Using Bacteria For Good - Koch's Postulates In Action

Part 1: Introduction

Bacteria are the most successful lifeforms on earth. They are found everywhere on the planet, even inside other living things. The initial reaction to this fact is disgust, because everyone thinks of bacteria as infections or as parasites. But the reality is quite the opposite-- only a few of the many types of bacteria are capable of causing disease, and many even help us. In this lab we will have a hands-on experiment with bacteria and see for ourselves how it can help us, in this case by turning milk into yogurt. The framework of this lab will be done by Koch's postulates, a series of tests and observations that will determine if a microorganism is responsible for a certain condition in another organism. This method is used by many biologists and scientists today for the same purpose (but with cases more complex than yogurt.)

In this lab we'll be using several different materials and instruments:

Materials

• milk
• yogurt
• ampicillin
• E. coli

Instruments

• test tubes
• inoculation loops
• agar plates

We will label 4 tubes: 1. Negative control (just milk), 2. Positive control (just yogurt), 3. Yogurt + ampicillin, 4. E. coli. We'll add 10 µl of ampicillin to tube 3, using sterile technique by minimizing potential of contamination during transfer. Next we'll dip an inoculation loop into some yogurt, and swirl the loop into tube 2. Then we'll use the same loop to dip in the yogurt again, and swirl in tube 3. With a new loop, we'll get some E. coli and put it into the yogurt in tube 4. Then we'll put them all in the mixer, and then in the waterbath.


Part 2: Experiment




Part 3: Discussion
After 24 hours, These were the results for each tube:

• 1. negative control (milk): white; gooey/gelatinous; smelled horrible, like sour milk and mold. pH of 7.
• 2. positive control (yogurt): slightly off-white; liquidy, milky; smelled like normal yogurt. pH of 4.
• 3. yogurt + ampicillin: slightly off-white; milky; smelled like old yogurt. pH of 6.
• 4. milk + E. coli: slightly off-white; liquidy; smelled like normal yogurt. pH of 4. 
• *. control yogurt (from cup, not in waterbath): white; gelatinous; smelled like yogurt. pH of 7.

 These data show that the negative control of normal milk turned sour without doing anything special to it, except for putting it in the waterbath. The positive control of yogurt turned liquidy in the waterbath, which shows that heat of the waterbath counteracts the bacteria and turns the yogurt to a milky texture. This was supposed to turn to all yogurt, but we probably didn't give it enough time. The yogurt with ampicillin shows that the antibiotic killed the active cultures in the yogurt, so the milk remained milk. It had a slightly sour smell to it which means that the milk had soured.The milk and E. coli had nothing special happen to it, because the E. coli had no effect on the milk. This is because not all types of bacteria turn milk into yogurt.

This lab shows that with some yogurt, you can turn milk into more yogurt. It also shows that not all bacteria curdles milk, and antibiotics can kill yogurt cultures. It also shows that milk sours when it's not kept cold, or if it stays around for too long. 

Some possible sources of error are:

• Test tube material-- could have skewed the observed color of the subject. May have been responsible for the slightly off-white appearance.
• Nothing is "sterile." Different types of bacteria could have gotten into any test tube through the air. This is unavoidable.
• We didn't set the waterbath temperature, so we don't know if it was right or not. We assumed it was right.
• 24 hours might not have been long enough to get the full effect of curdling.