Communicable Diseases

• communicable disease- can be passed from one organism to another of the same or different species
• pathogen - microorganism that causes disease. includes bacteria, fungi, viruses and protoctista
• vectors - a living or non-living factor that transmits a pathogen from one organism to another
• Bacteria:

  • rod shaped - bacilli
  • spherical - cocci
  • comma shaped - vibrios
  • spiralled - spirilla
  • corkscrew - spirochaetes
• Bacteria cell wall:

  • there are two different types of bacterial cell wall that are identified by gram staining
  • gram staining is a method of staining bacteria using a dye called crystal violet
  • the gram-staining characteristics of bacteria are denoted as positive or negative depending on whether they take up the stain
  • gram positive bacteria look purple-blue under the light microscope e.g. MRSA
  • gram negative bacteria appear red under a light microscope e.g. E. Coli
• Viruses:

  • non-living infectious agents
  • 0.02-0.3 micrometers in diameter - 50x smaller than bacteria
  • DNA or RNA surrounded by a protein coat
  • Viruses invade living cells. the genetic material in the virus will take over the very biochemistry of the cell and use it to produce more viruses
  • all naturally occurring viruses are pathogens
  • Bacteriophages: viruses that attack bacteria, used for scientific research to identify and treat diseases
• Protoctista:

  • eukaryotic organisms
  • include single cell organisms and cells grouped into colonies `
  • some protista are pathogenic
  • e.g. malaria, sleeping sickness, amoebic dysentery and giardia
• Fungi:

  • Eukaryotic single and multicellular organisms
  • causes irritation in animals
  • most fungi are saprophytes, but some are parasitic
  • fungal affections often affect leaves
  • fungi reproduce by spores which can spread quickly
• Viruses - damage

  • take over cell metabolism
  • insert viral genetic material into host DNA
  • uses host cell to make more viruses which then burst out of the cell
  • destroys the cell and viruses go on to infect more cells
• Protoctista - damage:

  • take over cell and digest its components
  • do not take over genetic material
  • break open and destroy cells as new generation emerges
• Fungi - damage:

  • digest and destroy living cells as food
  • symptoms stem from this process and body reponse
  • may also produce toxins which affect the host
• Bacteria - damage:

  • enter cells or live on surface
  • produce toxins that are a by-product of the normal functions of the bacterial cell
  • toxins cause:
  • damage to cell membranes
  • interference with host cell's genetic material
  • damage or inactivation of enzymes
• Plant defences against pathogens:

  • waxy cuticle
  • bark on trees
  • cellulose cell walls
  • sacrifice infected area by shutting it off
• When a plant is attacked:

  • receptors in the cell responds to molecules from the pathogens or to chemicals produced when the plant cell wall is attacked
  • this stimulates signaling molecules to be released (e.g. nitric oxide) which causes genes to be switched on in the nucleus . This then triggers cell responses such as:
  • produce defensive chemicals
  • produce chemicals that send an alarm signal to unaffected cells to trigger their defences
  • produce the polysaccharides callose and lignin to strengthen the cell wall
• Plant physical defences:

  • when a plant is attacked, it produces high levels of the polysaccharide callose
  • callose has beta-1,3 linkages and beta-1,6 linkages
  • callose is placed between the cell walls and the cell membranes of cells next to infected cells, acting as a barrier against the pathogen
  • lignin is added, and callose blocks sieve plates in the phloem to seal off the infected area
  • callose is deposited in the plasmodesmata, sealing infected cells off from healthy cells
• Plant Chemical Defences:

  • plants produce powerful chemicals to repel insect vectors or kill invading pathogens:
  • insect repellants: pine resin or citronella
  • insecticides: pyrethrin or caffeine
  • antibacterials: phenols, gossypol, defensins, lysosomes
  • antifungal compounds: phenols, gossypol, caffeine, saponins, chitinases
  • anti-oomycetes: glucanases
  • toxins: cyanides
• Non specific defences:

  • prevents the entry of pathogens
  • the skin has a flora of healthy microorganisms that outcompete pathogens for space
  • skin produces sebum, this is an oily substance that inhibits the growth of pathogens
• Mucous membranes:

  • e.g. found in the airways
  • secrete sticky mucus
  • microorganisms get stuck in the mucus
  • mucus contains lysozymes that destroy bacterial and fungal cell walls
  • mucus contains phagocytes
• Lysozymes/acid:

  • antimicrobial enzymes found in tears and urine
  • acid found in stomach; low ph denatures proteins
• Expulsive reflexes:

  • coughing and sneezing expels pathogen-laden mucus from airways
  • vomiting and diarrhoea expels contents of the gut
• Blood clotting:

  • platelets come into contact with collagen and adhere to surfaces
  • they release thromboplastin which catalyses prothrombin to turn into thrombin
  • thrombin catalyses fibrinogen to turn into fibrin which forms the clot
• Serotonin:

  • causes smooth muscle to contract, narrowing blood vessels and reducing blood supply to wounded area
• Scabbing:

  • once the clot has formed, it will dry out and prevent pathogens entering
  • epidermal cells begin to grow below the scab
  • blood vessels regrow
  • collagen fibres are deposited to give the tissue strength
  • the scab will slough off and the wound is healed
• Inflammatory response:

  • localised response to pathogens resulting in inflammation, characterised by pain, heat, redness and swelling
  • when tissues are damaged, mast cells are activated and release chemicals such as histamines and cytokines to attract phagocytes or make blood vessels dilate/leak
• Phagocytosis:

  • phagocyte attracted by chemicals produced by pathogen
  • phagocyte recognises pathogen as non-self and binds to it
  • phagocyte engulfs pathogen to form phagosome; lysosome moves towards phagosome and combines with it, forming phagolysosome
  • enzymes break down pathogen
  • digested pathogen absorbed by phagocyte; antigens combine with major histocompatibility complex in cytoplasm
  • MHC/antigen complex is displayed on phagocyte, making it an APC
• Fevers:

  • normal body temp: 37 degrees, controlled by hypothalamus
  • cytokines stimulate hypothalamus to increase body temperature
  • higher temperatures inhibit pathogen reproduction
  • specific immune system works faster
• Phagocytes:

  • build up at the site of infection, forming pus
  • neutrophils and macrophages
  • neutrophils do not form APCs but macrophages do
• Cytokines:

  • produced by phagocytes
  • act as cell-signalling molecules and inform other cells that the body is under attack
  • also stimulate specific immune response and increase body temperature
• Opsonins:

  • enhance phagocytosis by marking antigen for immune response
  • receptors on phagocytes will bind to common opsonins, helping the phagocyte engulf that pathogen
  • antibodies such as IgG and IgM are strong opsonins
• Antigens:

  • molecules found on the surface of all cells
  • body recognises difference between self-antigens and non-self antigens
  • toxins also act as antigens
• Antibodies:

  • Y shaped glycoproteins called immunglobulins
  • bind to specific antigens on pathogen/toxin
  • each one is specific to an antigen
  • can act as opsonins
  • made up of 4 identical polypeptide chains, two long (heavy) and two short (light)
  • held together by disulfide bridges
  • binding site made of 110 amino acids - variable region
  • lock and key mechanism
• Antibodies are produced by B plasma cells and can:

  • act as an opsonin
  • bind to antigens to prevent them entering other cells
  • act as agglutinins
  • act as anti-toxins
• T helper cells:

  • have CD4 receptors on membrane which bind to the antigens on APCs
  • produce interleukins which stimulate B cells to produce antibodies, other T cells to be produced, macrophages to ingest pathogens
• T killer cells:

  • destroy the pathogen with the antigen on its surface or infected body cells
  • use perforin to make holes in pathogenic membranes
• T memory cells:

  • part of immunological memory
  • divide rapidly when exposed to a pathogen to form T killer and T helper cells
• T regulator cells:

  • suppress the immune system
  • stop an immune response once a pathogen has been eliminated
  • ensure body recognises its own antigens and doesn't set up an autoimmune response
• B effector cells:

  • divide to produce plasma cells
• B Plasma cells:

  • produce antibodies for a specific pathogen and release them into circulation
  • only live for a few days but release 2000 antibodies per second
• B memory cells:

  • part of immunological memory
  • will recognise antigens they've seen before
  • allows for a rapid secondary immune response
• Cell-mediated immunity:

  • macrophages digest pathogen
  • antigens are processed and placed on macrophage surface
  • T helper cell complimentary to antigen binds to macrophage and becomes activated
  • activated t helper cell releases interleukins which stimulate other T cells to divide into memory cells, produce interleukins for phagocytosis, cause B plasma cells to divide, develop into T Killer Cells