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The different types of pathogen that can cause communicable diseases in plants and animals
To include: bacteria – tuberculosis (TB), bacterial meningitis, ring rot (potatoes, tomatoes), virus – HIV/AIDS
(human), influenza (animals), Tobacco Mosaic Virus (plants), protoctista – malaria, potato/tomato late blight, fungi
– black sigatoka (bananas), ring worm (cattle), athlete’s foot (humans)
...
Not all parasites cause disease but they all gain energy from
the host
...

Pathogens cause communicable diseases
...

Bacteria are prokaryotes that can be classified by:
 Their basic shapes – they may be rod shaped (bacilli), spherical shaped (cocci), comma shaped (vibrios), spiralled
(spirilla) and corkscrew (spirochaetes)
 Their cell walls – two main types of bacterial cell walls have different structures and react differently to Gram staining
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g
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g
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coli)
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Some bacterial toxins
damage the host cells by breaking down the cell membrane, some damage or inactivate enzymes and some interfere with
the host cell genetic material so the cells cannot divide
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Viruses are non-living infectious agents which are 0
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3ɥm in diameter
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Viruses take over the cell
metabolism
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and evolving by
developing adaptations to their host
...

Bacteriophages are viruses that attack bacteria by
taking over bacteria cells and using them to replicate
itself and destroy the bacteria at the same time
...

Medical scientists consider viruses to be the ultimate
parasites
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Only a small percentage act as pathogens
...
Pathogenic protists may need a vector to transfer them to their hosts or they may enter the body directly through
polluted water
...
Fungi cannot photosynthesise and digest their food extracellularly before
absorbing the nutrients
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Some are patristic (feed on living),
which are pathogenic and cause disease
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When they reproduce, they produce millions of spores which can spread huge distances so they can
spread rapidly and widely through crop plants
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Name
Bacteria
Tuberculosis
(TB)
Bacterial
meningitis

Ring rot
Virus

Infective agent

What does it do?

Treatment and prevention

Mycobacterium/
M
...

bovis
Streptococcus
pneumonia/
Neisseria
meningitidis

Damages and destroys lung tissue
and weakens the immune system
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No cure
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HIV/AIDS

Human
immunodeficiency
virus (HIV)
(Causes AIDS)

Influenza

Orthomyxoviridae
spp
...


Infects tobacco plants and 150
other species e
...
peppers,
cucumbers and petunias
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Plasmodium

Gives flu-like symptoms which can
weaken people causing life
threatening conditions
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The hypha
penetrates and digests the cells,
turning them black
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Grows and digests the warm, moist
skin between toes causing cracking
and scaling which is itchy and may
become sore
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Kills epithelial cells in gas exchange
system, leaving airways open to
secondary infection

No vaccine and no cure but anti-retroviral
drugs slow progress

No cure but vulnerable people are given
vaccines to prevent spread
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No cures but there are resistant crop
strains
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Control the vector: Anopheles
mosquito can be destroyed by insecticides
and by removing standing water where
they breed
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Anti-malarial
drugs available to prevent malaria
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Careful management and
chemical treatments reduce infection risk
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God care and fungicide
treatment controls spread of disease
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Antifungal creams
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g
...

Direct transmission in animals:
 Direct contact – bodily fluids, skin-to-skin contact, microorganism from faeces onto hands
 Inoculation – break in skin, animal bites, wounds and punctures and sharing needles
 Ingestion – taking in contaminated food or drink, transferring pathogens from hands to mouth
Indirect transmission in animals:
 Fomites – inanimate objects
 Droplet infection (inhalation) – droplets expelled as you talk, cough or sneeze which contain pathogens
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g
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g
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It
is an airborne infection
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Cannot live
outside the body for long so isn’t spread as quickly as a
cold
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Damp, overcrowded conditions, alcohol and
drug abuse, HIV patient, weak immune
system
More males and elderly people get
meningitis, overcrowded conditions, having
other infections and being born to an infected
mother (genetics)
Symptomless and latently infected tubers and
over-crowding

Exchange of bodily fluids e
...
blood
...

Cannot live outside the body so isn’t spread through
saliva
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Having unprotected sex and sharing
contaminated needles with an infected
person
Younger children and older adults are more
likely to get it
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Spread by the vector: Female Anopheles mosquito
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When the infected
mosquito feeds on a human, the parasite passes from
the saliva to the blood where it reproduces in the liver
(hepatocyte cells) and enters the red blood cells (where
gametes are produced)
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Spores spread by wind and water from infected plants to
uninfected ones
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Walking barefoot, people who have impaired
immune systems and diabetes and warm,
moist conditions

Direct contact with infection and by skin particles left on
towels, shoes, floors etc

Warm, humid and windy conditions and
overcrowding

Plant defences against pathogens
To include production of chemicals and plant responses that limit the spread of the pathogen (e
...
callose
deposition)
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Receptors in the cells respond to molecules from the pathogens, or to
chemicals produced when the plant cell wall is attacked, stimulating the release of signalling molecules that switch on
genes in the nucleus
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High levels of a polysaccharide called callose is produced which contains β-1,3 linkages and β-1,6 linkages between the
glucose monomers
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Callose

papillae act as barriers, preventing the pathogens entering the plant cells around the site of infection
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Lignin is added, making the mechanical barrier thicker and
stronger
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Callose is deposited in the plasmodesmata between infected cells and their neighbours, sealing them off from the healthy
cells and helping to prevent the pathogen spreading
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They can be extracted and synthesised to help
control insects, fungi and bacteria:
 Insect repellents – pine resin and citronella from lemon grass
 Insecticides – pyrethrins made by chrysanthemums which act as insect neurotoxins and caffeine which is toxic to insects
and fungi
 Antibacterial compounds including antibiotics – phenols made in different plants and antibacterial gossypol produced by
cotton
 Antifungal compounds – phenols made in different plants and caffeine
 Anti-oomycetes – glucanases which are enzymes made by some plants that break down glucans (polymers in cell walls )
 General toxins – some chemicals found in plants that break down, forming cyanide compounds when the plant cell is
attacked which is toxic to most living things
The primary non-specific defences against pathogens in animals
Non-specific defences to include skin, blood clotting, wound repair, inflammation, expulsive reflexes and mucous
membranes (no detail of skin structure is required)
...

Blood clots seal the wound, preventing pathogens getting in
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The clot dries out, forming a hard, tough scab
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Collagen fibres are deposited to give the new tissue strength
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Inflammation occurs when there is damage or irritants from pathogens on the site of the wound
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A widespread infection causes a whole-body rash
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The raised temperature prevents pathogens
reproducing
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Tissue fluid causes swelling (oedema) and pain
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g
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Vomiting
and diarrhoea expels the contents of the gut along with any infective pathogens
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Mucus also
contains phagocytes removing remaining pathogens
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Normal body temperature is around 37°C and is maintained by the hypothalamus in the brain
...
This inhibits pathogen reproduction as most
pathogens reproduce best as o below 37°C
...

The structure and mode of action of phagocytes
To include neutrophils and antigen-presenting cells and the roles of cytokines, opsonins, phagosomes and
lysosomes
...
There are two main types –
neutrophils and macrophages
...

Phagocytes are the secondary defence against pathogens because they are
involved when the pathogen enters the body
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Sometimes pus is seen in a spot, cut or
wound
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Phagocytes are able to pass from the blood into the tissue fluid because they have a lobed and narrow nucleus, the cells
can change shape allowing them to squeeze between cells, move through pores and fit in walls of capillaries
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1
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2
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There are a
number of different opsonins, but antibodies such as immunoglobulin G (IgG) and immunoglobulin M (IgM) have the
strongest effect
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The phagocyte recognises the non-human proteins on the pathogen as non-self and receptor on their cells surface
membrane binds to the common opsonins
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The phagocyte engulfs the pathogen into its cytoplasm
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The pathogen is enclosed in a vacuole called a phagosome where phagocytosis occurs
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The phagosome moves towards and fuses with a lysosome to form a phagolysosome
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The lysins (enzymes) in the lysosome digest the pathogen, breaking down its products which are absorbed into the
cytoplasm
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8
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They also increase body temperature and stimulate the specific immune system
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Macrophages take longer but undergo a more complex process
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The MHC complex moves the pathogen
antigens to the macrophage’s own surface membrane, becoming an antigen-presenting cell (APC)
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The structure, different roles and modes of action of B and T lymphocytes in the specific immune response
To include the significance of cell signalling (reference to interleukins), clonal selection and clonal expansion,
plasma cells, T helper cells, T killer cells and T regulator cells
...

B lymphocytes mature in the bone marrow
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The main types are:
 T helper cells – have CD4 receptors on cell surface membrane which binds to surface antigens on APCs, produces
interleukins (type of cytokine) which stimulates the activity of B cells increasing antibody production, stimulating
production of other types of T cells and attracting and stimulating macrophages to ingest pathogens with antigen-antibody
complexes
 T killer cells – destroys the pathogen carrying the antigen , produces perforin which kills the pathogen by making holes in
the cell membrane making it freely permeable
 T memory cells – live for a long time, part of immunological memory (remember antigen)
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Stop immune response one pathogen
has been eliminated and makes sure body recognises self-antigens and doesn’t set up an autoimmune response
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In humoral response, the body responds to antigens found outside the cells, e
...
bacteria and fungi, and antigens found to
APCs
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1
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2
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3
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The receptor and interleukin have a complementary shape which activates the B cell
5
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6
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The cloned plasma cells binds to the antigens and disables them or acts as opsonins or agglutinins – primary response
8
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In cell-mediated response, T lymphocytes respond to the cells of an organism that have been
changed in some way (e
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virus infection), by antigen processing or by mutation (e
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cancer cells) and to cells from
transplanted tissue
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The receptors on some of the T helper cells are complementary to the APCs, formed when macrophages engulf and digest
pathogens in phagocytosis, and become activated when they fit with the antigens
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The cloned T cells may:
 Develop into T memory cells, rapidly responding to invading pathogens
 Produce interleukins, stimulating phagocytosis
 Produce interleukins, stimulating B cells to divide
 Develop into T killer cells, destroying infected cells

The primary and secondary immune responses
To include T memory cells and B memory cells
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Secondary immune response is the relatively fast production of very large quantities of the correct antibodies the second
time a pathogen is encountered as a result of immunological memory – the second stage of a specific immune response
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They recognise an antigen and produce a clone which can
change into plasma cells which make antibodies against the antigen
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They remain in circulation and provide immunological memory
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Antibodies are Y-shaped glycoprotein molecules called immunoglobulins, which bind to a specific antigen on the pathogen
or toxin that has triggered the immune response
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There are disulphide bonds within the polypeptide chains and holding the light and heavy chains
together
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The constant region binds to phagocytes
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It is a different shape on each antibody and gives the antibody its specificity
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When an antibody binds to an antigen it forms
an antigen-antibody complex
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Agglutination is the
prcoess in which chemcials called agglutins cause pathogens to clump together so they are easier for phagocytes to engulf
and digest
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The hinge region allows flexibility
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Most pathogens can no longer effectively invade the host cells once they are part of an antigen-antibody
complex
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Antibodies can act as anti-toxins, binding to the toxins produced by pathogens and making them harmless
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Active immunity may be lost once the individual leaves the infectious areas because there are no repeat infections so there
is no further exposure to the antigen
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A booster is needed to maintain active immunity
...

Active immunity
Exposure to antigen; immunity achieved by injecting
antigen or through infection
Specific response made by individual achieving
immunity
Immune system activated by antigen; immune memory
in effect
Immune response can be maintained via stimulation of
memory cells i
...
a booster
Immune state develops over a period of weeks

Passive immunity
No exposure to antigen; immunity achieved by injecting
antibodies or antigen-reactive T cells
Specific immune response made by the donor of antibodies or
T cells
No immune system activation; no immune memory
Immunity cannot be maintained and decays rapidly
Immunity develops immediately

Artificial immunity is immunity acquired as a result of the deliberate exposure of the body to antibodies or antigens in nonnatural circumstances
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g
...

Artificial passive immunity is immunity which results from the administration of antibodies from another animal against a
dangerous pathogen e
...
anti-tetanus injections and injections against snake bites
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g
...

Natural active immunity is immunity which results from the response of the body to the invasion of a pathogen e
...
being
exposed to pathogen that causes disease like chickenpox and colds
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g
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Artificial active immunity
Vaccine containing a dead or weakened pathogen
Injected before a person is infected with a disease
Body needs time to synthesise antibodies
Long lasting immunity
Booster needed for certain diseases because the first
injection usually induces a slow and low level of antibodies
Natural active immunity
Antigens enter the body naturally through contact with a
pathogen/infection
Needs time to produce antibodies
Produces own antibodies

Artificial passive immunity
Serum containing specific antibodies
Injected at the time when a person is at high risk of getting
the diseases
Ready-made antibodies give immediate immunity
Short lasting immunity
Normally doesn’t need a booster dose as the first injection
is usually sufficient
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g
...
An autoimmune response is the
response when the immune system acts against its own cells and destroys healthy tissue in the body
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Immunosuppressant drugs, which prevent the immune system working, may be used as treatments but they deprive the
body of its natural defences against communicable diseases
...

Can attack any organ in the body including
kidneys, liver, lungs or brain

Lupus

Treatment
Insulin injections, pancreas transplants,
immunosuppressants
No cure
...
Anti-inflammatory drugs, steroids
and immunosuppressants

The principles of vaccination and the role of vaccination programmes in the prevention of epidemics
To include routine vaccinations and reasons for changes to vaccines and vaccination programmes (including
global issues)
...
An epidemic is when a
communicable diseases spread rapidly to a lot of people at a local or national level
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At the beginning of an epidemic, mass vaccination can prevent the spread of the pathogen into the wider population
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Antibodies produced need to match the new
strain/antigen
...
This is known as herd immunity, as there is minimal opportunity for an outbreak to occur
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Some communicable diseases cause problems at a global level as they cannot yet be prevented by vaccination
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It’s not always possible to produce an effective vaccine for diseases such as theses because
there are different strains and different antigens due to mutations caused by genetic variation
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The pathogen may be concealed in cells and may only be in circulation for a short time, such as a
virus
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They are used to treat and cure
symptoms, making people feel better
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This allows models of potential drug molecules to be built up which are targeted at
particular areas of a pathogen
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Many
drugs have been based on bioactive compounds discovered in plants, microorganisms or other forms of life
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The
human genome can be analysed relatively rapidly and cheaply, giving a growing understanding of the genetic basis of
many diseases
...
Genotypes and drugs
react so looking at the genome of patients and the genome of the invading pathogen before deciding treatment is more
common now and can reduce cancer by looking at tumours and treating mutations with drugs
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Mammals have also been genetically modified to produce
therapeutic proteins in their milk
...

The benefits and risks of using antibiotics to manage bacterial infection
To include the wide use of antibiotics following the discovery of penicillin in the mid-20th century and the increase
in bacterial resistance to antibiotics (examples to include Clostridium difficile and MRSA) and its implications
Antibiotics interfere with the metabolism of the bacteria without affecting the metabolism of the human cells – selective
toxicity
...

Antibiotics reduce the death rate due to communicable diseases
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It comes from a mould,
Penicillium chrysogenum, famously discovered by Alexander Fleming in 1928, when he found it growing on his
Staphylococcus spp
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Howard Florey and Ernst Chain developed an industrial process for making the new drug,
which has since saved millions of lives around the world
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Unfortunately, antibiotics are becoming
less effective in the treatment of bacterial diseases as bacteria are becoming resistant to more and more antibiotics
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difficile
...

If a random mutation during bacterial reproduction produces a bacterium that is not affected by the antibiotic that is the one
which is best fitted to survive and reproduce, passing on the antibiotic resistant mutation to the daughter cells
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Antibiotic-resistant bacteria are a concern because it is harder to treat and may become untreatable because there is a
potential for a disease outbreak
...
At the moment,
bacterial resistance is building faster than new antibiotics can be found
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However, this may accelerate the natural selection of antibiotic-resistant strains of both human and animal pathogens
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Antibiotic-resistant infections can be reduced in the long term by taking measures such as minimising the use of antibiotics
and ensuring that every course of antibiotics is completed to reduce the risk of resistant individuals surviving and
developing into a resistant strain population
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