chapter 12 p1 Flashcards
Communicable diseases are caused by
infective organisms known as pathogens.
Each pathogen has particular characteristics that affect the way they are spread and the ways we can attempt to prevent or cure the diseases they cause.
A communicable disease can be passed from one organism to another.
In animals they are most commonly spread from one individual of a species to another, but they can also be spread between species.
Communicable diseases in plants are spread
directly from plant to plant
Vectors
which carry pathogens from one organism to another, are involved in the spread of a number of important plant and animal diseases.
Common vectors include water and insects.
Types of pathogens:
bacteria, viruses, fungi, and protoctista
Bacteria
There are probably more bacteria than any other type of organism.
A small proportion of these bacteria are pathogens, causing communicable diseases.
Bacteria are prokaryotes, so they have a cell structure that is very different from the eukaryotic organisms they infect. They do not have a membrane-bound nucleus or organelles.
Bacteria can be classified in two main ways:
By their basic shapes
By their cell walls
By their basic shapes
they may be rod shaped (bacilli), spherical (cocci), comma shaped (vibrios), spiralled (spirilla), and corkscrew (spirochaetes).
By their cell walls
the two main types of bacterial cell walls have different structures and react differently with a process called Gram staining.
Following staining Gram positive bacteria look purple-blue under the light microscope, for example methicillin-resistant Staphylococcus aureus (MRSA).
Gram negative bacteria appear red, for example the gut bacteria Escherichia coli (E.coli).
This is useful because the type of cell wall affects how bacteria react to different antibiotics a compound that kills or inhibits the growth of bacteria).
viruses p1
- Viruses are non-living infectious agents.
- At 0.02-0.3 pm in diameter, they are around 50 times smaller in length than the average bacterium.
- The basic structure of a virus is some genetic material (DNA or RNA) surrounded by protein.
- Viruses invade living cells, where the genetic material of the virus takes over the biochemistry of the host cell to make more viruses.
- Viruses reproduce rapidly and evolve by developing adaptations to their host, which makes them very successful pathogens.
viruses p2
All naturally occurring viruses are pathogenic.
They cause disease in every other type of organism.
There are even viruses that attack bacteria, known as bacteriophages.
They take over the bacterial cells and use them to replicate, destroying the bacteria at the same time.
People now use bacteriophages both to identify and treat some diseases, and they are very important in scientific research.
Medical scientists consider viruses to be the ultimate parasites.
Protoctista (protista)
The protoctista (now widely known as protista) are a group of eukaryotic organisms with a wide variety of feeding methods.
They include single-celled organisms and cells grouped into colonies.
A small percentage of protoctista act as pathogens, causing devastating communicable diseases in both animals and plants.
The protists which cause disease are parasitic - they use people or animals as their host organism.
Pathogenic protists may need a vector to transfer them to their hosts - malaria and sleeping sickness are examples - or they may enter the body directly through polluted water - amoebic dysentery and Giardia are examples of these.
- Fungi p1
- Fungal diseases are not a major problem in animals, but they can cause devastation in plants.
- Fungi are eukaryotic organisms that are often multicellular, although the yeasts which cause human diseases such as thrush are single-celled.
- Fungi cannot photosynthesise and they digest their food extracellularly before absorbing the nutrients.
- Many fungi are saprophytes which means they feed on dead and decaying matter.
- However some fungi are parasitic, feeding on living plants and animals.
Fungi p2
These are the pathogenic fungi which cause communicable diseases. Because fungal infections often affect the leaves of plants, they stop them photosynthesising and so can quickly kill the plant.
When fungi reproduce they produce millions of tiny spores which can spread huge distances, this adaptation means they can spread rapidly and widely through crop plants.
Fungal diseases of plants cause hardship and even starvation in many countries around the world.
Pathogens - modes of action
Damaging the host tissues directly:
Many types of pathogen damage the tissues of their host organism.
It is this damage, combined with the way in which the body of the host responds to the damage, that causes the symptoms of disease.
Different types of pathogens attack and damage the host tissues in different ways:
Different types of pathogens attack and damage the host tissues in different ways:
Viruses take over the cell metabolism.
Some protoctista also take over cells and break them open as the new generation emerge,
Fungi digest living cells and destroy them.
bacteria produce toxins
Viruses take over the cell metabolism.
The viral genetic material gets into the host cell and is inserted into the host DNA.
The virus then uses the host cell to make new viruses which then burst out of the cell, destroying it and then spread to infect other cells
Ring rot
a bacterial disease of potatoes, tomatoes, and aubergines caused by the Gram positive bacterium Clavibacter michiganensis.
It damages leaves, tubers and fruit. It can destroy up to 80% of the crop and there is no cure.
Once bacterial ring rot infects a field it cannot be used to grow potatoes again for at least two years.
Tobacco mosaic virus (TV)
a virus that infects tobacco plants and around 150 other species including tomatoes, peppers, cucumbers, petunias and delphiniums.
It damages leaves, flowers and fruit, stunting growth and reducing yields, and can lead to an almost total crop loss.
Resistant crop strains are available but there is no cure.
Potato blight (tomato blight, late blight)
caused by the fungus-like protoctist oomycete Phytophthora infestans.
The hyphae penetrate host cells, destroying leaves, tubers and fruit, causing millions of pounds worth of crop damage each year.
There is no cure but resistant strains, careful management and chemical treatments can reduce infection risk.
Black sigatoka
a banana disease caused by the fungus Mycosphaerella fijiensis, which attacks and destroys the leaves.
The hyphae penetrate and digest the cells, turning the leaves black.
If plants are infected it can cause a 50% reduction in yield.
Resistant strains are being developed - good husbandry and fungicide (a chemical that kills fungi) treatment can control the spread of the disease but there is no cure.
