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Parasites: Our Tiny Companions Ecology and symbiosis Grades 4 to 9 Objectives: To provide lively, personally relevant examples of symbiosis. To provide experience culturing bacteria, and testing common anti-bacterial agents. To introduce students to the importance of sterility in lab procedures. Overview I spent two summers working at Lawrence Berkeley National Labs with a microbiologist named Richard Schwartz through the Teacher Research Associates Program (TRAC). The research he was conducting on factors influencing cell growth seemed very difficult to present to my students, so I decided to focus on making a basic bacteria lab come to life, by providing a lively context for it. The background information which begins the lesson is intended for student use, and draws largely from an excellent little book called Furtive Fauna, by Roger Knutson, Penguin Books, 1993. The bacteria lab emphasizes sterile lab techniques, and is useful in showing students the importance of working carefully and following proper procedures. The other two activities offer students some other experiences with parasites, in relation to human evolution and in plants. Parasites: Our Tiny Companions Who are you taking along for the ride today? We, like all large animals, share our bodies' resources with many smaller forms of life. We are hosts. We often hear that living things are interconnected. In many cases this connection is indirect, such as our dependence on oxygen produced by plants. In some cases, however, these connections are real physical ties which link one organism to another. Some of these organisms only help themselves, and hurt their host. This is parasitism. Others help themselves, while not hurting or helping their host. This is called commensalism. Still others actually help themselves and their host. This is called mutualism. Let's do an inventory of some of our companions. Fleas: In the olden days, before frequent baths and visits to the laundromat, most people had fleas with them all the time. In fact, we humans have our very own flea, Pulex irritans, evolved just to live with us! Nowadays, we are more often bothered by fleas from our pet dog or cat, which are from a slightly different species. During the middle ages, it was fleas from rats which spread the plague in Europe, which killed one in three people. Fleas are very small insects. They hatch from eggs as tiny worm-like larvae, and live in your rug or carpet, eating dirt and dander (little flakes of skin you shed.) When they have grown a bit, they spin a tiny cocoon, and transform into a flea, just like a caterpillar turning into a butterfly! Then they wait until they sense the heat or vibrations of a passing host, and they hop for it. When they bite, they inject a little saliva into us to keep the blood from clotting. This causes us to itch. Since they steal our blood and are a pain, they are parasites. Mosquitos: Not quite a parasite, since they don't really live with us. But they still manage to steal our blood. They are, like the flea, insects (Their name means "little fly" in Spanish). They lay their eggs in standing water, where you can see their larvae wriggling around. When they hatch, they leave the water and fly around looking for a host like you! The worst thing about mosquitos is they sometimes carry even smaller parasites in their gut. In Africa, the Anopheles mosquito carries a protozoan which causes a disease called malaria. This protozoan is a one-celled organism, which lives part of its life in mosquitos, and part in humans. This disease kills thousands of people each year. But people who have sickle cell anemia do not suffer from malaria. If they just have one gene for sickle cell they are protected. People without the gene get sick and die from malaria. Those with the gene survive and reproduce, which explains why it is widespread among people from West Africa, where malaria is a big killer. Mites: These tiny little creatures are related to spiders. They are smaller than the period on this page. Some of them, called follicle mites, live head down in the follicles at the base of our eyebrows. They seem to be harmless. Lice: These little insects come in three varieties. Head lice enjoy the dry forest of your scalp, where they eat your skin. Body lice like your armpits. Pubic lice enjoy the moist jungle of your pubic area. They are not caused by being dirty. They are spread by contact, which is why kids get them in school. Pubic lice are larger than head lice, and they have big claws which they use to hang on to hairs with. This gives them their common name, crabs. Bacteria: Now we are really getting tiny. You could lay 800,000 of them from end to end across your palm. They can only be seen in the most powerful microscopes. They live both in and on you! There are as many on your body as the number of people who ever lived! Most are harmless, therefore commensal. One type, called LACTOBACILLUS, actually helps you, since it digests milk. These are MUTUALISTIC. Some are annoying. They grow well with warmth and moisture, so when you sweat they multiply. As they process their food, which is your sweat, they release odor. A few, such as staphyllococcus (staph for short), can get under your skin and cause infections. The overuse of antibiotics is leading to the development of resistant strains of these bacteria, which are very hard to kill. Tooth amoeba: This little creature actually helps you by eating up the bacteria which grow in your mouth. Though they only have one cell, they are much bigger than the tiny bacteria. They reproduce by dividing in half.   Parasite Activities Bacteria All Around Us Objectives: To allow students to grow and actually see bacteria colonies. To introduce sterile laboratory techniques. To make students aware of the widespread presence of bacteria. To test common antibacterial agents. Materials: 2 covered agar plates per group. 8 inoculating loops. 8 alcohol burners. lysol, pinesol, anti-bacterial soap and/or other anti bacterial agent. Preparation and procedure If you don't know how to prepare nutrient agar plates, consult a lab book. Basically powdered agar is added to boiling water, then poured into the agar plates, which are quickly covered. For minimal contamination this should be done wearing gloves, and in a sterile hood. They should then be autoclaved. Since most schools, including mine, lack these facilities, just work quickly, hope for the best, and explain to your students your breach of proper lab procedures. Activity One: Bacteria, Bacteria, How Can We See You If You Are So Small? Have students work in groups of from two to four. Each group should receive two agar plates. Have a class discussion about where bacteria might be found, in their environment, and on their body. They should decide where on their body or in the room they would like to test for the presence of bacteria. Then they should sterilize the inoculating loop in flame, allow it to cool, and drag it across the surface they wish to test. You mightinstruct a student to scrub their hands as well as they could, then take a sample fromunder their fingernails -- almost always a source of bacteria.The loop should then be quickly dragged across the surface of the agar, which should be exposeded and covered as quickly as possible, to avoid airborne contamination. They should only put one sample on each agar plate, to avoid confusion. The second agar plate is left shut as a control. The bacteria should grow, if present, within 48 hours of inoculation. It will grow faster at 98 degrees F, but will grow just fine at room temperature. (Normal lab cultures are incubated at around 98 degrees F). You will get white and/or orange circles spreading on the plate, each of which began as a single bacteria. (They are, in effect, clones of the priginal.) In order to be visible, millions of bacteria must be present. Many of the bacteria produce smelly gases, which explains body odor, and the smell coming from swamps and stagnant ponds. If you find fuzzy colonies, sprouting spores, you have cultivated mold. Remember that the discovery of penicillin resulted from such contamination on the agar plate of Alexander Fleming, and his observation that the mold killed the bacteria he was actually trying to grow. Students should not become overly phobic about germs or bacteria. Most of them are harmless. Even if ingested on our food, they are usually digested easily. Problems arise when bacteria are allowed to multiply, for example, on contaminated meat. Then, if the bacteria are not killed by thorough cooking, they can cause food poisoning. This is a problem with salmonella bacteria on chicken and eggs. Recent studies have shown most chicken to be somewhat contaminated with bacteria, so it should always be cooked thoroughly. This is also why it is important to thoroughly clean a cutting board which has been used for meat. If salad is cut on the same board, the vegetables may become contaminated, and they may be eaten raw. For the same reason, hands should be washed before cooking and eating. Hands should also be washed regularly throughout the day, as most viruses are passed from one person to another on their hands. The hands then often transport the virus into the body through the eyes, which are rubbed more often than we realize. Activity Two: Now that we have found you, how can we make you go away? Now that your students are thoroughly disgusted with their filthy surroundings, you may interest them in testing antibacterial agents, such as Lysol. Make up agar plates as you did before. Inoculate them with known sources of bacteria (perhaps even thecolonies from their previous experiment). Then treat them with dilute solutions of Lysol, Pinesol, regular dish detergent, anti-bacterial hand soap. Have each group test a different antibacterial. Each group should use the same source of bacteria, so results can be compared. Each group should likewise do a control with no antibacterial agent. Observe for 72 hours. Note: Overuse of antibiotics is leading to the development of strains of bacteria which are completely resistant to them. The miracle of penicillin may expire within our lifetimes. As students have observed, many of generations of bacteria can grown in a fairly short time, allowing for rapid evolution. When antibiotics are used, bacteria which are resistant survive and reproduce. There are now certain bacterial infections that can only be treated by surgical removal of the infected flesh. Resistant strains of tuberculosis (TB) are making that disease a threat once more as well. Additional questions for students suggested by a Brazilian colleague, Wanderly Carvalho: "Is it possible to calculate the number of colonies in each agar plate ?" "What sort of factors interfere in the colonies growth? " "Did the antibacterial agents used in the test worked well? If not, what kind of troubles can they bring to us?" "Is the food we eat also good for bacteria and fungus?" Extensions and Resources: Have students calculate the number of bacteria they have grown in an hour if they divide every five minutes. Try two, four, and 24 hours. Point out the factors which limit bacterial growth in nature, however, such as limited food, self-pollution, etc. The Videodiscovery videodisc, Science Sleuths, has a simulation activity called the BiogenePicnic, which leads students to discover the source of contamination at a company picnic. For a good, short reading on Fleming's discovery of penicillin, see Marvels of Science, 50 Fascinating 5 minute Reads, by Kendall Haven, published in 1994, by Libraries Unlimited, Englewood, CO Trials of Life, Living Together: A vivid, often disgusting portrait of parasitism and symbiosis can be found in this episode of David Attenborough's series Lifesense: Another video series which addresses parasites, which aired on the Arts and Entertainment channel a few years ago. It traces the coevolution of a variety of plant and animal species, which have adapted to living with human beings. Sickle Cell Anemia Sickle Cell Anemia is an important topic in my school as many students are of African descent, and many carry the sickle cell gene. Every year, there are several students in the school who suffer from the disease. Sickle cell anemia is a disease which results from misshapen red blood cells. These cells do not carry as much oxygen as normal red blood cells, so sufferers experience shortness of breath, fatigue, and muscle and bone pain. They also experience periodic episodes of extreme pain, which can require hospitalization. Life expectancy is shorter than normal. The disease occurs when a person has two recessive genes for it. If you have only one gene, you are a carrier, and could pass the disease on to another. The gene can be detected through a simple blood test before two people decide to have children. If two carriers of the gene have a child, the chances are one in four the child will get both recessive genes and suffer from the disease. What does this have to do with parasites? Sickle cell actually survived in West Africa because of a parasite! Research has shown that people who have one or both genes for sickle cell do not perish from malaria. Malaria is a tropical disease caused by a protozoan carried by the Anopheles mosquito. Since malaria kills many people, the gene for sickle cell provides an actual advantage for people in this region, which explains why it remains widespread there. Naturally, this advantage does not serve African American students here, so I encourage them to get their blood screened and find out if they are carriers before they have children. Looking at Sickle Cells Get slides of sickled blood cells from a biological supply house to examine microscopically. Sickle cells are crescent shaped, rather than the smooth discs of normal red blood cells. This results in them not carrying all the oxygen they should, and also in getting caught in capillaries and not flowing smoothly. Sickle Cell Punnett Squares In the context of studying Mendelian genetics, do Punnett squares to predict the results of various genotypes.The square at the right illustrates the result of a cross of two carriers. What would be the outcome of a cross between a carrier and someone with both recessive genes? You Have a Lot of Gall! Parasitism is not limited to humans, or even to animals. Anywhere in nature there is food, animals and plants will try to exploit it. Galls are growths on plants which are the of an insect laying its eggs there. The larvae hatch and burrow into the plant. Once there, they irritate the plant, which tries to insulate itself by forming the gall, a sort of scar tissue. The larvae feed inside the gall, undergo metamorphosis, and emerge as an adult, to start the process all over again. The life cycle of a fly, Eurosta solidaginus, is typical. A female lays her eggs on the Canada goldenrod in the spring. In June the eggs hatch, and the young bore through the stem's outer wall, whereupon a ball-shaped gall forms around them. In the fall, the larvae make a tunnel through to the outermost layer of plant tissue to provide a route for its departure as an adult, then retreats back to the center of the gall. Protected through winter, it remains in its larval stage until the spring, when it is time to pupate. When the pupal stage is complete in late spring, a balloon-like bladder forms on the front of the fly's head, which it inflates to push the front off the pupa case. It then crawls through the exit tunnel and batters through the final barrier of plant tissue by inflating and deflating the balloon, which is absorbed afterward into the insect's head. The adult flies soon mate, and the cycle begins again. Without the gall to protect the larvae over the winter, the species would not survive. Galls result from parasitism, as the insects are diverting the host plant's food and energy. They are not usually very harmful, though. If the insect killed the host plant, both would perish. Most parasites take just enough food so that their host is weakened, but not seriously hurt. Many different plants have galls. Oaks are especially prone to them. Of the 2,000 types of galls found in North America, 800 are found on oaks. Willows and poplars also have galls. Some galls are found in the stems, others on the leaves. Gall Dissection Obtain a variety of galls from various plants. Give each group of students a selection of galls. They should begin by observing and sketching the galls. Have them look for entry and exit holes. Then open the gall with a sharp knife or saw. If possible, refer to insect guides to classify the inhabitants. Links: Sickle Cell Defiers: A Website for people with Sickle Cell Insect Resources: Gordon's Entomological Page That Gunk on Your Car This lesson is made available for the educational use of individual teachers. All rights are reserved. If you use this lesson, drop me a line with your feedback! Thanks.