Biology
Home > Biology > Core > The search for better health > Search for better health: 3. Identifying microbes that cause disease
9.4 Search for better health: 3. Identifying
microbes that cause disease
| Syllabus reference (October 2002 version) | ||
|---|---|---|
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
|
Students learn to:
|
Students:
|
Extract from Biology Stage 6 Syllabus (Amended October
2002). © Board of Studies, NSW
[Edit: 4 Jun 09]
perform an investigation to model Pasteur's experiment to identify the role of microbes in decay
-
Perform Pasteur's experiment that
showed that something from the air causes meat broth to go
bad. As you conduct your experiment consider which
variables need to be kept constant and be able to explain
which are the dependent and which are independent
variables.
Procedure:
- Use a meat extract cube to make a clear broth.
- Use two conical flasks instead of Pasteur's balloon flasks. Fit the flasks with one-holed stoppers. Use glass tubing bent into an S-shape to replace Pasteur's swan-necked flask. Place a straight piece of glass tubing in the other flask.
- Put some broth into both flasks and boil gently for fifteen minutes.
- Leave both flasks, not in direct sunlight for several weeks. Every two or three days compare the contents of the two flasks. Look for cloudiness, scum, bubbles and mould colonies. Record your results.
- Assess the accuracy of your observation and the relative importance of the data gathered. You could do this by comparing your experiment with the description of Pasteur's experiment, which follows.
- When Pasteur did his experiment, the broth in the swan-necked flask remained clear for several weeks, while that in the open flask quickly became cloudy and smelly.
- Both flasks were open to the air. In the swan-necked flask, air could move freely through the neck of the flask just as it did in the straight-necked flask, but the much heavier micro-organisms, in the air, were trapped in the bottom part of the S-curve.
- This experiment showed that for the broth to grow micro-organisms and start to decay, there had to be access to air containing the spores of micro-organisms.
describe the contribution of Pasteur and Koch to our understanding of infectious diseases
Louis Pasteur
- The role of Pasteur in identifying the causes of disease
was:
- that he disproved the theory of spontaneous generation, which was widely held at the time. Before Pasteur's work, people believed that maggots and fungi grew naturally from non-living material
- that he showed that micro-organisms came from pre-existing micro-organisms.
Louis Pasteur (1822-1895)
National Health Museum,
USA
Robert Koch
- About 150 years ago, Pasteur's work in identifying, under the microscope, the organism that caused fermentation, led some people to suggest the infectious diseases were caused by microscopic pathogens. Others argued that bacteria found in sick animals followed the infection, rather than causing it.
- The work of Robert Koch (1843 - 1910) provided the proof that was needed to convince people that microscopic pathogens cause disease. His first experiments were with the disease anthrax in sheep. Later, he obtained similar results for tuberculosis and cholera.
- First Koch found bacteria in sheep infected with anthrax. Then, he placed the bacteria on agar plates in Petri dishes so that many colonies of the bacteria were produced. He used bacteria from these colonies to infect healthy sheep and found that they became infected.
- After his experiments with anthrax, Koch was able to
state a series of steps that are needed to identify the
micro-organism responsible for a particular disease. These
steps are called Koch's postulates.
- Step 1: All infected hosts must contain the suspect organism.
- Step 2: A pure culture of the suspect organism must be obtained.
- Step 3: A healthy organism infected with the pure culture must have the same symptoms as the original host.
- Step 4: The suspect organism must be isolated from the second host, grown in pure culture and prove to be identical to the first culture.
- Koch's postulates can be used to identify the causative organism of an infectious disease. The symptoms of the disease are carefully identified and then the blood of sufferers is examined to determine possible causative organisms. A particular micro-organism will be suspected if other sufferers have the same micro-organism present in their blood and a mechanism can be identified to allow transfer of the micro-organism. Identification of the organism in more sufferers will confirm the causative organism. Sometimes, a suspected causative organism can be confirmed by infecting a test organism.
gather and process information to trace the historical development of our understanding of the cause and prevention of malaria
Background on malaria
Malaria is a disease caused by a protozoan of the genus Plasmodium. It has a complicated life cycle requiring a mosquito of the Anopheles genus to carry the Plasmodium to its host. The disease is common in tropical areas where the Anopheles mosquito lives. The female mosquito requires a blood meal to complete the reproduction cycle of the mosquito. During the blood meal the Plasmodium (sporozoites) are transferred from the mosquito salivary glands into the blood system of the host. The sporozoites travel to the liver via the blood system and enter cells in the liver. After 12 days a new form of the protozoan called merozoites are released and these enter blood cells. At the same time toxins are released. This causes the sweats and fever that are associated with the disease. Some of the merozoites develop into gametocytes and may be sucked up by another mosquito in another blood meal. In the gut of the female mosquito the gametocytes become gametes and are fertilised. This forms sporozoites which will travel to the salivary glands of the female mosquito and await the next blood meal to enter another host.
The disease was known from the start of recorded history but it took many researchers to uncover the complicated life cycle above. Sir Ronald Ross (1857 - 1932) was a British medical officer working in India. For thousands of years, people had been puzzled about the way in which malaria spread but they knew that malaria was common in areas close to swampy land. In the late 1800s, people were beginning to wonder if mosquitoes could spread malaria. Ross collected mosquitoes and painstakingly dissected them under a microscope. He discovered the micro-organism that was known to cause malaria, inside the bodies of Anopheles mosquitoes. This led to the realisation that insects could carry pathogens, that is, they can be vectors of disease.
- Use reference books or the Internet to
gather information to trace how scientists
identified the cause and prevention of malaria.
A useful Internet site:
History of Malaria
Division of
Laboratory Medicine at Royal Perth Hospital - Process this information to show how their work contributed to our understanding of the cause and prevention of malaria. Trends and patterns could be well illustrated through the use of an annotated timeline. The table below is a guideline.
| Date | Development |
|---|---|
| 18 BC | The disease malaria was described by the Romans. Malaria was thought to come from swamps so the name means 'bad air' |
| 1820 | Quinine used to prevent the disease |
| 1880 | Charles Laveran a French army doctor observed the malarial parasite |
| 1886 | Golgi observed asexual reproduction in the protozoan Plasmodium and identified two species |
| 1898 | Giovanni Grassi named the Anopheles mosquito as the carrier of the malarial parasite |
| 1897 | Ronald Ross discovered that Plasmodium was the protozoan that caused the disease malaria. |
| 1940 | Chloroquinine the first synthetic anti-malarial drug was used |
distinguish between:
- prions
- viruses
- bacteria
- protozoans
- fungi
- macro-parasites
and name one example of a disease caused by each type
of pathogen
| Pathogen
|
Description
|
Examples of diseases it causes
|
|---|---|---|
| Prions
|
Protein that has been altered from its
normal structure and can then alter other proteins to
develop more prions, so that the change spreads like a
chain reaction.
|
|
| Viruses
|
Consist of DNA or RNA enclosed in protein,
live inside living cells. They are so small that they
cannot be seen with a light microscope.
|
|
| Bacteria
|
Very simple cells with no internal
membranes.
|
|
| Protozoans
|
Microscopic single-celled organisms with
internal membranes.
|
|
| Fungi
|
Heterotrophic organisms. Some (e.g.
yeasts) are unicellular, others consist of long
branching threads.
|
|
| Macro-organisms
|
Organisms that are visible to the naked
eye, also called parasites.
|
|
identify the role of antibiotics in the management of infectious disease
- Antibiotics play an important role in the management of infectious diseases. Antibiotics were discovered by Alexander Fleming in 1928. They are naturally occurring compounds produced by one organism to prevent the growth of bacteria. Before the discovery of antibiotics, many people died of what we now would think of as simple infections.
process information from secondary sources to discuss problems relating to antibiotic resistance
Background
Unfortunately, the overuse of antibiotics has led to the
selection of more virulent bacteria that are resistant to
antibiotics.
When antibiotics were first introduced, they had a dramatic effect on the pathogens that cause disease. Over time, it became apparent that the effects of the antibiotics were beginning to become less potent. This was because of the development of drug resistance in the pathogen. Each time an antibiotic is used, there may be some individual pathogens that have a natural resistance to the drug. These naturally resistant individuals are left to breed the next generation and pass on the genetic information that made them resistant. The next time the drug is used, it will have no effect. Overuse of antibiotics has resulted in "superbugs". These strains are resistant to antibiotics and include vancomycin resistant golden staph (Staphylococcus aureus). These organisms are not destroyed by our strongest antibiotics. Scientists are developing new antibiotics such as Zyvox to deal with multi-resistant bacteria. In the future, unless new antibiotics are produced, common infections will once again be responsible for many deaths.
Many household products and cleaning agents now contain antibiotics. These do not kill all bacteria so act as a selecting agent for antibiotic resistant bacteria. These can increase in number without competing with other bacteria.
The use of antibiotics in farm animals also has the same effect of selecting for antibiotic resistant bacteria. Some farm industries put human antibiotics into the feed of their animals. Thus increasing the build up of antibiotic resistant bacteria. During the production of meat, animals are given antibiotics to prevent infections. When the meat reaches the table, it may still contain these animal antibiotics. This could lead to more antibiotic resistant bacteria.
It is important to complete a course of antibiotics even when the symptoms are gone. This will ensure that the bacteria have been completely destroyed. Not finishing antibiotics can lead to the selection of antibiotic resistant strains.
-
Process this information to discuss the
problems of antibiotic resistance. Trends and patterns
could be illustrated by the unending cycle between the
introduction of new antibiotics and the development of
resistance in bacteria.
Some good starting points are the websites below.
The development of antibiotic resistance
Sumanas Inc.Antibiotic resistant bacteria
University of Wisconsin, USA.
identify data sources, gather, process and analyse information from secondary sources to describe one named infectious disease in terms of its:
- cause
- transmission
- host response
- major symptoms
- treatment
- prevention
- control
- To address this dot point, it is a good idea to select an
infectious disease that interests you or that represents a
significant problem in your community.
Use the list presented in the dot point to determine the type of information that you need to collect. Gather the information from a range of sources including digital technologies, locally available health brochures or phamphlets and the Internet. Process the accuracy of the information by looking for information that is consistently represented across a range of reputable publications. Analyse your information by developing accurate generalisations into short notes.
A good example of a named infectious disease is malaria. The following is a description of the disease.
| Factors
|
Description
|
|---|---|
| Cause
|
The parasitic protozoan,
Plasmodium
|
| Transmission
|
Anopheles mosquito is the
insect vector. Blood from a malaria victim contains
Plasmodium sex cells. These form zygotes in
cysts in the stomach wall of the mosquito and mature
into sporozoites. When a cyst bursts, the
sporozoites travel to the mosquito salivary glands,
from where they are transferred to the victim of the
mosquito bite. The sporozoites travel to the liver,
multiply and then enter the red blood cells, where
they also multiply. When the infected cells burst,
they cause the malarial fever. Male and female
gametes are produced from these sporozoites, which
are then taken in the blood the next time a mosquito
bites.
|
| Host response
|
When in the blood cells the host
produces antibodies against Plasmodium
|
| Major symptoms
|
Chills, fever, sweating, delirium and
headache
|
| Treatment
|
Anti-malarial drugs such as quinine and
chloroquinine
|
| Prevention
|
Cover up after dark and use personal
insecticide, mosquito nets
|
| Control
|
Draining swamps, spraying with
insecticides.
|
Malaria: An Online Resource Royal Perth Hospital, Western
Australia
|