Factors that Affect Bacterial Growth

We are launching into a unit on bacteria, and so we began with the question of what factors affect bacterial growth. 
Even if you don't have a microscope, you can see bacterial growth though this simple demonstration.

First dissolve one chicken bouillon cube in 1 1/3 cups of hot water from the faucet. Equally divide the solution between four small glasses. 

Now, to one of the glasses, add a teaspoon of salt and label it. 

To the second glass, add a teaspoon of vinegar and label this glass. 

Label the third glass "cold" and put it in the refrigerator and label the last glass "control". 

Leave all of the glasses uncovered. After two days, compare the clearness of the liquids.

take a piece of white paper and write a word on it with a colored marker. Put the paper behind each of the glasses, starting with the "control" glass and note in your science notebook whether you can see the word clearly, mostly see it, barely see it or see it not at all through the glass.

During the two days the glasses sat, bacteria entered the liquids on dust and the bacteria fed on the chicken bouillon, growing and multiplying, clouding the bouillon. The cloudier the bouillon, the more bacterial growth.

The results?

control

The control was the cloudiest. 

control

salt

The salt and...

salt

cold

the refrigerated glasses 

cold

were a little less cloudy, so they prevented some bacterial growth. 

The vinegar glass had even less bacterial growth, because it was pretty clear. 

This makes sense since during the Middle Ages, before refrigeration, many foods were salted or pickled. 
Other ways of preserving food were invented, such as dehydration, as bacteria must also have moisture to survive. Later canning was developed, in which the bacteria was killed by heat and then sealed so that no new bacteria could enter the food.

Oh, and if you do this science demonstration, don't forget to dump the glasses and with them thoroughly with soap and hot water to get rid of the bacteria you have cultured.