Ch 4 Cell Structure

ISBN: 978-0-8153-6514-3Microbiology: A Clinical Approach, by Tony Srelkauskas © Garland ScienceMicrobiology: A Clinical Approach (2nd
Edition) © Garland Science
CHAPTER 4
AN INTRODUCTION TO CELL STRUCTURE AND
HOST-PATHOGEN RELATIONSHIPS

WHY IS THIS IMPORTANT?
 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

OVERVIEW

CLASSIFICATION OF
ORGANISMS
 All living organisms can be classified as either:
 Prokaryotes – lack membrane-bound organelles
 Eukaryotes – contain membrane-bound organelles

CLASSIFICATION OF
ORGANISMS
 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
 In some cases, the genus can have several species.
 An example is Clostridium tetani and Clostridium
botulinum
 The genus and species names of microorganisms
are italicized when written

BACTERIA: Overview
 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: Size, Shape, and
Multicell Arrangement
 Bacteria can be of different shapes, sizes, and
arrangements
 The most common shapes are:
 Bacillus (rod-shaped)
 Coccus (spherical)
 Spiral (spiral-shaped)

Panel b: © CDC/ Dr. Richard Facklam; Panel c: © CDC/ Dr. Mike Miller

Panel b: Macmillan Publishers, Ltd: Review in Microbiology, Volume 2, Issue 6, © (2004).; Panel c: © CDC

Panel b: © CDC/ Janice Haney Carr/ Jeff Hageman, M.H.S.;Panel c: © CDC/ Dr. Thomas F. Sellers/Emory University

BACTERIAL STAINING:
Types of Stain
 Bacteria can be stained in the following ways:
 Simple stains – stain using only one color
 Differential stains – stain using more than one
color

THE GRAM STAIN
 The Gram stain takes advantage of the
differences in the cell walls of bacteria
 The Gram stain process divides bacteria into
four major groups:
 Gram-positive
 Gram-negative
 Gram-variable
 Gram nonreactive

THE GRAM STAIN

THE NEGATIVE (CAPSULE)
STAIN
 The negative technique stains the background
surrounding encapsulated bacteria
 It can be used to identify the presence of a
capsule (important in virulence factors) as the
background staining makes the capsule highly
visible

THE NEGATIVE STAIN
© CDC/ Courtesy of Larry Stauffer, Oregon State Public Health Laboratory

THE FLAGELLA STAIN
 The flagella stain identifies the presence of
flagella, which are used for motility
 Motility is important for infection as it allows
the invading organisms to move from the
initial site of infection

THE FLAGELLA STAIN
© CDC/ Dr. William A. Clark

THE ZIEHL-NEELSEN
ACID-FAST STAIN
 This stain is used to detect Mycobacterium
species such as M. tuberculosis (the cause of
tuberculosis) or M. leprae (the cause of
leprosy)
 These organisms have mycolic acid (a
virulence factor) in their cell walls, making the
cell wall difficult to penetrate

THE ZIEHL-NEELSEN
ACID-FAST STAIN
 It uses heat as part of the process to permit the
entry of the stain
 It is called acid-fast because positive cells
retain color even after washing with acid

THE ZIEHL-NEELSEN
ACID-FAST STAIN
© CDC/ Dr. George P. Kubica

THE ENDOSPORE STAIN
 Endospores are small, tough, dormant
structures that can form in certain bacteria
 Heat is needed to make the endospore wall
permeable to the stain

THE ENDOSPORE STAIN
© CDC/ Courtesy of Larry Stauffer, Oregon State Public Health Laboratory

HOST-PATHOGEN
RELATIONSHIPS
 Infectious diseases have been major causes of
death and suffering throughout history
 Pathogens are organisms that cause infectious
diseases

HOST-PATHOGEN
RELATIONSHIPS
 Infectious diseases are complex and involve a
series of shifting interactions between host and
pathogen
 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

HOST-PATHOGEN
RELATIONSHIPS
 For the host, the interactions depend on:
 The host having useful functioning defenses
 The host’s susceptibility to infection
 The degree of compromise found within the host
immune system

PATHOGENICITY
 In most cases, the interactions between the
body and microbes cause no harm
 Some microbes have a mutualistic relationship
with the host
 In these cases, the microbes provide something
beneficial to us and we provide something
beneficial to them
 However, some harmless organisms can
become opportunistically pathogenic

OPPORTUNISTIC PATHOGENS
AND PRIMARY PATHOGENS
 Opportunistic pathogens cause infection by
taking advantage of a host’s increased
susceptibility of infection
 Primary pathogens are those that cause disease
in healthy individuals

OPPORTUNISTIC PATHOGENS
AND PRIMARY PATHOGENS
 Characteristics of primary pathogens are as
follows:
 They have evolved mechanisms that can overcome
host defenses
 Once inside, they can multiply rapidly
 Some primary pathogens are restricted to humans

DISEASE AND
TRANSMISSIBILITY
 Successful infection requires the following
from a pathogen:
 The ability to multiply in sufficient numbers
 The ability to transmit to new hosts

DISEASE AND
TRANSMISSIBILITY
 Pathogens can be transmissible through:
 Coughing – transmits respiratory infections
 Diarrhea – transmits digestive infections
 Infections that kill too quickly inhibit
transmission

BACTERIAL PATHOGENICITY
AND VIRULENCE
 Pathogens must be able to accomplish the five
requirements for infection:
 Entry (getting in)
 Establishment (staying in)
 Defeat the host defenses
 Damage the host
 Be transmissible

AND VIRULENCE
 Virulence refers to how harmful a pathogen is
to the host
 Virulence depends on genetic factors of the
pathogen
 These genes are often turned on only in the host

AND VIRULENCE
 Pathogens carry virulence genes in clusters
called pathogenicity islands
 These can be located on plasmids
 Plasmids can be transferred between cells

QUORUM SENSING
 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:
 An example of this is toxin production in
Salmonella

BIOFILMS
 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
 Biofilms can build up on medical devices such
as:
 Catheters
 Internal pacemakers
 Prosthetic devices
 Biofilms are one of the causes of plaque build-
up on teeth

BIOFILMS

THE HOST CELL
 There are several differences between
prokaryotic and eukaryotic cells
 Many of the structures of eukaryotic cells play
a role in infection

THE HOST CELL

EUKARYOTIC CELLS:
The Plasma Membrane
 The eukaryotic plasma membrane is made up
of a phospholipid bilayer
 It is a fluid matrix containing a variety of
proteins and other molecules

EUKARYOTIC CELLS:
The Plasma Membrane

EUKARYOTIC CELLS:
Role of the Plasma Membrane in Infection
 Because the plasma membrane is the barrier
between the inside and the outside of the cell,
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

EUKARYOTIC CELLS:
Role of the Plasma Membrane in Infection

EUKARYOTIC CELLS:
Cytoplasm
 The eukaryotic cytoplasm consists of:
 A semifluid material that is mainly water and
dissolved substances
 Membrane-bound structures called organelles
 Structures not bound by membranes

EUKARYOTIC CELLS:
Role of the Cytoplasm in Infection
 Cytoplasm is involved in a variety of
infections
 It has a major role in viral infections
 Many viruses replicate in the host cell
cytoplasm

EUKARYOTIC CELLS:
Host Cell Cytoskeleton
 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

EUKARYOTIC CELLS:
Host Cell Cytoskeleton
 There are three types of cytoskeleton
components:
 Microfilaments
 Intermediate filaments
 Microtubules

EUKARYOTIC CELLS:
Role of the Cytoskeleton in Infection
 Many pathogens use the cytoskeleton as part
of the infection process
 Shigella use microfilaments to move laterally
between cells of the intestine

EUKARYOTIC CELLS:
Role of the Cytoskeleton in Infection

EUKARYOTIC CELLS:
Cilia
 Cilia are made up of microtubules that can be
projected outward from the cell surface
 The lower respiratory tract is lined with
ciliated cells that help move trapped particles
upward and out of the respiratory tract

EUKARYOTIC CELLS:
Role of Cilia in Infection
 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

EUKARYOTIC CELLS:
Ribosomes
 Ribosomes are the site of protein synthesis
 They are found either free in the cytoplasm or
attached to the endoplasmic reticulum
 Ribosomes in eukaryotic cells have a different
structure to those prokaryotic cells

EUKARYOTIC CELLS:
Role of the Ribosome in Infection
 Eukaryotic ribosomes are very important in
viral infections:
 The virus takes over the host cell ribosome
function
 It is then used only to make new virus

EUKARYOTIC CELLS:
Role of the Ribosome in Infection
 Prokaryotic ribosomes are damaged by the
antibiotics erythromycin and streptomycin
 Eukaryotic ribosomes are not affected by these
antibiotics

EUKARYOTIC CELLS:
Mitochondria
 Mitochondria produce ATP in eukaryotes
 Mitochondria are very similar to bacteria
 Bacterial prokaryotes integrated into
eukaryotic cells as mitochondria – part of the
endosymbiotic theory

EUKARYOTIC CELLS:
ER & Golgi Apparatus
 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

EUKARYOTIC CELLS:
ER & Golgi Apparatus
 The Golgi apparatus has the three following
functions:
 Modifying and packaging products coming from
the ER
 Renewing the cell’s plasma membrane
 Producing lysosomes

EUKARYOTIC CELLS: Role of the
ER & Golgi Apparatus in Infection
 Both structures are involved in the
biosynthesis and assembly of viruses
 The ER is also involved in the adaptive
immune response to infection

EUKARYOTIC CELLS:
Lysosomes
 Lysosomes are filled with destructive enzymes
and chemicals
 They create vesicles
 They are responsible for destroying pathogens
that enter the cell
 They also act in recycling host cell
components

EUKARYOTIC CELLS:
Proteasomes
 Proteasomes are organelles that participate in
the degradation of proteins
 They are also involved in recycling protein
components

EUKARYOTIC CELLS:
Role of the Proteasome in Infection
 Proteosomes degrade the proteins of
pathogenic cells
 The degraded proteins trigger an immune
response against the pathogen

EUKARYOTIC CELLS:
Peroxisomes
 Peroxisomes are organelles that participate in
the degradation of fatty acids
 They contain catalase, and enzyme that
counteracts hydrogen peroxide–a
by-product of fatty acid break down

EUKARYOTIC CELLS:
The Nucleus
 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

EUKARYOTIC CELLS:
Role of the Nucleus in Infection
 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

EUKARYOTIC CELLS:
Endocytosis & Exocytosis
 Endocytosis involves bringing things into the
cell through the formation of vesicles
 Exocytosis involves moving things out of the
cell which is also done through the formation
of vesicles

EUKARYOTIC CELLS:
 There are three ways that endocytosis operates
within a cell:
 Pinocytosis
 Phagocytosis
 Receptor-mediated endocytosis

EUKARYOTIC CELLS:

EUKARYOTIC CELLS: Role of
Endocytosis & Exocytosis in Infection
 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

EUKARYOTIC CELLS: Role of
Endocytosis & Exocytosis in Infection
 Phagocytosis is a type of endocytosis that can
be used to defend against infection
 Many pathogens have found ways to defeat
phagocytosis

Ch 4 Cell Structure

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