Testing Koch's Postulates to Observe the Presence of Bacteria that cause Milk to become Yogurt

Introduction
Bacteria are present throughout the world, some beneficial others harmful. Microbial pathogens were a cause of great concern during the 19th century as people would sicken suddenly growing ill and die. Scientists had already theorized that this was the caused by microorganism that could freely travel and infect individuals. A scientist, Koch postulated that to test for the cause of a disease from a particular microorganism 4 rules would have to be observed.
1) The microorganism must be found in all organisms suffering from the disease.
2) The microorganism must be isolated from a diseased organism and grown in pure culture.
3) The cultured microorganism should cause disease when inoculated into a healthy individual.
4) The microorganism must be again isolated from the inoculated organism and then identified as identical to the original sample extracted and grown in culture.
These are used to attribute a microbe as the cause of a disease, and then used to develop a cure, or treatment.
Milk is a perfect medium for bacterial growth and contains many forms of spoilage bacteria (that cause milk to sour) and pathogenic bacteria. In order to deal with these milk is pasteurized by scalding and then later cooled. The pathogenic bacteria are killed, and most of the spoilage bacteria as well. Yogurt forming bacteria break down lactose into pyruvic acid and later again into lactic acid. Lactic acid reduces the pH of the milk causes a protein (casein) to denature. This causes milk to curdle and solidify into yogurt. The decrease in pH has the side effect of not allowing other microorganisms to grow, thus allowing yogurt to be safely consumed. The bacteria Streptococcus thermophilus and Lactobacillus bulgaricus are the primary producers of lactic acid, though others are added for flavor. Due to the threat of pathogenic bacteria widespread antibiotics, starting with penicillin, have become increasingly common. People have begun using antibiotics with an unnecessary frequency. Antibiotics when ingested into the body kill or restrain the growth of bacteria. However antibiotics cannot distinguish between pathogenic bacteria and good bacteria, such as the E. coli in one's intestines. Yogurt is a probiotic containing many bacteria that support the body. In the lab we will be using ampicillin as an antibiotic control. Using Koch's postulates we will see if the bacteria from our yogurt sample are the cause of milk curdling and solidifying into yogurt.  We will begin with 4 tubes: negative control, positive control, ampicillin and E. coli. The negative control contains milk alone, the positive control contains a mixture of milk and yogurt, the ampicillin contains yogurt combined with the antibiotic ampicillin, and the final tube contains E. coli bacteria. We will begin by adding a small amount of yogurt from a sample into the ampicillin and positive control tubes. We will add bacterial colonies to each tube: yogurt bacterial to the positive control and ampicillin tubes, and E. coli to the E. coli tube. Following this all tubes will be placed in a 37 degree Celsius water bath to allow for pasteurization. Following this we will observe and record results. Our controls are positive and negative tubes. The negative tube contains milk alone without any yogurt, while the positive control contains milk with yogurt. Our variable is the ampicillin tube, which adds ampicillin to see the effect on the bacteria. These bacteria should be fewer in number but the strain that may remain would be resistant to ampicillin.

Video:
http://www.youtube.com/watch?v=vVhIArPkyi0

Analysis:
The milk in the negative control tube and positive control tube both produced some amount of yogurt, though the amount in the negative control was excessively minute. The ampicillin control stayed completely fluid as did the E. coli control. The ampicillin killed the bacteria in the yogurt sample that we added, preventing the curdling. In the E. coli sample the bacteria was not the appropriate bacterium to cause the milk to solidy into yogurt, thus causing it to remain fluid. However it is possible that the E. coli bacteria proccessed some part of the milk because the smell was rather different from the original milk.

Sources of Error:
Possible sources of error include the contamination of the negative control. The negative control should not have solidified into yogurt, due to the lack of bacteria to cause the reaction. However a small amount of yogurt was produced in the process. We were unsure exactly why the yogurt was produced but the amount was significantly less compared to the positive control.