WJEC Biology for A2: Student Bk

YOU SHOULD KNOW › › › ››› That the human body acts as a host for other organisms ››› Examples of diseases caused by viruses, bacteria and protoctista ››› The characteristics of infection by microbes causing cholera, TB, smallpox, influenza and malaria ››› The treatment of cholera, TB, smallpox, influenza and malaria Key Terms Infection : A transmissible disease often acquired by inhalation, ingestion or physical contact. Endemic : Disease occurring frequently, at a predictable rate, in a specific location or population. Carrier : An infected person, or other organism, showing no symptoms but able to infect others. Disease reservoir : The long-term host of a pathogen, with few or no symptoms, always a potential source of disease outbreak. St u dy point Many organisms live in or on the body in a mutualistic or parasitic relationship with us. They may cause disease or defend us from disease. St u dy point The word ‘mutualism’ refers to organisms of two species that live together to their mutual advantage. Sometimes the word ‘symbiosis’ is used. Disease The body as a host It has been said that we should not think of ourselves as individuals, but as colonies. That is because although we have 10 13 cells in our bodies, we have at least that number of other individual organisms living in or on the body. They may be internal, living in our cells, tissue fluid and our guts, or external, on our skin and hair and at body openings. They include microbes, such as fungi, protoctista and over 1000 bacterial species in the gut flora. We may carry other larger parasites, including insects, such as fleas, or worms, such as tapeworms. Many of these organisms are parasites and have the potential to cause disease if, for example, they secrete toxins, if their populations increase too much or if they are transferred to an inappropriate part of the body. But many contribute to our general health and with those we have mutualistic relationships. ▪ The bacteria E. coli in the large intestine synthesise vitamin K, but in the stomach and small intestine, they can cause gastro-intestinal disease. ▪ The mites in the hair follicles of our eyelashes eat dead cells. They stick to eye make- up, so if you don’t remove your mascara, you risk their population building up too much and causing inflammation. ▪ Entamoeba is the protoctistan that grazes on the dead cells of our gums. If you don’t brush your gums enough when you clean your teeth, they will reproduce in large numbers and cause gingivitis. The bacteria and fungi that colonise our skin prevent the build-up of harmful microbes, as do many of the gut flora. If these useful organisms are removed, for example when antibiotics are used to kill bacteria, then other organisms, such as yeast, increase in number and cause disease. There is an ecological balance in a healthy body, in which the many different organisms keep each other in check. Important diseases All groups of organisms suffer from infection and disease. Even bacteria can be infected by viruses. Susceptibility to pathogens is a property of life, as one organism provides a habitat for another. Some microbial diseases with significant social and medical consequences are described here. Cholera ▪ Cholera is caused by the Gram-negative, comma- shaped bacterium Vibrio cholerae. It can only reproduce when it is inside its human host. ▪ Cholera is endemic in parts of the world. People become infected through contaminated food or water. They become carriers and, acting as reservoirs of disease, can contaminate other water supplies and spread the disease. Vibrio cholerae 261 Option A: Immunology and disease