Communicable diseases, prevention, and the immune system

• How the tobacco mosaic virus causes disease

  1. Affects plants. Mainly transmitted via infected sap

  2. Contains ssRNA, which is directly transcribed by host cell to assemble new virions

  3. Virions enter other cells via plasmodesmata then enter xylem & phloem

  4. Causes stunted growth & mottled leaves
• Groups of pathogens that can cause communicable diseases

  • Bacteria
  • Fungi
  • Protoctista
  • Viruses
• How Mycobacterium tuberculosis causes disease

  1. Triggers inflammatory response by infecting phagocytes in lungs

  2. Infected phagocytes are sealed in waxy-coated tubercles so bacteria remain dormant. First infection has no symptoms

  3. If another factor weakens immune system, bacteria become active & destroy lung tissue
• How HIV results in the symptoms of AIDS

  1. Attachment proteins bind to complementary CD4 receptor on TH cells

  2. HIV particles replicate inside TH cells, killing or damaging them

  3. AIDS develops when there are too few TH cells for the immune system to function

  4. Individuals cannot destroy other pathogens & suffer from secondary diseases/infections. May cause death
• How the influenza virus causes disease

  1. Transmitted via: droplet infection, contact with mucus containing virus, zoonotic infection, contact with fomites

  2. Injects viral RNA into ciliated epithelial cells of throat & lungs. Viral RNA hijacks cell biochemistry to produce new virions. Cell lysis releases virions

  3. 5-7 days of headache, coughing, sneezing, sore throat, vomiting, fever, muscular/joint pain
• What causes malaria

  1. Female Anopheles mosquito acts as vector for Plasmodium spp. protoctista when it transfers saliva to another organism during feeding

  2. Parasite reproduces asexually in red blood cells in liver, causing lysis
• What causes potato/tomato late blight and ring rot of potatoes

  1. Blight: The protoctista Phytophthora infestans behaves similarly to a fungus. Mainly transmitted via spores

  2. Ring rot: Sepedonicus subspecies of the bacterium Clavibacter michiganensis. Mainly transmitted by planting infected seeds/contact with fomites. Plant-to-plant transmission is rare
• How the tobacco mosaic virus causes disease
• How Mycobacterium tuberculosis causes disease
• What causes malaria
• How HIV results in the symptoms of AIDS
• What causes bacterial meningitis
• How the influenza virus causes disease
• What causes potato/tomato late blight and ring rot of potatoes
• Causes of bacterial meningitis

  • Meningococcal bacteria A, B, C, W, X, Y, Z
  • Pneumococcal bacteria
  • Haemophilus influenzae type b (Hib) bacteria
• Bacterial meningitis is transmitted by

  Droplet infection & direct contact with saliva e.g. kissing
• Plant-to-plant transmission of the bacterium Clavibacter michiganensis is rare
• Describe 3 fungal infections
• How are communicable pathogens transmitted indirectly?
• How do living conditions affect disease transmission?
• How are communicable pathogens transmitted directly?
• Name 4 physical barriers to pathogen entry in plants
• What is necrosis?
• Describe 2 mechanical responses to infection in plants
• Bacterial meningitis affects meninges (protective membranes around the brain)
• What causes bacterial meningitis?
• Subspecies of the bacterium Clavibacter michiganensis are mainly transmitted by

  • Planting infected seeds/ contact with fomites
• Effects of living conditions on disease transmission

  • Overcrowding increases direct transmission
  • Climate determines which organisms can survive e.g. malaria is more prevalent in tropical countries
  • Social factors influence how quickly people are treated, which can increase/decrease direct transmission
• 3 fungal infections

  • Leaf-spot disease black sigatoka in banana plants caused by the sac fungus Mycosphaerella fijiensis
  • Ringworm caused by about 40 types of fungi
  • Athlete’s foot in humans (tinea pedis) caused by a range of fungi
• Ways communicable pathogens are transmitted directly

  • Inhalation (droplet infection)
  • Skin-to-skin contact or exchange of fluids
  • Penetrate skin actively using enzymes or passively through wounds, hair follicles or sweat glands
• Bacterial meningitis is usually spread by

  Carriers of the bacteria who are not ill & occasionally by individuals with meningitis
• Ways communicable pathogens are transmitted indirectly

  • Consumption of contaminated food & drink
  • Via a vector e.g. mosquitoes transmit Plasmodium parasite
  • Spores
• Injury activates intracellular enzymes in plants that kill cells near the site of infection to prevent pathogen from spreading. Necrosis of woody tissue is known as canker
• Mechanical responses to infection in plants

  • Guard cells close stomata
  • The thick polysaccharide callose is produced & deposited between the cell wall & plasma membrane to increase entry distance/ limit spread
• Physical barriers to pathogen entry in plants

  • Cellulose cell walls
  • Lignified layer
  • Waxy upper cuticle
  • Old vascular tissue is blocked to prevent pathogens from spreading inside the plant
• Process of inflammation

  1. Damaged vessels release histamines, causing vasodilation

  2. Blood flow & permeability of blood vessels increase

  3. White blood cells & plasma move into the infected tissue
• Expulsive reflexes are the body's attempt to force foreign substances out
• Chemical defences plants use against pathogens

  1. Terpenoids (essential oils) act as antibacterials

  2. Phenols inhibit insects from attacking by interfering with digestion

  3. Alkaloids deter herbivores from feeding because they taste bitter

  4. Defensins inhibit transport channels

  5. Hydrolytic enzymes break down cell wall of invading organisms
• Types of white blood cell involved in phagocytosis

  • Neutrophils
  • Macrophages (can become antigen-presenting cells)
• Barriers to infection in animals

  • Skin is tough keratin layer
  • Blood clotting prevents pathogens from entering through skin lesions
  • Hydrochloric acid in stomach kills bacteria
  • Harmless bacteria in gut & on skin surface increase interspecific competition with pathogens
  • Mucous membranes trap pathogens and may secrete antimicrobial enzymes