Microbiology- a clinical approach by Anthony Strelkauskas et
al. 2010
Chapter 4: : An introduction to cell structure and host
pathogen relationships
http://www.wired.com/news/images/full/5thplace_f.j
pg
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You have to understand cell structure for
success in studying microbiology in general
and the process of infection in particular.
Understanding the relationship between host
cells and pathogens is required for
understanding the processes of infection and
disease.
© National Institutes of Health, United States Department of Health and Human Service.
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All living organisms can be classified as
Prokaryotes
Eukaryotes
Organelles
No Organelles
No nuclear membrane Nuclear
membrane
Bacteria
Archaea
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Biologists classify microorganisms by their genus
name and their species name.
◦ An example is Clostridium tetani. Clostridium is the genus
and tetani is the species.
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In some cases, the genus can have several species.
◦ An example is Clostridium perfringens and Clostridium
botulinum.
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The genus and species names of microorganisms are
italicized when written.
The genus name is abbreviated by its first letter.
◦ C. perfringens
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Bacteria are the smallest living organisms and
are microscopic.
Bacteria are immensely diverse and very
successful organisms that colonize all parts of
the world and its inhabitants.
Bacteria are referred to as pathogens if they
cause disease.
◦ Primary
◦ opportunistic
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Bacteria can be of different
shapes, sizes, and
arrangements.
The most common shapes are:
◦ Bacillus (rod shaped)
◦ Coccus (circular shaped)
◦ Spirilla (spiral shaped)
Common multicell arrangements
are:
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Diplo
Tetrad
In chains
In clusters
Capsule stain
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Positive stains – stain the
organism
◦ e.g. methylene blue
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Negative stains – stain the
background
◦ used to demonstrate capsule, an
important virulence factor
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Simple stains – stain using only
one color
Differential stains – stain
using more than one color
◦ Gram stain, a classical and still
highly important routine stain
◦ Acid fast stain (Ziehl Neelson)
for Mycobacterium spec.
◦ Spore stain for Bacillus spec. and
Clostridium spec.
Ziehl Neelson stain
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Infectious diseases have been major causes of
death and suffering throughout history.
Infectious diseases are complex and involve the
ongoing shifting interaction between the host
and the pathogen.
◦ HOST
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PATHOGEN
Many factors affect this relationship.
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For the pathogen, the
interactions depend on:
◦ The pathogen’s ability to
evade or overcome the
host’s defense
◦ The pathogen’s ability to
increase in numbers
◦ The pathogen’s ability to
identify transmission
mechanisms to new hosts.
◦ The pathogen’s ability to
compete with normal
microbiota.
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For the host, the
interactions depend on:
◦ The host having useful
functioning defenses
(including a healthy normal
microbiota)
◦ The host’s susceptibility to
infection
◦ The degree of compromise
found within the host
immune system.
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Primary pathogens cause disease in healthy
individuals
◦ They include viruses and bacteria and others.
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Opportunistic pathogens cause infection by
taking advantage of a hosts’ increased
susceptibility of infection.
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They have evolved mechanisms that can
overcome host defenses.
Once inside the host, they can multiply rapidly.
They need to be transmitted
◦ Examples are coughing, diarrhea, biting too
◦ If host dies too quickly spreading will be impaired
Some primary pathogens are restricted to
humans.
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Entry (getting in)
Establishment (staying in)
Defeat the host defenses
Damage the host
Be transmissible
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Virulence refers to how harmful a pathogen is to
the host.
◦ Virulence depends on genetic factors of the pathogen.
◦ These genetic elements are often turned on only in the
host.
 Quorum sensing in Pseudomonas aeruginosa
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Pathogens carry virulence genes in clusters
called pathogenicity islands.
 Type III secretion apparatus in Salmonella
◦ These can be located on plasmids.
◦ Plasmids can be transferred between cells.
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Organisms sense their environment using
special sensing proteins. This is called quorum
sensing.
This sensing is based on population densities.
Certain genes are only turned on when there
are enough cells present.
Formation of biofilm is under the control of
quorum sensing.
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Bacteria can grow in aggregated assemblies within
their host. These assemblies are called biofilms.
Biofilms are clinically important because:
◦ They can capture and retain nutrients (allowing
continued growth).
◦ They impede uptake of antibiotics and
disinfectants.
◦ They inhibit phagocytosis.
Biofilms can build up on medical devices such as:
◦ Catheters
◦ Heart valves
◦ Prosthetic devices
Biofilms are one of the causes of plaque build-up on
teeth.
http://www.onemedplace.com/blog/wpcontent/uploads/2008/06/catheter_biofilm.gif
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Eukaryotic cells are found in humans and are
involved in infection.
There are several differences between
prokaryotic and eukaryotic cells.
Many of the structures of eukaryotic cells play
a role in infection.
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Structure involved in infection process
Some specialized proteins
in some
in some
in most
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List structures that are only found in
prokaryote cells (though not necessarily in all):
What is the equivalent of the mitochondrium in
prokaryote cells?
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The eukaryotic plasma membrane is made up of a phospholipid
bilayer.
It is a fluid matrix containing a variety of proteins and other
molecules.
It must be breached if pathogens are to gain entrance.
It contains specific receptors used by viruses to attach to host
cells.
It can become the envelope for certain types of viruses.
Rhinovirus infecting a host cell via membrane surface receptor
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Cytoplasm is involved in a variety of infections.
It has a major role in viral infections.
Many viruses replicate in the host cell
cytoplasm.
Some bacteria escape into the cytoplasm.
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The cytoskeleton gives eukaryotic cells
structural integrity.
The cytoskeleton is involved in how cells are
joined together to form tissue.
The components of the cytoskeleton play a role
in cellular mitosis and meiosis.
There are three types of cytoskeleton
components:
◦ Microfilaments
◦ Intermediate filaments
◦ Microtubules
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Many pathogens use
the cytoskeleton as
part of the infection
process.
Shigella use
microfilaments to
move laterally between
cells of the intestine.
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Cilia are made up of microtubules that can
be projected outward from the cell
surface.
The lower respiratory tract which is lined
with ciliated cells that work together
with mucus-producing cells to move
trapped particles upward and out of the
respiratory tract.
Pathogens can attack the cilia and
destroy their trapping capability.
In some respiratory diseases, such as
pertussis (whooping cough), the pathogens
(in this case Bordetella pertussis) attach
to host ciliated cells as an initial part of
the infection
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Ribosomes are the site of protein synthesis.
They are found either individually or attached
to the endoplasmic reticulum.
Ribosomes in eukaryotic cells have a different
structure to those prokaryotic cells.
Eukaryotic ribosomes are very important in
viral infections but not by choice.
◦ The virus takes over the host cell ribosome function.
◦ It is then used only to make new virus.
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Both systems of membranes that form
flattened sacs and platelike
structures.
The endoplasmic reticulum (ER) can be
smooth (without ribosomes) for lipid
synthesis, or rough (with attached
ribosomes) for protein synthesis.
Both structures are involved in the
biosynthesis and assembly of viruses.
The ER is also involved in the adaptive
immune response to infection.
The Golgi apparatus has the following
functions:
◦ Modifying and packaging products coming
from the ER
◦ Renewing the cell’s plasma membrane
◦ Producing lysosomes
http://micro.magnet.fsu.edu/cells/endoplasmicreticu
lum/images/endoplasmicreticulumfigure1.jpg
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Lysosomes are filled with destructive enzymes
and chemicals.
They fuse with cellular vesicles.
They are responsible for destroying foreign
materials that enter the cell.
They also act in recycling host cell components.
Lysosomes fuse with phagocytic vesicles and
destroy invading pathogens.
Many pathogens can defeat this defense.
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Proteasomes participate in
the degradation of proteins
that are made in the cell.
They are also involved in
recycling protein components.
Proteosomes degrade the
proteins of pathogens that
multiply in the cytpoplasma.
The degraded proteins
trigger the adaptive immune
response against the
pathogen.
Some pathogens block
proteasome activity.
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The nucleus is the location of the
cellular DNA of eukaryotic cells.
The nucleus is bound by a double
phospholipid bilayer membrane.
It is the site for DNA replication
during cell division.
The transcription of messenger
RNA also occurs here.
The nucleus of the host cell is
important in many infections,
particularly those caused by DNA
viruses.
◦ Copies of the viral DNA are made in
the nucleus.
◦ These copies are then moved into
the cytoplasm to be used for the
construction of new virus molecules.
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Endocytosis involves
bringing things into the cell
through the formation of
vesicles.
◦ Pinocytosis: liquids
◦ Phagocytosis: particles and
cells
◦ Receptor-mediated
endocytosis
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Exocytosis involves moving
things out of the cell which
is also done through the
formation of vesicles.
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Many pathogens enter the host cell through the
formation of vesicles.
This method provides protection for the pathogen
from the host immune response.
Some pathogens bind to host cell receptors that
trigger endocytosis. This is particularly true of
viruses.
Phagocytosis is a type of endocytosis that can be
used to defend against infection.
Many pathogens have found ways to defeat
phagocytosis.
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All living organisms can be divided into
prokaryotes and eukaryotes.
Prokaryotes are very simple cells that do not
contain a nucleus or cytoplasmic membraneenclosed organelles like those seen in
eukaryotic cells.
Bacteria are classified by genus and species and
have distinct sizes, shapes, and arrangements.
There are several staining techniques that can
be used to classify and identify different
bacteria.
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Pathogens are organisms that cause disease in
humans.
Infection is a complex process that involves
both the pathogen and the host.
Pathogens can be primary (causing disease even
though the host defenses are intact) or
opportunistic (causing disease when host
defenses are diminished).
Many of the structures of the eukaryotic host
cell have important functions in the infection
process.
Virulence genes are
arranged into
A. Reservoirs
B. Pathogenicity islands
C. Clusters
D. Plasmids
E. Individual
chromosomes
Biofilms
A. Are aggregations of
many bacterial cells
B. Form on a base layer
of fibrin, albumin,
and immunoglobulins
C. Protect bacteria
D. All of the above