Infection and Response

• Pathogens


  Microorganisms such as viruses and bacteria that cause infectious diseases in animals and plants
• Pathogens


  • They depend on their host to provide the conditions and nutrients that they need to grow and reproduce
  • They frequently produce toxins that damage tissues and make us feel ill
• We can avoid diseases by reducing contact with pathogens
• Body's defences against pathogens

  • Barriers
  • Immune system is triggered which is usually strong enough to destroy the pathogen and prevent disease
• Vaccination


  Enhancing the body's natural system to protect against unusual or dangerous diseases
• Since the 1940s a range of antibiotics have been developed which have proved successful against a number of lethal diseases caused by bacteria
• Many groups of bacteria have now become resistant to these antibiotics
• The race is now on to develop a new set of antibiotics
• Communicable (infectious) diseases


  Diseases caused by viruses, bacteria, protists and fungi that are spread in animals and plants
• Pathogens


  Microorganisms that cause infectious disease, including viruses, bacteria, protists and fungi
• Pathogens


  • They may infect plants or animals and can be spread by direct contact, by water or by air
• Bacteria and viruses


  • They 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
• Measles 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, and 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, giving 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, causing fever, abdominal cramps, vomiting and diarrhoea due to the bacteria and the toxins they secrete
• In the UK, poultry are vaccinated against Salmonella to control the spread
• Gonorrhoea


  A sexually transmitted disease (STD) with symptoms of a thick yellow or green discharge from the penis and pain on urinating, caused by a bacterium that was easily treated with the antibiotic penicillin until many resistant strains appeared
• Gonorrhoea is spread by sexual contact
• Rose black spot


  A fungal disease where purple or black spots develop on leaves, which often turn yellow and drop early, affecting the growth of the plant as photosynthesis is reduced, 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


  A disease caused by pathogens that are protists, with a life cycle that includes the mosquito, causing 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
  • Trachea and lungs produce mucus
  • Blood clots
  • Stomach secretes hydrochloric acid
  • Ear wax
  • Tears
  • Nostril hairs
  • Eyelashes
• White blood cells


  • Ingest pathogens
  • Produce antibodies to destroy particular bacteria or viruses
  • Produce antitoxins to counteract toxins released by pathogens
• Vaccination


  Introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate the white blood cells to produce antibodies, so that if the same pathogen re-enters the body the white blood cells respond quickly to produce the correct antibodies, preventing infection
• Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body
• The use of antibiotics has greatly reduced deaths from infectious bacterial diseases
• The emergence of strains resistant to antibiotics is of great concern
• Doctors should only prescribe antibiotics when necessary - and not for viruses
• It is important that if you are prescribed antibiotics you take the whole course, otherwise a few bacteria inside your body will reproduce, increasing the chance of some developing resistance
• Painkillers and other medicines are used to treat the symptoms of disease but do not kill pathogens
• Development of antibiotic resistance

  1. Antibiotics kill individual pathogens of the non-resistant strain

  2. Individual resistant pathogens survive and reproduce, so the population of the resistant strain increases

  3. Now, antibiotics are no longer used to treat non-serious infections, such as mild throat infections, so that the rate of development of resistant strains is slowed down
• Discovery and development of drugs

  1. Traditionally drugs were extracted from plants and microorganisms

  2. Most new drugs are synthesised by chemists in the pharmaceutical industry, but the starting point may still be a chemical extracted from a plant

  3. New medical drugs have to be tested and trialled before being used to check that they are safe and effective

  4. Preclinical testing is done in a laboratory using cells, tissues and live animals

  5. Clinical trials use healthy volunteers and patients, starting with very low doses and increasing if the drug is found to be safe
• In double blind trials, some patients are given a placebo