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| Question | Answer |
| Pathogens are microorganisms - such as bacteria and viruses - that cause disease. Bacteria release toxins, and viruses damage our cells. White blood cells can ingest and destroy pathogens by producing antibodies that destroy the infectious microorganisms, and antitoxins to neutralise toxins created as a by-product. | Bacteria and viruses are the main types of pathogen. Bacteria are microscopic organisms. They come in many shapes and sizes, but even the largest are only 10 micrometres long - that's 10 millionths of a metre. Bacteria are living cells and, in favourable conditions, can multiply rapidly. Once inside the body, they release poisons or toxins that make us feel ill. Diseases caused by bacteria include: food poisoning cholera typhoid whooping cough. |
| Viruses are many times smaller than bacteria. They are among the smallest organisms known and consist of a fragment of genetic material inside a protective protein coat. spherical shaped virus showing a cross-section through the core A hepatitis C virus showing DNA enclosed in a protein coat. Viruses can only reproduce inside host cells and they damage the cell when they do this. A virus can get inside a cell and, once there, take over and make hundreds of thousands of copies of itself. Eventually the virus copies fill the whole host cell and burst it open. The viruses are then passed out in the bloodstream, the airways, or by other routes. Diseases caused by viruses include: influenza (flu) colds measles mumps rubella chicken pox AIDs. | White blood cells can: ingest pathogens and destroy them produce antibodies to destroy particular pathogens produce antitoxins that counteract the toxins released by pathogens. In a written examination, it is easy to get carried away with metaphors about invaders and battles: stick to the point. Note that: the pathogens are not the disease - they cause the disease white blood cells do not eat the pathogens - they ingest them antibodies and antitoxins are not living things - they are specialised proteins. |
| Pathogens contain certain chemicals that are foreign to the body. These chemicals are called antigens. Certain white blood cells, called lymphocytes, can produce specific antibodies to kill a particular pathogen. Antibodies Antibodies are proteins. They can neutralise pathogens in a number of ways. For example, they can: bind to pathogens and damage or destroy them coat pathogens, clumping them together so that they are easily ingested by white blood cells called phagocytes. Each lymphocyte produces a specific type of antibody - a protein that has a chemical 'fit' to a certain antigen. When a lymphocyte with the appropriate antibody meets the antigen, the lymphocyte reproduces quickly and makes many copies of the antibody to kill the pathogen. | Vaccination causes the body to produce enough white blood cells to protect itself against a pathogen. Antibiotics are effective against bacteria, but not against viruses. Some strains of bacteria are resistant to antibiotics. So people can be immunised against a pathogen through vaccination. Different vaccines are needed for different pathogens. For example, the MMR vaccine is used to protect children against measles, mumps and rubella (German measles). Vaccination involves putting a small amount of an inactive form of a pathogen, or dead pathogen, into the body. Vaccines can contain: live pathogens treated to make them harmless harmless fragments of the pathogen dead pathogens. These all act as antigens. When injected into the body, they stimulate white blood cells to produce antibodies against the pathogen. If the person does get infected by the pathogen later, their body can respond in the same way as if they had had the disease before. If a large proportion of the population is immune to a particular pathogen, the spread of that pathogen is greatly reduced. |
| One simple way to reduce the risk of infection is to maintain personal hygiene and to keep hospitals clean. In the 19th century, Ignaz Semmelweiss realised the importance of cleanliness in hospitals. Semmelweiss insisted that doctors should wash their hands before examining patients, something that was not common at the time. This policy greatly reduced the number of deaths from infectious diseases in his hospital. Unfortunately, although his ideas were successful, they were ignored at the time because people did not know that diseases were caused by pathogens that could be killed. | Painkillers helps to relieve the symptoms of an infectious disease, but they do not kill the pathogens involved. For example, paracetamol, aspirin and morphine block nerve impulses from the painful part of the body, or block nerve impulses travelling to the part of the brain responsible for perceiving pain. |
| Antibiotics are substances that kill bacteria or stop their growth. They do not work against viruses because they live and reproduce inside cells. It is difficult to develop drugs that kill viruses without also damaging the body’s tissues. Penicillin, the first antibiotic, was discovered in 1928 by Alexander Fleming. He noticed that some bacteria he had left in a Petri dish had been killed by naturally occurring penicillium mould. Since the discovery of penicillin, many other antibiotics have been discovered and developed. Different antibiotics work in different ways. It is important that specific bacteria should be treated using specific antibiotics. | penicillin- breaks down cell walls erythromycin- stops protein synthesis ciprofloxacin- stops DNA replication |
| Bacterial strains can develop resistance to antibiotics. This happens because of natural selection. In a large population of bacteria, there may be some cells that are not affected by the antibiotic. These cells survive and reproduce, producing even more bacteria that are not affected by the antibiotic. | MRSA is the acronym for 'methicillin-resistant Staphylococcus aureus'. It's very dangerous because it's a strain of bacterium that is resistant to most antibiotics. To slow down or stop the development of other strains of resistant bacteria, we should always: avoid the unnecessary use of antibiotics complete the full course. |
| The appearance of resistant strains of bacteria means that vaccinations and antibiotics may no longer work. As people are not immune to it, and there is no effective treatment, a resistant strain will spread rapidly. New antibiotics must be developed as a result. | Development of resistance - Higher tier The main steps in the development of resistance are: antibiotics kill individual pathogens of the non-resistant strain resistant individual pathogens survive and reproduce the population of the resistant pathogens increases. The rate of development of resistant strains of bacteria can be slowed down. One way to do this is to avoid using antibiotics for infections that are not serious, such as mild throat infections. |
| The action of antibiotics and disinfectants can be investigated using cultures of microorganisms (populations of microorganisms that have been grown for a purpose). It is important that the cultures are uncontaminated by other microorganisms, so sterile conditions are needed: the Petri dishes, nutrient agar jelly and other culture media must be sterilised the inoculating loops used to transfer microorganisms must be sterilised (usually by passing the metal loop through a Bunsen burner flame) the lid of the Petri dish is sealed with sticky tape to stop microorganisms from the air getting in and contaminating the culture. | Safety in the lab Bacteria grow and reproduce more quickly when they are warm than when they are cold. It would be dangerous to incubate (keep and grow) cultures at temperatures close to body temperature (37°C) because doing so might allow the growth of pathogens harmful to health. So the maximum temperature used in school and college labs is 25°C. However, higher temperatures can be used industrially, and these produce faster growth. |
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