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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

Pasteur's experiment
  • 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.

Pasteur's experiment (external website)

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describe the contribution of Pasteur and Koch to our understanding of infectious diseases

Louis Pasteur

Robert Koch

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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.

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
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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.
  • scrapie in sheep
  • spongiform encephelopathy in cattle (mad cow disease)
  • Creutzfeldt-Jakob (CJD) disease in humans
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.
  • influenza
  • measles
  • a common cold
  • herpes
  • AIDS
  • Warts
  • Hepatitis
  • Foot-and-mouth disease
  • Plum pox virus
Bacteria
Very simple cells with no internal membranes.
  • Boils
  • Cholera
  • Legionnaire's disease
  • Tuberculosis
  • Crown gall blight
Protozoans
Microscopic single-celled organisms with internal membranes.
  • Amoebic dysentery
  • Giardia,
  • Malaria,
Fungi
Heterotrophic organisms. Some (e.g. yeasts) are unicellular, others consist of long branching threads.
  • Ringworm
  • Tinea
  • Thrush
  • Many plant diseases such as damping off in seedlings
Macro-organisms
Organisms that are visible to the naked eye, also called parasites.
  • fleas
  • ticks
  • tapeworms
  • bilharzia worms
  • hydatid worms
  • liver fluke
  • many plant parasites, e.g. aphids
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identify the role of antibiotics in the management of infectious disease

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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.

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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

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 (external website) Royal Perth Hospital, Western Australia

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