Animal and plant pathogens

• Communicable are caused by pathogens.
• Pathogens are microorganisms which cause diease.
• Pathogens include: bacteria, viruses, fungi and protists.
• A communicable disease can be passed from one organism to another.
• Bacteria

  Bacteria are prokaryotes and can be classified in two main ways:

  • 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. Gram positive bacteria appear purple-blue and gram negative bacteria appear red.
• Viruses

  Viruses are non-living infectious agents. Their basic structure 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 develop adaptations to their hosts. They cause disease in every other type of organism, even bacteria (bacteriophages). They take over bacterial cells and use them to replicate, destroying the bacteria at the same time. These bacteriophages can be used to both identify and treat some diseases.
• Protists


  Protists are a group of eukaryotic organisms with a wide variety of feeding methods. A small percentage of protists act as pathogens, causing devastating communicable diseases in both animals and plants. Protists which cause disease are parasitic- they need people or animals as their host organism. Pathogenic protists may need a vector to transfer them to their hosts or they enter the body through polluted water.
• Fungi


  Fungi are eukaryotic organisms. Fungi cannot photosynthesise and they digest their food extracellularly before absorbing the nutrients. Some fungi are parasitic. Fungal infections often affect the leaves of plants which stops photosynthesis so can quickly kill the plant. When fungi reproduce, they produce millions of spores which can spread over huge distances so they spread rapidly and widely through crop plants.
• Modes of action: Damaging tissues directly, Viruses

  Viruses take over 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.
• Modes of action: Damaging tissues directly, Protist

  Some protists also take over cells and break them open as the new generation emerge, but they do not take over the genetic material of the cell. They simply digest and use cell contents as they reproduce.
• Modes of action: Damaging tissues directly, Fungi

  Fungi digest living cells and destroy them. This combined with the response of the body to the damage caused by the fungus gives the symptoms of disease.
• Modes of action: Producing toxins which damage host tissues, Bacteria


  Most bacteria produce toxins that poison or damage the host cells in some way, causing disease. Some bacterial toxins damage the host cells by breaking down the cell membranes, some damage or inactivate enzymes and some interfere with the host cell genetic material so the cells cannot divide. The toxins are a by-product of the normal functioning of the bacteria.
• Modes of action: Producing toxins which damage host tissues, Fungi


  Some fungi produce toxins which affect the host cells and cause disease.