resident microflora, despite the large
numbers of organisms potentially able to
reach this region during breathing.
Normal Flora of Respiratory Tract
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The presence of a population of normal
nonpathogenic microorganisms in the
respiratory tract is essential for normal
organ function and often prevents the
colonization of pathogens.
Harmful Microbial Interactions
with Humans
Definitions
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Pathogenicity: the ability to cause disease
in a host
The degree of pathogenicity is called
virulence
Attenuation is loss of virulence.
Entry of the Pathogen into the
Host
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Pathogens gain access to host tissues by
adherence to mucosal surfaces through
interactions between pathogen and host
macromolecules. Table 21.3 gives major
adherence factors used to facilitate
attachment of microbial pathogens to
host tissues.
Pathogen invasion starts at the site of
adherence and may spread throughout
the host via the circulatory systems.
Colonization and Growth
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A pathogen must gain access to nutrients
and appropriate growth conditions before
colonization and growth in substantial
numbers in host tissue can occur.
Organisms may grow locally at the site of
invasion or may spread through the body.
If extensive bacterial growth in tissues
occurs, some of the organisms are usually
shed into the bloodstream in large
numbers, a condition called bacteremia.
Process of
Microbial
Infection that
Causes a Disease
Virulence
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Virulence is determined by invasiveness,
toxicity, and other factors produced by a
pathogen (Figure 21.16). Various
pathogens produce proteins that damage
the host cytoplasmic membrane, causing
cell lysis and death.
Virulence is measured by the LD50 (the
dose of an agent that kills 50% of the
animals in a test group) or ID50
Virulence Factors
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Pathogens produce a variety of enzymes
that enhance virulence by breaking down
or altering host tissue to provide access
and nutrients.
Still other pathogen-produced virulence
factors provide protection to the
pathogen by interfering with normal host
defense mechanisms. These factors
enhance colonization and growth of the
pathogen.
Exotoxins
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The most potent biological toxins are the
exotoxins produced by microorganisms.
Each exotoxin affects specific host cells,
causing specific impairment of a major
host cell function.
Mechanism of cholera toxin will be
discussed.
Mode of Action For
Cholera Toxin
Mode of Action For
Cholera Toxin