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Challenging misconceptions about microbes Introduction
Introduction This misuse of terminology is not confined to students. Common, everyday reference to pathogens by adults usually fails to distinguish between the two. This leads to a number of problems, both in school and beyond. For instance, the fact that antibiotics act on bacteria and not viruses is not widely understood. This sometimes leads to concern when antibiotics are not prescribed by a doctor for an illness caused by a virus (such as the common cold, or influenza). Building models of bacteria and viruses can help to address this misconception. Students will realise that the two are not the same. A more difficult concept, and one that is unclear in many text books, is the notion that viruses are not living things. While showing the characteristic life-process of reproduction (when assisted by another cell), they do not do anything else - they do not respire, feed, move, excrete or sense their environment. So are they alive? And if not, why do they contain DNA? Older theories about the origin of decay and infection include ideas such as spontaneous generation. In certain conditions, perhaps if food is left for a period in warm, moist conditions, it will start to rot. With certain foods (for example, meat), maggots may ultimately be observed in it. Spontaneous generation would merely require these conditions for microorganisms to appear from 'nowhere'. It was once thought that this could also explain the appearance of the maggots. Students often have a similar conception of the origin of 'germs' in rotting material, (although perhaps nowadays it is commonly understood by them where the maggots come from). Students are likely to have met the experiment based on Pasteur's discovery of microorganisms as the cause of food spoilage. Pasteur worked on the causes of wine going sour. He discovered that if the wine was in contact with the atmosphere by way of a tube with a deep 'S' bend (which prevented the settling of microbe-laden dust on the surface), then spoilage did not happen. There is a simple, school laboratory version of this experiment that allows students to discover that only when the liquid with direct 'vertical' contact with the air does spoilage take place. Whilst this may help to displace the notion of microorganisms (or 'germs') arising spontaneously in 'dirty' places, further work at 14-16 is likely to be necessary. A related topic that can help here is the study of disease. Again, this is likely to have been met earlier. Students are likely to have already learned that specific microorganisms are responsible for certain diseases. At 14-16 this can be reinforced by a study of how diseases can be transmitted. The UK foot and mouth disease epidemic of spring 2001 can be used here as a context. Students could look at the precautions taken – particularly the restrictions on livestock movement and access to farms. Students could trace the story of the outbreak – from contaminated meat for human consumption, to pig swill (made from waste human food), to an infected pig (or pigs), to a truck load of pigs arriving at an abattoir. From here, visiting lorries (some taking animals away from the abattoir) picked up the infection and carried it to various parts of the country. Human carriers, as well as airborne infection, then played a part, with devastating results. Following this through, preferably with news cuttings or web references, will help to reinforce the idea that infectious diseases require a specific pathogen. Further reinforcement can come through teaching about the body's defences against disease. Active immunity (antigen-antibody interaction), vaccination and antibiotic use all require an understanding of the specificity of pathogens, and their link with particular diseases. A further misconception that tends to persist into adulthood is the idea that all bacteria are harmful. Again, this will perhaps have been dealt with earlier. Specific contexts to address this misconception at 14-16 include the following:
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