Yogurt Lab Intro

     It was not until a little over a hundred years ago that scientists discovered bacteria were capable of causing disease. Bacteria had been identified in sick people much earlier, but it took many years of experiments and guesswork to prove that bacteria were the actual infecting agents. German physician Robert Koch developed a series of tests to prove anthrax was caused by bacteria, but his method can be used to prove that any microbe causes a specific disease. These tests, termed "Koch's postulates," are as follows:
1. The microbe is found in organisms with the disease but is not found in healthy organisms.
2. The microbe is isolated from the diseased subject and grown in culture.
3. The cultured microbe causes disease when introduced to a healthy organism.
4. The microbe is again isolated from the host and shown to be identical to the original.

     Bacteria have several characteristics that make them effective disease-causing agents. Most importantly, they are tiny prokaryotic cells much smaller than the eukaryotic cells that make up animals and other living organisms, allowing them to infiltrate a host. Also, the thick cell walls of bacteria make them very resilient.
     Using milk as a model test subject, we will use apply Koch's postulates to determine whether microbes in yogurt cause milk to thicken and turn into yogurt. By adding yogurt to milk and inoculating, we will see if the milk turns into a substance similar to the original yogurt.
     We will use four test tubes for our experiment: a negative control with milk only; a positive control with milk and yogurt; one with milk, yogurt, and ampicillin; and one with milk and E. coli. The tube with just milk is a control because it shows that milk will not turn into yogurt just by fermenting for a day. The tube with yogurt and ampicillin is a similar control because ampicillin kills all the bacteria in the yogurt, so the milk should not be spoiled. The E. coli tube should spoil the milk, but it won't necessarily turn the milk into yogurt. For the procedure, we will transfer the bacteria using sterile inoculating loops. Then, we will let the tubes sit in a hot water bath overnight to stimulate bacterial fission.
     I predict that the positive control (milk and yogurt) is the only tube where the milk will turn into yogurt. The yogurt itself was once milk and, therefore, must have the yogurt-making bacteria in it.