F6 MICROBES AND
DISEASE
F.6.1 – F 6.10
F.6.1Pathogen entry
F.6.1 List six methods by which pathogens are
transmitted and gain entry to the body
1.
2.
3.
4.
5.

Openings on surface of skin (i.e. injuries)
Eating contaminated foods
Sexual contact (unprotected)
Drinking contaminated water
Inhalation (air-borne droplets)
Vector-based (carried by animals)
F.6.2 Distinguish between intracellular and
extracellular bacterial infection
Using Chlamydia and Streptococcus as examples
Pathogenic bacteria can be grouped into three
categories on the basis of their invasive properties
1.
2.
3.
Extracellular bacteria
Facultative intracellular bacteria
Obligate intracellular bacteria
Extracellular bacteria

Extracellular bacteria
 Do
not invade cells
 proliferate in the extracellular environment which is
enriched with body fluids.


Some extracellular bacteria don’t penetrate body
tissues (e.g. V. cholerae) but adhere to epithelial
surfaces and cause disease by secreting endotoxins.
Streptococcus is an extracellular bacteria.
Facultative intracellular bacteria

Invade host cells when it gives them selective
advantage
 Bacteria
are shielded from humoral antibodies and can be
eliminated only by a cellular immune response.

However these bacteria must possess specialized
mechanisms to protect them from the harsh environment
of the lysosomal enzymes encountered within the cells.
 Salmonella
and Mycobacterium spp: Very resistant to
intracellular killing by phagocytic cells.
Obligate intracellular pathogens




Obligate intracellular parasites cannot reproduce
outside their host cell, meaning that the parasite's
reproduction is entirely reliant on intracellular resources.
Obligate intracellular parasites of humans include:
viruses
Certain bacteria, including:
Chlamydia, and closely related species.[6]
 Rickettsia
 Coxiella
 Certain species of Mycobacterium such as Mycobacterium
leprae

Contents of this slide: http://en.wikipedia.org/wiki/Intracellular_parasite
Streptococcus pyogenes




Gram positive
Facultative anaerobes
Only known reservoirs are the skin and mucous
membranes of the human host.
Defense against the human immune system:
The capsule of Streptococcus pyogenes is chemically similar
to that of it's host's connective tissue, and therefore, is
nonantigenic.
 it's cytoplasmic membrane has antigens similar to human
cardiac skeletal and smooth muscle.
 It is equipped with a wide variety of virulence factors to
invade human tissue and escape from immunity.

Chlamydia & Streptococcus
Chlamydia trachomatis

Gram negative, Obligate intracellular
Aerobic
small needle-like projection called a type III secretion
apparatus.
Resides in vacuoles

There is a Chlamydia pneumoniae, it is not an STD.



it is one of the main causative agents of pneumonia, and
bronchitis.
 It has also been linked with artherosclerosis, and multiple
sclerosis.

Chlamydia trachomatis effects
Women




PID (Pelvic
Inflammatory Disease)
Infertility
Ectopic pregnancy
one of the primary
causes of blindness
Men





Infertility
infections often begin in
the urethra, with
urethritis,
epididymitis and
prostatitis
Attach to sperm
one of the primary
causes of blindness
F.6.3 Distinguish between:
Endotoxins and Exotoxins
Exotoxin versus Endotoxin
Exotoxin





V. cholera (-)
C. botulinum (+)
C. perfringens (+)
Staphylococcus aureussome strains
Streptococcus
pyogenes
Endotoxins



Gram negative
Lipopolysaccharide
components.
Escherichia coli,
Salmonella, Shigella,
Pseudomonas, Neisseria,
Bordetella pertussis (and
Vibrio cholerae.)
F.6.4 Evaluate methods of controlling
microbial growth including:
Irradiation
Pasteurization
Antiseptic
disinfectants
Irradiation




Irradiation involves destroying pathogens using
radiation (usually gamma radiation – UV radiation is
less effective)
It is useful for sterilizing food, medical instruments and
equipment
Ionizing (gamma) radiation produces free radicals
which may affect the flavor of food, although the food
itself does not become radioactive
However some people are reluctant to buy irradiated
food, fearing possible effects
Pasteurisation




Pasteurization involves heating a liquid to a
specified temperature for a specific duration to kill
pathogens
It does not effect the composition or nutritional
value of the liquid itself
After pasteurization, it is important to refrigerate
the liquid to delay spoilage (it may be only
bacteriostatic for non-pathogenic bacteria)
Pasteurization is commonly used to maintain and
improve the quality of milk and milk products
Disinfectants



Disinfectants are chemicals that kill bacteria (but not
endospores)
They are bacteriocidal in concentrated forms (e.g.
bleaches, detergents) but may be only
bacteriostatic when dilute (e.g. chlorination in pools)
Concentrated forms can be damaging to mucous
membranes and skin
Antiseptics



Mild chemicals that are less effective than
disinfectants, but also less toxic to living tissue
They are used to kill microbes and prevent further
growth on the surface of tissues and wounds, in
order to prevent infection
However they cannot be taken internally or used on
foods and will have an unpleasant taste
F.6.5 Outline the mechanisms of action of
antibiotics, including:
Inhibition of synthesis of cell walls, proteins and
nucleic acids.
1.
2.
3.
4.
Antibiotics interfere with some aspect of prokaryotic
metabolism that is different to that of eukaryotes,
thus targeting the bacteria without affecting the host
Antibiotics can target enzymes involved in the
synthesis of bacterial cell walls (e.g. penicillin)
Antibiotics can inhibit bacterial protein synthesis by
interfering with 70S ribosomes (e.g. erythromycin,
tetracycline)
Antibiotics can target enzymes involved in the
bacterial synthesis of nucleic acids (e.g. rifampicin)
F.6.6 Outline the lytic life cycle of the
influenza virus


The lytic cycle is a method of viral replication which
involves the independent replication of viral DNA /
RNA as part of virion assembly
This is in contrast to the lysogenic cycle, where the
viral DNA is integrated into the host genome and is
replicated with the cell's own DNA
The lytic life cycle of the influenza virus can
be summarized in a number of key steps:
1.
2.
3.
4.

When the virus attaches to cell surface receptors it is internalized in an
endosome, uncoated and released into the cytoplasm
Influenza is a retrovirus and the RNA of the viral genome is transported
to the nucleus whereby it is replicated (reverse transcription is not
involved)
The viral RNA acts as an mRNA template for the translation of viral
core proteins, which assemble as a capsid around viral RNA in the
cytoplasm
Newly constructed virus particles recognize regions of the cell
membrane where viral proteins have attached and are released from
these points
Unlike most lytic viruses, the influenza virus does not promote the lysis of
the cell as part of its release
Viral RNA is (-) strand that must be copied to a (+) strand before it can be translated in
cytoplasm
F.6.7 Define epidemiology
Epidemiology is the study of the occurrence,
distribution and control of diseases
F.6.8 Discuss the origin and epidemiology
of one example of a pandemic
In the late 20th century (1997) there was a
pandemic outbreak of avian bird flu





Infectious Agent: The outbreak was caused by the H5N1
virus
Mode of Transmission: The virus originated in domestic
poultry but spread to humans through exposure to the birds
and their feces
Distribution: The outbreak originated in Asia, and although
it mainly remained there, there were cases noted in other
parts of the world
Methods of Control: Containment strategies were
employed for the infected, infected birds were killed and
global records of occurrences were kept
Problems of Control: Migrating wild birds could spread
virus, problems coordinating international communities,
questions of equitable access to care
F.6.9 Describe the cause, transmission and
effects of malaria.
as an example of a disease caused by a
protozoan




Malaria is caused by the protozoan Plasmodium and
is transmitted to humans by the Anopheles mosquito
(females only)
The life cycle of the Plasmodium parasite requires
both a primary human host (adult stage) and
intermediary mosquito host (juvenile stage)
When a mosquito ingests infected human blood, the
parasite reproduces and matures in the mosquito's gut
before migrating to the salivary glands
When a mosquito feeds on another human, the
parasite is injected into the blood and travels to the
liver for further development



After a period of time, the parasite migrates to
erythrocytes (red blood cells), which then rupture as the
parasite spreads
Symptoms of malaria include fever, nausea and chills –
these symptoms may be cyclic due to the parasitic life
cycle (period of dormancy in liver)
Additionally, the frequent rupturing of red blood cells
can lead to the development of anemia
Life Cycle of the Malarial Parasite
(Plasmodium falciparum)
F.6.10 Discuss the prion hypothesis for
the cause of spongiform encephalopathy's
•Prion proteins aggregate together to form amyloid fibrils capable of causing
disease
•Diseases caused by prion proteins are called spongiform encephalopathies,
because they cause holes to form within the brain (neurodegeneration)
•Examples of prion diseases include:
•Mad cow disease (affects cows)
•Scrapie (affects sheep)
•Creutzfeld-Jacob disorder (CJD) and kuru (affects humans)
•Mad cow disease has been used as an excuse for protectionism by some
national governments, refusing beef imports to protect local interests
•A prion (proteinaceous infectious particle) is a protein that
has refolded abnormally into a structure that is capable of
causing disease.
•It is also able to convert normally folded protein molecules
into the abnormal form (mechanism of conversion not well
understood)
•Infectious prion proteins have a higher content of betapleated sheets, which increases structural stability making
them more resistant to denaturation
•This makes treatment of prion proteins extremely difficult
(there are currently no known cures)
Folding and Replication of Prion
Protein