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:
perform an investigation to model Pasteur's experiment to identify the role of microbes in decay
describe the contribution of Pasteur and Koch to our understanding of infectious diseases
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.
A useful Internet site:
History of Malaria Division of Laboratory Medicine at Royal Perth Hospital
|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|
and name one example of a disease caused by each type of pathogen
||Examples of diseases it causes
||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
||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.
||Very simple cells with no internal
||Microscopic single-celled organisms with
||Heterotrophic organisms. Some (e.g.
yeasts) are unicellular, others consist of long
||Organisms that are visible to the naked
eye, also called parasites.
identify the role of antibiotics in the management of infectious disease
process information from secondary sources to discuss problems relating to antibiotic resistance
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.
There are many sites on the internet that cover this topic. Here is one to start with.
The development of antibiotic resistance Sumanas Inc.
- host response
- major symptoms
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.
||The parasitic protozoan,
||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
||When in the blood cells the host
produces antibodies against Plasmodium
||Chills, fever, sweating, delirium and
||Anti-malarial drugs such as quinine and
||Cover up after dark and use personal
insecticide, mosquito nets
||Draining swamps, spraying with
Malaria: An Online Resource Royal Perth Hospital, Western Australia