INFECTION AND RESPONSE

• Pathogens


  Microorganisms that cause infectious disease
• Types of pathogens


  • Viruses
  • Bacteria
  • Protists
  • Fungi
• Pathogens may infect plants or animals and can be spread by direct contact, by water or by air
• Bacteria and viruses may reproduce rapidly inside the body
• Bacteria


  May produce poisons (toxins) that damage tissues and make us feel ill
• Viruses


  Live and reproduce inside cells, causing cell damage
• Diseases caused by viruses, bacteria, protists and fungi can be spread in animals and plants
• The spread of diseases can be reduced or prevented
• Students should be able to explain how diseases caused by viruses, bacteria, protists and fungi are spread in animals and plants
• Students should be able to explain how the spread of diseases can be reduced or prevented
• Pathogens


  Microorganisms that cause infectious disease. They may be viruses, bacteria, protists or fungi. They may infect plants or animals and can be spread by direct contact, by water or by air
• Bacteria may produce poisons (toxins) that damage tissues and make us feel ill
• Viruses live and reproduce inside cells, causing cell damage
• Measles

  A viral disease showing symptoms of fever and a red skin rash. It is a serious illness that can be fatal if complications arise. The measles virus is spread by inhalation of droplets from sneezes and coughs
• HIV

  Initially causes a flu-like illness. Unless successfully controlled with antiretroviral drugs the virus attacks the body's immune cells. Late stage HIV infection, or AIDS, occurs when the body's immune system becomes so badly damaged it can no longer deal with other infections or cancers. HIV is spread by sexual contact or exchange of body fluids such as blood which occurs when drug users share needles
• Tobacco mosaic virus (TMV)


  A widespread plant pathogen affecting many species of plants including tomatoes. It gives a distinctive 'mosaic' pattern of discolouration on the leaves which affects the growth of the plant due to lack of photosynthesis
• Salmonella food poisoning


  Spread by bacteria ingested in food, or on food prepared in unhygienic conditions. In the UK, poultry are vaccinated against Salmonella to control the spread. Fever, abdominal cramps, vomiting and diarrhoea are caused by the bacteria and the toxins they secrete
• Gonorrhoea


  A sexually transmitted disease (STD) with symptoms of a thick yellow or green discharge from the vagina or penis and pain on urinating. It is caused by a bacterium and was easily treated with the antibiotic penicillin until many resistant strains appeared. Gonorrhoea is spread by sexual contact. The spread can be controlled by treatment with antibiotics or the use of a barrier method of contraception such as a condom
• Rose black spot


  A fungal disease where purple or black spots develop on leaves, which often turn yellow and drop early. It affects the growth of the plant as photosynthesis is reduced. It is spread in the environment by water or wind. Rose black spot can be treated by using fungicides and/or removing and destroying the affected leaves
• Malaria


  The pathogens that cause malaria are protists. The malarial protist has a life cycle that includes the mosquito. Malaria causes recurrent episodes of fever and can be fatal. The spread of malaria is controlled by preventing the vectors, mosquitos, from breeding and by using mosquito nets to avoid being bitten
• Non-specific human defence systems against pathogens


  • Skin
  • Nose
  • Trachea and bronchi
  • Stomach
• Immune system


  Tries to destroy pathogens that enter the body
• How white blood cells help defend against pathogens

  • Phagocytosis
  • Antibody production
  • Antitoxin production
• Vaccination


  Involves introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate the white blood cells to produce antibodies. If the same pathogen re-enters the body the white blood cells respond quickly to produce the correct antibodies, preventing infection
• Antibiotics


  Medicines that help to cure bacterial disease by killing infective bacteria inside the body. It is important that specific bacteria should be treated by specific antibiotics
• The use of antibiotics has greatly reduced deaths from infectious bacterial diseases. However, the emergence of strains resistant to antibiotics is of great concern
• Antibiotics cannot kill viral pathogens
• Painkillers and other medicines are used to treat the symptoms of disease but do not kill pathogens
• It is difficult to develop drugs that kill viruses without also damaging the body's tissues
• Discovery and development of potential new medicines

  1. Preclinical testing using cells, tissues and live animals

  2. Clinical trials using healthy volunteers and patients

  3. Very low doses of the drug given at the start of the clinical trial

  4. If the drug is found to be safe, further clinical trials are carried out to find the optimum dose

  5. In double blind trials, some patients are given a placebo
• Monoclonal antibodies


  Produced from a single clone of cells. The antibodies are specific to one binding site on one protein antigen and so are able to target a specific chemical or specific cells in the body
• Uses of monoclonal antibodies

  • Diagnosis such as in pregnancy tests
  • In laboratories to measure the levels of hormones and other chemicals in blood, or to detect pathogens
  • In research to locate or identify specific molecules in a cell or tissue by binding to them with a fluorescent dye
  • To treat some diseases: for cancer the monoclonal antibody can be bound to a radioactive substance, a toxic drug or a chemical which stops cells growing and dividing. It delivers the substance to the cancer cells without harming other cells in the body
• Monoclonal antibodies create more side effects than expected. They are not yet as widely used as everyone hoped when they were first developed
• Methods to detect plant diseases

  • Stunted growth
  • Spots on leaves
  • Areas of decay (rot)
  • Growths
  • Malformed stems or leaves
  • Discolouration
  • The presence of pests
• Methods to identify plant diseases

  • Reference to a gardening manual or website
  • Taking infected plants to a laboratory to identify the pathogen
  • Using testing kits that contain monoclonal antibodies
• Plant pathogens


  • Viral
  • Bacterial
  • Fungal
  • Insects
• Plant ion deficiencies


  • Stunted growth caused by nitrate deficiency
  • Chlorosis caused by magnesium deficiency
• Physical plant defence responses

  • Cellulose cell walls
  • Tough waxy cuticle on leaves
  • Layers of dead cells around stems (bark on trees) which fall off
• Chemical plant defence responses

  • Antibacterial chemicals
  • Poisons to deter herbivores
• Mechanical plant adaptations


  • Thorns and hairs deter animals
  • Leaves which droop or curl when touched
  • Mimicry to trick animals