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Learn about BacteriaTable of Contents
Teaching Tip: Bacteria Introduction Bacteria is the name given to the group of single-celled microorganisms that do not have chlorophyll and do not have a distinct cell nucleus enclosed by a membrane. Instead the nuclear material (a single strand of DNA) is folded and clumped in the interior of the cell. Microorganisms that do not have a distinct nuclear membrane are called prokaryotic organisms. Bacteria are classified in the kingdom Monera. Bacteria reproduce most commonly by binary fission where a single parent bacterium divides to form two independent bacteria. This type of reproduction is called asexual because there is no exchange or combination of nuclear material between two organisms. Fission occurs rapidly in as little as 20 minutes. Under perfect conditions a single bacterium could grow into over one billion bacteria in only 10 hours! Some bacteria can also reproduce asexually by forming thick-walled endospores that are very resistant to conditions of extended heat, cold, or dryness. An endospore is formed within the cell body of a bacterium. Usually a bacterium forms only one endospore and that endospore will produce only a single bacterium. Endospores are difficult to kill except by strong chemicals or high heat. Bacteria are classified by their shape. There are three basic bacteria shapes: sphere, rod, and spiral. The spherical bacteria are round or oval in shape. The rod bacteria are longer in length and look like a rectangle or long oval. The spiral bacteria have long bodies with a twist that forms a spiral pattern when connected to each other. Bacteria grow in colonies, groups of thousands of individual bacterium. Different colonies can be identified by their shape, texture, and color. Teaching Tip: Anti-Bacterial "Agents" Some types of bacteria cause disease and sickness. These kinds of bacteria are called pathogens. They reproduce very rapidly, like all bacteria. How do people stop bacteria from growing and spreading? They control it in two ways: by killing the bacteria cells, and by stopping the bacteria from reproducing. An agent is a solution or method which either kills or stops reproduction. Bactericides are agents that kill bacteria cells. Static agents inhibit cell growth and reproduction. There are a variety of ways to kill bacteria or keep it from reproducing. Physical methods: Sterilization. The application of heat to kill bacteria. Includes incineration (burning), boiling, and cooking. Pasteurization. The use of mild heat to reduce the number of bacteria in a food. Cold temperatures. Refrigeration and freezing are two of the most common methods used in homes, for preserving food's life span. Chemical methods: Antiseptics. These agents can be applied directly to living tissues, including human skin. Disinfectants. These agents are not safe for live tissues. Disinfectants are used to clean toilets, sinks, floors, etc. Preservatives. These are used in almost every processed food that is available today. They inhibit bacterial growth in foods. For a project, consider counting the different preservatives listed in the ingredients of packaged foods. This might be a good project to do if your children go to the grocery store with you! Keep a list of the different preservatives, and keep track of which ones are used most often. Some food preservatives are:
Antibiotics. These kill the bacteria cells that are inside the body, without harming normal cells. Antibiotics often are able to cure once-fatal diseases, such as scarlet fever. However, they can kill good bacteria along with the bad.
If you have 2-3 petri dishes and some agar nutrient, you can study the effects of some disinfectants. Follow the instructions on your agar bottle to prepare an agar solution. Cover the bottom of the petri dishes with the agar solution. Store the dishes upside down in your refrigerator until you are ready to use them. Decide on a location either indoors or outdoors where you wish to study the bacteria in the air. Take the covers off of the culture dishes and set the open dishes in the location you have selected. Leave the culture dishes exposed for about an hour and then cover them. Label the culture dish covers appropriately, such as "Garden Air 1" and "Garden Air 2". Cut small squares of paper (blotter paper works well), label them (i.e. "I" for iodine, "A" for alcohol, etc.), and soak each in a different household chemical you wish to test for anti-bacterial properties. If you have time, you might also experiment with natural antibacterial agents, such as tea tree oil or red pepper. Wipe off any excess liquid and set each of the squares on a different spot in one of the culture dishes with tweezers. The second culture dish is your "control". It will show you what an air bacteria culture looks like without any household chemicals. Store the dishes in a dark place like a closet where they will be undisturbed for a few days. After 3-7 days take both culture dishes and carefully observe the bacteria growth in each dish, leaving the covers on. Take notes of your observations and make drawings. Some of the questions to answer in your observations are: In the control culture, how many bacteria colonies do you see? Do any colonies have a different appearance, texture, or color, indicating a different type of bacteria? How much of the dish is covered with bacteria? In the sensitivity square test culture, how many bacteria colonies do you see? Have the bacteria covered this dish to the same extent as the control culture? What effect have each of the chemicals had on the bacteria growth? Did a particular chemical kill the bacteria or just inhibit its growth? Project: Good Bacteria Generally when people think of "bacteria", they think of harmful germs. However, not all forms of bacteria are bad! "Good" bacteria are an essential aid to our digestion process, and organic materials such as dead trees would not be broken down into dirt, if not for bacteria. Bacteria helps enrich soil by causing decay in dead plants and other organic matter. Nitrogen-fixing bacteria in soil holds nitrogen from the air. This helps plants obtain nitrogen nutrients, through their root systems. Some foods contain helpful bacteria. The distinctive flavors of sauerkraut, vinegar, yogurt, and cheese are due to good bacteria. Sauerkraut is made from cabbage, which lactic acid bacteria causes to ferment. The same type of bacteria causes fermentation in vinegar. Cheese is made out of curds, which are formed when certain types of bacteria cause milk to clot. Yogurt is a useful means for replacing good bacteria when your body is fighting off infection or is losing good bacteria to antibiotic treatment. You can enjoy a tasty product of good bacteria and build science lab skills at the same time, by making yogurt at home. You'll need to use a starter (available at grocery or health food stores), or else one cup of plain, unflavored yogurt that has live cultures in it. Slowly heat four cups of milk until it is hot, not boiling or scalding. The temperature should be around 95-120 degrees to kill or deactivate some of the harmful bacteria. Cool slightly, until milk is warm, and then add the cup of active yogurt or the starter. Put the mixture in a large bowl or else in two pint or quart jars, and cover them. Make sure that the bowl or jars are sterilized--either run them through the dishwasher or wash them with very hot water. There are two different methods for culturing the yogurt mixture. You can put the bowl or jars into a clean plastic cooler, and fill the cooler with hot water to just below the top of the culture containers. You will need to occasionally refill the cooler with hot water, so that the temperature of the yogurt stays consistent. The other method is to wrap the containers in a heating pad and towels, setting the heating pad on low to medium heat. After 3 1/2-4 hours, check the mixture. It should be set up--have a smooth, creamy consistency like store-bought yogurt. If the mixture is not set up yet, continue to heat it for another 1-2 hours. After it is the right consistency, add some flavoring--such as vanilla extract, chocolate syrup, or berries--and put the yogurt in the refrigerator. It should keep for a couple of weeks. For safety purposes we suggest that you do not eat any yogurt that has separated or has a non-typical consistency. Noteworthy Scientist: Joseph Lister (1827-1912) Joseph Lister was born in 1827, in Essex, England. He was the son of a famous physicist. Lister was educated in Quaker schools, although he later joined the Scottish Episcopal Church. He attended college in London and Edinburgh, and received a degree in Medicine in 1852. After graduation, he worked in Scotland as a surgeon. At that time, almost half of the patients that successfully underwent major surgery ended up dying from infection (usually sepsis). The popular theory was that exposing moist wounds to air caused sepsis. During the early 1860s, 45-50% of Lister's amputation cases died from infection. After experimenting with different unsuccessful prevention methods, Lister formulated the idea that infection was caused by a sort of "disease dust", rather than by the air itself. He realized the relationship between Louis Pasteur's germ discoveries and what was happening in the hospitals. Lister began to use a carbolic acid solution to cleanse wounds and equipment. Although carbolic acid was hard on people's skin and bodies, it was a successful method of preventing infection. In 1867, Lister informed the British Medical Association that his hospital ward had been free of sepsis for nine months. During the 1870s, the Germans adopted his antiseptic (from "anti-sepsis") methods during the Franco-Prussian war, with good results. However, British and American doctors opposed Lister's germ theory. Lister eventually won the debate by using antiseptic methods to successfully perform surgery that had usually resulted in death. After Lister's methods gained popularity, the post-surgery deaths from infection went from around 50% to less than 5%. Lister was made a baron in 1887. In 1891 he established the Institute of Preventive Medicine. He died in 1912. Science Links Learn about the structure of bacterial cells! www.cellsalive.com/cells/bactcell.htm |
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