CHAPTER 4
AN INTRODUCTION TO CELL STRUCTURE AND HOSTPATHOGEN RELATIONSHIPS
© National Institutes of Health, United States Department of Health and Human Service.
WHY IS THIS IMPORTANT?
• An understanding of cell structure is vital to
understanding microbiology, in particular, the
process of infection.
• Host-pathogen interactions must be understood
in order to understand the processes of
infection and disease.
OVERVIEW
CLASSIFICATION OF ORGANISMS
• All living organisms can be classified as
Prokaryotes
Eukaryotes
No Organelles
Organelles
(Bacteria)
(Everything else)
Biological Kingdoms
Taxonomy: A system for classifying biological organisms
Genus and species
Used to identify an
organism
CLASSIFICATION OF ORGANISMS
• Biologists classify microorganisms by their genus and
species names.
– An example is Clostridium tetani. Clostridium is the
genus and tetani is the species.
• The genus can have several species.
– E.g. Clostridium perfringens, Clostridium botulinum
and Clostridium difficile, etc.
• The genus and species names of microorganisms are
italicized when written.
• First letter of genus is capitalized
• The genus can be abbreviated to first letter:
Escherichia coli
E. coli
All E’s are NOT Created Equal
The genus can be abbreviated to first letter:
Escherichia coli
E. coli
Enterobacter aerogenes
Enterococcus faecalis
Staphylococcus aureus
Streptococcus pneumoniae
E. aerogenes
E. faecalis
S. aureus
S. pneumoniae
BACTERIA: Overview
• Microscopic
• Immensely diverse
• Successfully colonize all parts of the world
and its inhabitants.
• Considered Pathogens if they cause disease.
BACTERIA: Size, Shape and
Multicellular Arrangement
• Bacteria can be of different shapes, sizes, and
arrangements.
• The most common shapes:
– Coccus (circular)
– Bacillus (rod)
– Spirilla (spiral)
Multicellular Arrangements
Chain
Clusters, clumps
Many organisms have no arrangement.
BACTERIAL STAINING:
Types of Stain
• Bacteria can be stained in the following ways:
–
–
–
–
Positive stains – stain the organism
Negative stains – stain the background
Simple stains – stain using only one color
Differential stains – stain using more than one
color
THE GRAM STAIN
• Differentiates bacteria based on differences in
the structure of their cell walls.
• The Gram stain process divides bacteria into
four major groups:
–
–
–
–
Gram-positive
Gram-negative
Gram-variable
Gram nonreactive
Gram neg. bacilli
Gram pos. cocci
THE GRAM STAIN
THE
(CAPSULE)
STAIN
• Stains
theNEGATIVE
background surrounding
encapsulated
bacteria.
• Capsules serve as important virulence factors for some
organisms:
• Streptococcus pneumoniae (pneumonia, meningitis, etc.)
• Hemophilus influenzae (pneumonia, meningitis, etc.)
• Neisseria meningitidis (bacterial meningitis)
• Bacillus anthracis (anthrax)
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 ZIEHL-NEELSEN
ACID FAST STAIN
• 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 in their cell walls,
making the cell wall difficult to penetrate.
THE ENDOSPORE STAIN
• Endospores are small, tough, dormant structures that
can form in certain bacteria.
Crystal Violet Stain
Excluded from spores
Malachite Green
Spore Stain
HOST-PATHOGEN
RELATIONSHIPS
• Infectious diseases are complex and involve
the ongoing interaction between the host and
the pathogen.
– HOST
PATHOGEN
• Many factors affect this relationship.
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:
– Ability to evade or overcome the host’s defense
– Ability to increase in numbers
– Ability to transmit to new hosts.
Covered in greater detail next week.
HOST-PATHOGEN
RELATIONSHIPS
• For the host, the interactions depend on:
– The host having useful functioning defenses
– The host’s susceptibility to infection
Covered in greater detail in weeks 6 and 7.
PATHOGENICITY:
All Microbes Are Not Created Equal
Commensals, Opportunists and Pathogens
Commensals – Do not harm the host
Mutualists – Benefit the host while deriving benefit
from the host
Opportunists – Can become pathogens given the
proper conditions (injury, immune compromised, etc.)
Pathogens – Overtly cause disease
HOST-PATHOGEN
RELATIONSHIPS
• Microbial flora can protect us through
microbial antagonism.
– Many bacteria produce bacteriocins which are
localized bacterial antibiotics.
– Bacteriocins can kill invading organisms but do
not affect the bacteria that produce them.
Competition for space and other resources.
• In most cases, the interactions between the body and
bacteria cause no harm.
– Some bacteria have a mutualistic relationship with
the host.
• However, some harmless organisms can become
opportunistically pathogenic.
OPPORTUNISTIC PATHOGENS
AND PRIMARY PATHOGENS
• Opportunistic pathogens cause infection by
taking advantage of a hosts’ increased
susceptibility of infection.
• Characteristics of primary pathogens are as
follows:
– They cause disease in healthy individuals.
– They include viruses and bacteria.
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.
– Some pathogens can infect humans and other
animals (zoonosis)
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
• Symptoms of certain infections can provide
transmission mechanisms 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
•Entry
•Colonization
•Immune Evasion
•Propagation
•Transmission
PATHOGENICITY AND
VIRULENCE
• 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.
BACTERIAL PATHOGENICITY
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 enterotoxin
production in Salmonella.
BIOFILMS
• Bacteria can grow in aggregated assemblies 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
– Heart valves
– Prosthetic devices
Lead to nosocomial
(hospital acquired) infections
• Biofilms are one of the causes of plaque buildup on teeth.
THE HOST CELL
• There are several differences between
prokaryotic and eukaryotic cells.
• Animals, including humans, possess
eukaryotic cells.
• Many of the structures of eukaryotic cells play
a role in infection.
EUKARYOTIC CELLS:
Bacterial
anatomy will be
covered in week
3 (chapter 9).
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:
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.
EUKARYOTIC CELLS: Cytoplasm
• The eukaryotic cytoplasm consists of:
– A semi-fluid 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.
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:
Cilia
• 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 mucusproducing cells to move trapped particles
upward and out of the respiratory tract.
• Muco-cilliary elevator
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 individually or attached
to the endoplasmic reticulum.
• Ribosomes in eukaryotic cells have a different
structure to those prokaryotic cells.
– Difference used to develop antibiotics
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.
ER & Golgi Apparatus
• Both systems of membranes that form flattened sacs.
• The endoplasmic reticulum (ER) can be smooth (without
ribosomes) for lipid synthesis, or rough (with attached
ribosomes) for protein synthesis.
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
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.
Lysosomes
• Lysosomes are filled with destructive enzymes and
chemicals.
• They destroy foreign materials that enter the cell.
• They also act in recycling host cell components.
Role of the Lysosome in Infection
• Lysosomes fuse with phagocytic
vesicles and destroy invading
pathogens.
• Many pathogens can defeat this
defense.
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.
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.
Endocytosis & Exocytosis
• There are three ways that endocytosis operates within
a cell:
– Pinocytosis
– Phagocytosis
– Receptormediated
endocytosis
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.