Bacteria are single-celled prokaryotes which have existed for billions of years. They are relatively simple in comparison to other life forms, and are most likely the most abundant (one colony can contain billions of individuals) and diverse. Differences range from shape to grouping to physical composition of the organism.
The most well-known bacteria are pathogens, or disease-causing microorganisms. In the 19th century, scientists were beginning to gain knowledge of the existence of bacteria and their relation to illness, but their knowledge was not specific enough until the German physician Robert Koch devised a series of tests to identify bacteria's role in disease. Koch's Postulates, as they came to be known, state the following:
1. The pathogen must be found in all organisms suffering from a disease, but no others.
2. One must isolate the pathogen and culture it separately, free of contact from other bacteria or host organisms
3. When introduced to a healthy subject, the cultured bacteria must cause the same disease as the bacteria taken from the original, ill subject
4. The bacteria must once again be isolated from its host and identified as the same pathogen in step 1.
Because it is often dangerous to work with pathogens, researchers may use a safe alternative. There are "good bacteria" which people use to perform many daily functions, including the making of foods such as cheese, pickles, and yogurt.
Yogurt bacteria works roughly the same way on milk as a pathogen works on living organisms- the milk is infected and "consumed" by the bacteria. Like any other living thing, of course, bacteria require food. Yogurt bacteria consume lactose from milk, and create lactic acid as a by-product. It is this lactic acid which curdles the milk, changing it into yogurt. However, yogurt bacteria requires certain conditions in order to thrive. The milk to be used must be pasteurized- not boiled- to kill other bacteria which would spoil the milk before the yogurt bacteria completed its process. After pasteurization, the yogurt bacteria is introduced. After about 24 hours, the bacteria has consumed all the lactose, replacing it with lactic acid. This acid lowers the pH of the yogurt, which kills any spoilage bacteria surviving the pasteurization, as well as preventing the growth of new spoilage bacteria. Therefore, yogurt does not become simply spoiled milk, and is void of its rancid smell and taste.
1. Obtain four vials of milk:
- Only milk- the negative control. The negative control is used to test what happens to the milk when it is left untouched.
- Milk with yogurt- the positive control. This proves that there is something in yogurt that helps it create more yogurt from milk.
- Milk, yogurt, and ampicillin. Ampicillin is an antibiotic, which will kill all bacteria in the sample. If there is no yogurt in the vial with ampicillin, it will prove that bacteria do cause the change into yogurt.
- Milk and E. coli. The sample containing E. coli will answer the question of whether all bacteria can create yogurt, or if there is a bacteria specific to the process.
3. Leave the vials in the cold water bath overnight to allow the bacteria to work.
The negative control formed a small lump of curdled milk floating near the top of the vial, suspended in a clear liquid. It smelled slightly old, but not too pungent.
The positive control was liquid on the top fourth, but the bottom three-quarters had coagulated into yogurt. (We're fairly certain it was yogurt, it had the smell and consistency of yogurt although none of us tasted it.)
The vial containing ampicillin resembled the negative control; because the ampicillin killed the yogurt-producing bacteria, there were no traces of yogurt. My group had hypothesized that the ampicillin would kill off spoilage bacteria, but neglected the fact that it would kill the yogurt-producing bacteria as well.
The vial containing milk and E. coli appeared the same as the first and third vials. It also had a smell which is best described as "rancid".
Fortunately, my group didn't encounter any errors in our work (as far as we know), although there were multiple opportunities, such as cross-contamination between vials.
