A: Background Information
In this Yogurt Lab, the purpose of the experiment is to demonstrate how the existing bacteria found in plain yogurt is used to convert plain, fresh milk back into yogurt. Bacteria are the single most popular and thriving lifeform on Earth, taking up more total biomass than any other lifeforms. In addition, bacteria is also one of the three domains of life. It is classified as a prokaryote with no nucleolus, a single-celled organism often so small it is unable to been seen with the naked eye. However, bacteria can increase in number by binary fission, which is splitting in half. Overnight a single bacterium, on an agar plate, can produce millions or even billions more which turn into a visible colony of bacteria. Different kinds of bacteria need different requirements to grow. Some need oxygen from the air to grow to grow, those are called aerobes. Others can only grow in the absence of oxygen, those are called anaerobes. There are also some bacteria which grow with or without oxygen, and those are called facultative anaerobes. Bacteria which form yogurt are anaerobes. In milk, they break down the lactose into pyruvic, acid and that into lactic acid. This lactic acid makes the milk acidic by lowering the pH. These acidic conditions cause casein, a common protein in milk, to curdle and turn more solid. Since the conditions are acidic, it inhibits the growth of microorganisms that may cause the yogurt to spoil, which is why yogurt has such a great shelf-life. In addition, the well-known bacteria E. coli takes part in the process by breaking down the sugar into succinic acid, lactic acid, formic acid, acetic acid and ethanol. Although, when the word E. coli comes to mind, most people think of it in a pessimistic way. Bacteria is often misunderstood due to all the negative connotations people have gave it, when really, most bacteria is basically harmless. One of the most common misconceptions with bacteria is that it causes diseases, such as tuberculosis, cholera, and typhus, along with many others which have killed millions. That is why now-a-days we have antibiotics to help. There are many different types of antibiotics, for example either bactericidal, which kill bacteria or bacteriostatic, which prevent bacteria. However, often times people needlessly take these antibiotics for viral infections and discontinue them once they start to feel better. These decisions both allow the bacteria to become resistant to the antibiotics and then leave the resistant bacteria to start a new round of infection. In this lab, the antibiotic ampicillin is used as an extra control and a tool to help further the experiment. Ampicillin is a beta-lactam antibiotic which also inhibits Gram-positive bacteria and some Gram-negative bacteria, such as E. coli (which is also used in this lab). As you might be able to tell by the name, ampicillin is similar to penicillin and amoxicillin. So if you have allergies to penicillin or any other member of that family of antibiotics, avoid contact with ampicillin. Also, ampicillin could cause irritation or allergic recreations to the skin, respiratory system, and eyes so please use with cation and wear suitable protective clothes. In case of any contact, rinse immediately with water in addition to seeking medical help. Although in real life, those are only a small minority which capable of such thing. In fact, most bacteria are beneficial to our bodies and necessary for the manufacture of certain foods, like sour cream, cheese, pickles, and what we'll learn more about, yogurt. For example, yogurt is made by adding strains of bacteria into milk which ferments under environmental conditions and controlled temperatures. It was in the mid-19th century when the famous French scientist Louis Pasteur made extensive studies on the role of bacteria in fermentation where he noticed that germs had to have come in contact with the host first and could not have randomly appeared. During this time there was an assumption that the microbes were connected to a disease, however they were not sure. Eventually, the work of Pasteur and many before him payed off and paved the way for the work of Robert Koch. Koch, an 1800's German physician, studied anthrax, a deadly disease the infects both animals and humans. He used steps, now called the "Koch's Postulates" to prove that a particular microbe caused this disease. These postulates are:
1. The microorganism has to be found in all organisms which have the diseases, besides from the healthy ones.
2. The microorganisms have to be isolated from a diseased organism and grown in a clean culture.
3. The cultured microorganism should cause the healthy organism to be diseased.
4. The microorganism must be isolated again from the inoculated, experimental host and be proven identical to the original diseased agent.
Students will use these postulates to figure out the "yogurtness disease"!
B: Objective & Purpose
The objective and purpose of this lab is to ideally figure out which test tube got the "yogurtness disease" (turned into yogurt). There will be a negative control which is just milk, a positive control which is just yogurt, and two other choices. Koch's postulates will help prove or disprove whether or not the the microbes found (or added) to the yogurt are the cause of "yogurtness disease". And to make yogurt!
C: Procedures
In this lab, one will have to follow the procedures very carefully and properly. First, you will find 4 test tubes with milk filled about half way into them. Label all 4 test tubes as follows: Tube 1 - Negative Control (Just Milk), Tube 2 - Positive Control (Yogurt), Tube 3 - Yogurt + Ampicillin, Tube 4 - E. coli. Once finished labeling, add 10 ul of ampicillin to Test Tube 3 - Yogurt + Ampicillin, with proper technique. Then, add the ampicillin, find Test Tube 2 - Positive Control and take a fresh inoculation loop and dip it into the yogurt, then swirl the inoculation loop into the Test Tube 2 - Positive Control. Put Test Tube 2 - Positive Control back and get out Test Tube 3 - Yogurt + Ampicillin. Use the same loop as before to dip into the yogurt again, then swirl into Test Tube 3 - Yogurt + Ampicillin. When done, put the used inoculation loop into the bleach beaker. Afterward, use a fresh inoculation loop and pick an E. coli colony from the agar plate, then transfer it to Test Tube 4 - E. coli. You're almost done! Now make sure you have all your test tubes and place them in an incubator or water bath at 37° C for 24-48 hours. This is because 37° C is the normal body temperature (also known as 98.6° F). All test tubes stay at this temperature to try and reenact the experiment as if it was in a human body. After 24-48 hours, take the test tubes out of the incubator or water bath and record your results. Examine each tube and find out which one has the "yogurtness disease"!
The controls in this lab are the negative control (Test Tube 1), which contains only milk and the positive control (Test Tube 2), which contains milk and yogurt. These controls will be used to compare the data and observations from the other two variables and see how the answers differed or matched. My prediction for this lab is that the positive control (Test Tube 2 - Yogurt) will be the same as the variable of Yogurt and Ampicillin (Test Tube 3). I also predict that the negative control (Test Tube 1 - Milk) will be smelling unpleasant and look chunky since it has been sitting in 37° C for 24-48 hours. I believe that the variable of milk combined with E. coli will stay the same, however it will have a disease of some sort to it.
Observations:
Discussion:
A: Analyze and Explain Data
My lab table and I acquired a lot of good data from the lab. In Test Tube #1 - Negative Control with just milk, the texture was liquid, the color was white, it had a slight smell of milk and a pH of 6. In Test Tube #2 - Positive Control with milk and yogurt, the texture was liquid, the color was white, it smelled like yogurt and also had a pH of 6. In Test Tube #3 - Milk, Yogurt, & Ampicillin, the texture was liquid, the color was white, it had a slight smell of milk and a pH of 7. In Test Tube #4 - E. coli & Milk, the texture was liquid, the color was white, it smelled slightly sour and a pH of 7. In the cup of yogurt which was a yogurt control, the texture was thick, the color was white, it smelled like yogurt and a pH of 4.
B: Possible Sources of Error
There are a few possible sources of error which can happen in this lab. One important source of error which may throw off the whole lab is if the test tubes or inoculation loops may not clean. In addition, when attempting to get E. coli from the the agar plate, one may take try to take E. coli from an area which had none. Finally, the incubator or water bath which is suppose to be exactly 37° C (or 98° F) may not have been that temperature.
Easily one of the best in the class - A+ !!!
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