a) Koch's postulates are... 1. Isolate a microbe that can be found in all sick people (in this case the "sick people" is yogurt) but not in healthy people ("healthy people" = milk). 2. Culture that microbe. 3. Inoculate the healthy people with the microbe. 4. Culture the microbe from the new sick people and it should be the same microbe as the original one. The microbe, or bacteria, that we will be using in this experiment is yogurt bacteria. Bacteria are prokaryotes, meaning they do not have a nucleus to hold their DNA. Bacteria can be both good and bad. E. Coli are bacteria that live in our intestines and help us digest food, which is very beneficial to us. Other bacteria however can cause disease and antibiotics can be taken to kill these harmful bacteria. Yogurt bacteria are a good kind of bacteria - they help with digestion and promote general good health. Yogurt bacteria break down milk sugar (lactose) into pyruvic acid, and then, using enzymes, the pyruvic acid is broken down into lactic acid. The lactic acid causes the milk to be more acidic, thus denaturing the milk protein casein into a more solid form - yogurt!
b) The purpose of this experiment is to make yogurt out of milk and yogurt bacteria. We will also be testing Koch's postulates using microbial techniques (extracting and culturing bacteria and identifying if the bacteria are the same).
c) We will be culturing bacteria using specific techniques. We will be using sterile inoculating loops to transfer yogurt bacteria from the petri dish to the test tubes. Precautions will be taken to avoid contamination, such as keeping the lab area clean and not touching the inoculating loops to any potentially contaminating surfaces. A vortex will be used to thoroughly mix the contents of each test tube and an incubator will encourage the production of yogurt overnight.
d) The controls of this experiment are the milk only tube (negative control) and the milk + yogurt bacteria tube (positive control). The variables are the milk + yogurt + ampicillin tube, and the milk + E. Coli tube. I think the negative control will do nothing and the positive control will produce yogurt (as should happen). The first variable should not produce yogurt because the ampicillin will kill the yogurt bacteria which are necessary to denature the milk into yogurt. The second variable will probably result in some of the milk being digested by the E. Coli, because that's what E. Coli do - digest food.
Procedure:
See video posted.
Results/Observations:
Discussion:
a) Tube #1, the negative control which contained only milk, obviously did not turn into yogurt because it did not have the yogurt bacteria in it. The sour, tainted milk smell is the result of unrefrigerated milk gone bad - it did not have the yogurt bacteria which helps preserve freshness. Tube #2, the positive control, turned into yogurt, as expected. The acidity of 4 can be attributed to the lactic acid, which was created by the breakdown of lactose and pyruvic acid (see Introduction part "a"). Tube #3 did not produce yogurt and turned out sour because the ampicillin killed all the yogurt bacteria, which are essential in the creation of yogurt. Tube #4, which served to test whether or not any type of bacteria can create yogurt out of milk, did nothing - the milk turned rotten and stayed in liquid form. This proved that the E. Coli does not produce yogurt out of milk and clearly, yogurt bacteria is the necessary ingredient for this reaction to take place.b) Some possible sources of error include...
- Outside bacteria getting into the solution
- Not enough yogurt bacteria, ampicillin, and/or E. Coli being added to the appropriate tube
- Milk improperly pasteurized
Excellent - A+
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