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Biology – The Search for Better Health
Section 3: During the second half of the nineteenth century, the work of Pasteur
and Koch and other scientists stimulated the search for microbes as causes of
disease.

Describe the contribution of Pasteur and Koch to our understanding of
infectious diseases.
Louis Pasteur was a French chemist whose discovery of the different forms of
tartaric acid led to an interest in wine making. Fermentation was considered to be a
totally chemical process at that time. Pasteur however, used a microscope to
compare extracts of fermenting wine with those of souring wine. In fermenting wine
he found small spheres. He also observed that these small spheres could sprout
small buds – they were growing and must be alive. These spheres were yeast. In sour
wine, Pasteur found no yeast but did find rod-shaped structures that also grew.
Fermentation was a biological process.
In 1862 Pasteur conclusively disproved the theory of spontaneous generation, and
supported the view that new cells are produced by existing cells. He showed that
heating to 55 degrees celcius for thirty minutes could kill the organisms responsible
for souring wine. Pasteur later applied the technique to milk, and that became
known as pasteurisation and is still currently used as the standard treatment for
milk.
The link between germs (micro-organisms) and disease was finally established when
Pasteur determined that diseased silkworms contained a parasite and that farmers
could eliminate the disease using only healthy, disease free worms. Pasteur then
announced his germ theory of infection, and he carried out his famous swan necked
flask experiment to support his theory.
These experiments involved using flasks that had long-drawn-out necks (like those of
swans) that were not sealed. Meat broth was boiled in the flasks and as they cooled
the air was drawn in from outside. Any microorganisms present in the air did not
reach the broth as they were trapped in the narrow neck and the curve of the glass.
No bacterial or fungal growth was observed in these fl asks. Bacterial growth
occurred if the curve of the fl ask was broken off and the contents of the flask
exposed to the air. Furthermore, the tipping of a flask to allow the solution in it to
reach the curve where the micro-organisms were trapped resulted in bacterial
growth occurring. This added further evidence to discredit the theory of
spontaneous generation. It proved that the organisms that contaminated the broth
and caused it to decay must be carried in the air and not be spontaneously
generated.
Pasteur used the results of Koch for further experimentation, first with the
microorganism causing anthrax in sheep and later with cholera in chickens. During
these experiments, Pasteur discovered that chickens which suffered as mild attach of
cholera survived an injection of a highly virulent strain. Chickens without a prior
infection died. The mild strain had given chickens immunity to cholera. Pasteur had
discovered a principle called vaccination. He went on to develop vaccines for a
number of other diseases such as rabies where he used the vaccine on humans for
the first time. Pasteur had established the principle of immunity and provided an
effective way to prevent infectious disease.
Robert Koch investigated Anthrax, which was a fatal disease prevalent in sheep and
cattle. He examined He examined the blood of sheep that had died from anthrax and
identified rod-shaped bacteria that he isolated and grew in cultures. These cultured
bacteria were then injected into healthy sheep that subsequently developed
anthrax.
Koch also developed a technique for growing and observing pure cultures of the
anthrax bacterium. He placed some drops of water fluid from the eye of a freshly
killed ox on a cover slip. To this he added a tiny piece of spleen from a mouse killed
from anthrax. He inverted the cover slip and sealed it with Vaseline. Then he used a
microscope to view the culture. He observed that small rods were growing, hence
proving that they were alive.
He repeatedly showed that the anthrax spores he had obtained from the pure
cultures he had grown could cause the disease in other animals and kill them. These
experiments added further weight to the germ theory of disease as they showed
that a microorganism grown outside the body caused a disease.
Koch determined that each disease is caused by a specific microorganism The
principles he used to identify the specific micro-organism that was responsible for a
disease came to be known as Koch’s postulates and are still in use today to identify
the specific micro-organism that causes a particular disease. One of Koch’s
subsequent big breakthroughs was the discovery of the bacterium responsible for
tuberculosis, Mycobacterium tuberculosis.
Koch’s postulates
1. The same microorganism must be present in every diseased host.
2. The microorganism must be isolated and cultured in the laboratory and accurately
described and recorded.
3. When a sample of the pure culture is inoculated into a healthy host, this host must
develop the same symptoms as the original host.
4. The microorganism must be able to be isolated from the second host and cultured
and identified as the same as the original species.

Distinguish between prions, viruses, bacteria, protozoans, fungi, microparasites and name one example of a disease caused by each type of
pathogen.
Prions:
A prion or “proteinaceous infectious particle” is a protein that is capable of causing
disease. Unlike other types of pathogens, priors do not contain any genetic material
(DNA or RNA). They are smaller than smaller than all other pathogens. Normal prion
proteins are coded for by genes in an organisms DNA. It is unclear what the function
of these prion proteins is, but they are present mainly in the nerve cells of the brain.
Normal prion proteins do not cause disease and can be destroyed by heat.
A mutation to the gene that codes for these normal proteins causes the production
of a different form of this prion protein. This form has a different structure and is the
disease-causing prion. This prion is also resistant to most normal methods used to
break down proteins and cannot be destroyed by heating or treating with chemicals.
Infected brains that have been sitting in formaldehyde for years can still transmit
prion disease.
Infectious prions are capable of multiplying and are thought to do this when they
come into contact with the normal prion proteins, altering their structure and
changing them into infectious prion proteins. Because of their unique shape,
infectious prion proteins tend to stick to each other and form long fibres that are
toxic to nerve cells in the brain and kill them. This leads to the characteristic holes
that are found in the brain of an organism suffering from a prion disease.
There are a number of ways an organism can contract prion disease including:
- By eating tissue containing infectious prions.
- As a result of surgery where contaminated implements were used.
- Through contaminated growth hormone injections or contaminated corneal
transplants from dead donors.
- By inheriting the mutated gene that codes for the infectious prions
- By the spontaneous formation of infectious prions.
Diseases caused by prions are called spongiform diseases because the brain tissue of
individuals that are infected by these diseases is full of holes, like that of a sponge.
All of these diseases are fatal.
An example such a disease is kuru, a disease that was found in tribes in the Fore
highlands of Papua New Guinea. The symptoms include uncontrollable shaking,
continuous trembling, and grimacing of the face which led to the name ‘laughing
death’. It was transmitted by eating, during funeral ceremonies, the infected brain
tissue of dead relatives. Only women and children contracted the disease, as the
men did not take part in the burial ritual. The transmission of disease stopped when
the cannibalism practices of the tribe stopped.
Viruses:
Viruses are non-cellular pathogens that have both living and non-living
characteristics. They contain genetic material and are able to pass on hereditary
information (a characteristic of living organisms), are not composed of cells and can
be crystallised (characteristics of non-living things). They are the smallest known
reproducing ‘things’
Viruses are larger than prions and many times smaller than bacteria. A virus is made
up of a protective protein coat that encloses the genetic material, which may be
either DNA or RNA. Viruses that contain RNA are known as retroviruses. Viruses are
unable to reproduce on their own and can replicate only inside host cells. The viral
protein coat contains chemicals that allow the virus to attach itself to the surface of
the host cell. Once the virus attaches itself to the cell, it enters and takes of the cells
reproductive mechanisms to make copies of itself. The cell becomes so full of copies
that it dies and bursts, releasing the new viruses so they can repeat the process with
other host cells.
Bacteriophages (viruses that invade bacterial cells) reproduce in the same way, but
instead of entering the cell they simply inject their genetic material into the host cell.
The treatment of viral diseases is very difficult as any attempt to kill the virus will
also affect the host cells. Many diseases are caused by viruses, including influenza,
measles, AIDS (caused by the human immunodeficiency virus) and herpes.
Bacteria:
Bacteria are single-celled procaryotic organisms – they have a cell wall but no
membrane bound nucleus or organelles. Their genetic material is a single large
chromosome – a circular thread of DNA double helix.
Most bacteria have a capsule outside their cell wall. This is made of slimy gelatinous
material and important in determining the virulence of the bacterium. The virulence
of a bacterium with a capsule can readily cause disease because the slimy nature of
the capsule makes it more difficult for the body’s defence cells to capture and ingest
the problem. When the capsule is removed, the bacterial cell can be more readily
captured by the defence mechanisms.
They are larger than viruses but smaller than protozoans and vary is size from 0.5 to
100 micrometres. Bacteria are classified on the bases of their shape – they can be a
spherical shape (coccus), a rod shape (bacillus), a spiral shape (spirillum), a comma
shape (vibrio) or an oval shape (rickettsiae). Bacteria reproduce by asexual
reproduction using the process of binary fission (dividing in two). The time it takes
for the number of bacteria to double is known as the generation time. This varies for
different species and is between 10 minutes and 24 hours. This means that many
bacteria can be produced in short space of time.
Some bacteria are beneficial whereas others are not. Those that aren’t release toxins
or chemicals that are harmful to the host’s body (these are known as exotoxins).
These toxins can either inhibit protein synthesis, damage cell membranes, disrupt
transport of material across cell membranes, or interfere with normal nerve
function. Some examples of diseases that are caused by bacteria include tetanus,
meningococcal disease and food poisoning.
Protozoans:
Protozoans are single-celled eucaryotic organisms. They have a cell membrane, have
no cell wall and possess a membrane-bound nucleus / organelles. Only some of the
many types are pathogenic. They usually reproduce by the process of asexual binary
fission and range from 1-300 micrometers (smallest bacterium). They are motile.
Some protozoans possess a long whip-like tail called a flagellum to enable them to
move about. These protozoans are known as flagellates. Ciliates have many hair-like
projections called cilia surrounding the cell. These beat rapidly to propel them along.
Protozoans such as Amoeba possess projections of the cytoplasm, called
pseudopods, to move them around. Sporozoa are protozoans such as Plasmodium
that do not have structures for motion and reproduce by spores. Diseases caused by
protozoans include malaria, giardiasis and trypanosomiasis.
Fungi:
Fungi are eucaryotic organisms that possess a cell wall, but its composition is
different to that of plant cell walls. In addition, fungi do not contain chlorophyll and
are not capable of producing their own food. They can be unicellular, such as yeasts,
or multi-cellular, for example mushrooms. Most are composed of a system of
microscopic tubular filaments or threads known as hyphae, which branch and spread
to form a structure known as mycelium. Fungi range in size (micro to macroscopic),
and also in reproductive methods (asexual, sexual or even both). Most fungi are
saprophytic (live on dead organisms) and hence act importantly as decomposers.
Most pathogenic fungi are dermatophytes (live in skin, nails and hair), and a small
number cause diseases such as candidiasis (thrush) and athlete’s foot (tinea).
Micro-parasites:
Micro-parasites are parasites that are visible to the naked eye and are larger than
other pathogens. They are multicellular eucaryotic organisms that vary in size from
the tiniest louse to very long tapeworms. Some macro-parasites cause disease
directly, whereas others will act as vectors in the transmission of a disease. Macroparasites can be divided into two groups:
1. Endoparasites live inside the host’s body and include flatworms
(tapeworms) and roundworms. They cause diseases such as taeniasis
(tapeworm disease), and elephantiasis (caused by a filarial worm).
2. Ectoparasites are parasites that live outside the body, usually sucking
blood. Examples include mosquitoes and lice. Some of these parasites
inject toxins while feeding; these can cause inflammation, allergic
reactions and sometimes partial paralysis. Ectoparasites can also act as
vectors for the transmission of other pathogens. The flea is vector for the
bacterium Yersinia pestis, which causes bubonic plague.

Identify the role of antibiotics in the management of infectious disease
Antibiotics play an extremely important role in the management infectious disease
that is caused by bacteria. They are chemicals that are capable of destroying or
inhibiting the growth of the bacteria that cause disease. Antibiotics are chemicals
that target the bacteria without destroying the host and are not at all effective
against viruses. There are two types of antibiotics – Broad Spectrum Antibiotics and
Narrow Range Antibiotics.
The antibiotics act on the bacteria to destroy them in a number of different ways:
- Some antibiotics accumulate in the cells of the bacteria and prevent them
from forming a new cell wall when they are dividing, for example, penicillin.
- Some antibiotics destroy the cell membrane, thus effectively destroying the
bacteria, for example amphotericin.
- Some antibiotics interfere with protein synthesis so the bacteria are unable
to make essential compounds, resulting in the death of the cell. Erythromycin
is an example of an antibiotic that acts in this way.

Gather and process information to trace the historical development of our
understanding of the cause and prevention of malaria.
Malaria is one of the most prevalent infectious diseases in the world today, with
more than 300 million cases reported and 1.5-3 million deaths, mostly of African
children under 5 years old, each year. Malaria starts suddenly and is characterised by
intermittent violent chills and intense fevers, severe headaches, convulsions and
delirium. Anaemia is also a common symptom, as well as an enlarged spleen. Death
will result when the tissue dies from a lack of oxygen or serious brain/kidney
infections.
Understanding the Cause:
Step 1: Recognising the symptoms and hypothesising the cause
The symptoms of malaria have been reported since the beginnings of recorded
history. In Chinese methodology for example, three demons are pictured: One with a
hammer, one with cold water, and one with a stove. These demons were held
responsible for the headache, chill and fever suffered with malaria. The Greeks,
however realised that those who live in swampy areas had a higher chance of
developing the disease. Hence they believed that it may have been due to the
drinking of the swamp water, or the inhalation of the dirty air that resulted in the
disease.
Step 2: Discovering the micro-organism.
Pasteur’s and Koch’s work ignited the search for the cause of malaria. In 1880
Charles Laveran discovered the pathogen that causes malaria while looking at the
blood of malaria patients. This organism was a protozoan that he classified as
Plasmodium.
In 1885, Golgi, an Italian neurophysiologist, established that there were at least two
forms of the disease: one that causes a fever third day. In 1886, he observed that
each of these forms produced differing amounts of new parasites (merozoites) and
that the peak of the fever coincided with the release of the merozoites into blood.
Later, other scientists found that there was in fact two more malaria parasites and
the initial pathogen’s name was changed form Plasmodium to P. falciparum.
Step 3: Determining life cycle of the parasite and the mode of transmission.
In 1897, Ronald Ross, demonstrated that the malaria parasite could be transmitted
from infected patients to mosquitoes. He tested this hypothesis in birds and was
successful in showing that the mosquitoes were able to pass the malaria parasite
from bird to bird. He determined the main steps in the cycle of transmission of the
malaria parasite and identified that only a certain strain of mosquito transmitted the
malaria parasite.
Life Cycle of Malaria:
The mosquito ingests infected red blood cells, the cells are digested and the malarial
parasite is released in the intestine of the mosquito. The parasites migrate from the
intestine to the salivary glands where they remain ready to enter another host when
the mosquito next feeds.
Prevention of Malaria
Although drugs are available for the treatment of malaria, a complete cure is
difficult. This is because the parasite can remain dormant for many years in the liver
before becoming active again. Different drugs are used against the different stages
of the malarial parasite. Malaria is still one of the most serious infections in the
world and is particularly common in some tropical and sub-tropical areas. The
Anopheles mosquito, the main carrier of malaria is common in these areas.
Control of the disease is also becoming more difficult as mosquitoes become
increasingly resistant to chemicals such as DDT, that have been effective against
them in the past. Eradication of the vector for the malarial parasite is proving to be
virtually impossible. People who need to travel to malarial infested regions must
take all precautions to reduce their chance of coming into contact with mosquitoes.
History of Prevention:
The ancient Chinese used the anti-fever properties of the Qinghao plant in order to
treat malaria. In order to prevent it, however, the ancient Greeks and Romans
decided to build drains to remove stagnant water. After they had done this, the
incidence of these fevers fell.
In the mid-1600’s the first drug to treat malaria was produced. This was known as
quinine. It was extracted from the bark of the Peruvian cinchona tree. After it was
shown that the mosquito was responsible for the transmission of malaria,
procedures were followed to reduce the chance of being bitten by a mosquito. Many
areas where the mosquitoes were bred were drained, bodies of water were sprayed
with oil to prevent breeding and protective clothing was worn to reduce the risk of
being bitten.
Following this, a range of drugs were developed, each of which had a positive
immediate effect such as Aterbrin, but over time, the virus evolved and became
resistant, hence reduces the drugs effectiveness. Additionally, the use of DDT was
employed to eradicate large numbers of mosquitoes. This also resulted in the same
consequence where initially large populations were wiped out, but over time, the
species evolved and became resistant to the pesticide.
Overall, the best method of prevention is those that reduce the risk of being bitten.

Describe an infectious disease in terms of its cause, transmission, host
response, major symptoms, treatment, prevention and control.
Disease: Influenza
Cause: Influenza is caused by infection with the influenza virus. There are two types
(A and B) are the two main types of influenza viruses that infect humans, and each
contains RNA surrounded by a protein coat. They differ from each other as they
carry different antigens on their surfaces. There are a number of different strains of
both the influenza A and B viruses.
Transmission: The transmission of influenza virus can be either direct of indirect:
- Direct contact: The viral particles are inhaled through the nose and mouth in
droplets that have been exhaled by an infected person when they sneeze or
cough.
- Indirect contact: The infected person touches the respiratory tract, and then
touches some else, such as a handrail. A second person touches the handrail
and soon afterwards, then places their hand on their nose or mouth.
Host Response to the influenza virus: The immune response is initiated by the
presence of the virus in the body. This produces antibodies and other immuneresponse cells specific for the particular strain of the influenza virus that has infected
the body. The immune response is responsible for destroying the viral particles that
have invaded the body.
Major symptoms of Influenza: Fever, Headache, inflammation of the upper
respiratory tract, sore throat, muscle pain, coughing and sneezing as well as nasal
catarrh (inflammation the mucous membrane causing excess production of mucous).
Treatment: Influenza is caused by a virus and therefore there is no treatment that
will cure it. The main method of treatment is to relieve the symptoms, get plenty of
bed rest and drink extra fluids. Bed rest allows the body to fight the disease and then
recover. Aspirin or paracetamol can be given to help alleviate headaches, sore throat
and muscle pain and to reduce fever. Antibiotics are ineffective in the treatment of
viral diseases, but can be used if secondary bacterial infections develop.
Prevention: The primary method of prevention of influenza involves the use of
influenza vaccines. New vaccines are produced each year and are derived from the
influenza A and B viruses that circulated during the previous influenza season. If the
virus vaccine and the circulating virus are closely related, the effectiveness is fairly
high. As the influenza virus is constantly changing by mutation, the vaccines have to
be constantly updated. Other strategies include wearing protective masks, avoiding
crowded areas and ensuring that adequate nutrition and sleep is obtained.
Control: To reduce the spread of the disease through the population a number of
strategies could be employed. These include:
- Implementing immunisation programs along with education programs to
encourage at risk individuals to be vaccinated.
- Isolation infection individuals to reduce the spread of influenza throughout
the population – this would include infected individuals remaining at home to
stop the spread of the virus to their work or school colleagues.

Process information from secondary sources to discuss problems relating to
antibiotic resistance.
How resistance evolves:
When penicillin was first discovered, and then many other antibiotics produced, the
threat posed by infectious diseases was greatly reduced and there was a dramatic
drop in the number of deaths from diseases that were previously untreatable. With
the widespread use of antibiotics, a problem that threatens the successful treatment
of these diseases has developed. Bacteria, during the normal process of natural
selection have evolved strains that are resistant to many if not all of the antibiotics
that are used to treat infectious disease in the world today.
The way in which bacteria develop a resistance to the antibiotic can be explained by
Darwin’s theory of evolution by natural selection. The theory states that in any
population there is variation. In a particular environment, the organisms that have
the variation that is best suited to that environment survive and reproduce. This
produces a population in which most organisms are adapted to survival in that
particular environment. When antibiotics are administered to treat a bacterial
infection, some of the bacteria present may possess a natural resistance to that
particular antibiotic, and so they survive. They then reproduce and can quickly build
up a population that is resistant to the antibiotic. In conjunction with this, the
bacteria are also capable of passing this resistance on to other bacteria they
encounter, further building up the population of resistant bacteria.
A number of common practices is further accelerating antibiotic resistance. These
include:
- The overuse of antibiotics for treatment of many diseases - not just
bacterial. Examples are the prescribing of antibiotics for the treatment of
coughs, colds and the flu. These diseases are caused by viruses, which are not
affected by antibiotics. This practice just gives the bacteria more chances to
build up populations of resistant strains.
- A very common practice is to only take the antibiotics provided until all the
symptoms disappear and not finish the whole course. This is also a dangerous
practice as not all the bacteria present may be killed before the end of the
course tablets. This allows yet another chance for the bacteria to survive, and
reproduce more resistive strains.
- Food-producing animals such as chickens and pigs are fed antibiotics as part
of their diet to promote growth. This widespread use of antibiotics further
promotes the formation of antibiotic resistant populations of bacteria.
- The same process occurs when we use cleaning products that contain antibacterial ingredients.
Problems relating to antibiotic resistance:
The development of antibiotic resistance has been happening ever since the
discovery of penicillin and many antibiotics that were originally hailed as ‘miracle
cures’ are no longer effective.
Micro-organisms that cause diseases once easily cured, such as tuberculosis, have
developed resistant strains that are not responding to the cheaper ‘first-line’
antibiotics. As a result of this, the effects of these diseases are now more sever and,
because they take much longer to cure, the infectious period is longer, meaning that
there us a greater chance of passing on the resistant strain of the micro-organisms to
other members of the community.
When ‘second-line’ or ‘third-line’ antibiotics have to be used they are usually more
expensive and more toxic. The drugs needed to treat multi-resistant tuberculosis are
100 times more expensive than those used to treat the non-resistant forms, and in
countries where this is too expensive to use, the disease is untreatable and therefore
spreads.
The result now it that a number of bacteria are resistant to almost all known
antibiotics. Vancomycin is an antibiotic that is used only when all other treatments
have failed, but even its effectiveness is reduced due to the emergence of
vancomycin-resistant enterococci (VRE) infections. These are extremely difficult to
treat and eradicate, and usually experimental drugs are used as a last resort.
As a result, antibiotic resistance is a major problem for the treatment of some
diseases, as the current trend indicates that in the near future some diseases will
have no treatment, unless there is a significant breakthrough in producing drugs that
are more effective.
Some strategies to slow resistance:
- Antibiotics should only be prescribed for bacterial infections
- The antibiotic should target the pathogen and not be broad spectrum
- Normal hygiene practices and antibiotics should be taken for whole course.
- Do not use cleaning products that contain anti-microbial ingredients.