Course 1: Making Yogurt the Scientific Way

Lesson Goals

  • To review the nutrients and conditions required for bacterial growth.
  • To see how these requirements are met by typical culture media and conditions.


 Key Terms

  • Growth Factors
  • Luria-Bretani Medium



Hi everybody! Welcome back to Synthetic Biology One. Today I want to talk about the care and feeding of bacteria. What do bacteria need to grow and how can we give it to them?

Temperature. Normal bacteria are happy between room temperature and body temperature, so between 20 and 37 C. Sure there are some super freako bacteria that can survive at 100 C, but you won’t find them in a normal lab.

pH. Normal bacteria like a neutral pH around 7. There are plenty of examples of bacteria that prefer slightly acidic or slightly basic conditions. And there are extremophiles that can live in concentrated sulfuric acid. In general, every microbe prefers a specific range of pH values, and we keep this in mind when preparing media to feed them.

Oxygen. Some bacteria need it, some don’t. In some cases, it is even toxic. However, most commonly used bacteria appreciate oxygen and grow better when they are well mixed with air.

Carbon serves two purposes. It is the backbone for most of the molecules that make up a cell. And it is an energy source. Glucose, for example, is a high energy sugar that many bacteria can use both to build the cell, and to power the cell.

Nitrogen is a big part of many biological molecules, in particular proteins. Cells might get this from ammonia, nitrate, or amino acids that we add to their food.

Basic Elements. Many other elements are needed in small quantities to make up the molecules of life. I’m talking about Sulfur, Magnesium, Iron and so on. Bacteria can not make them out of thin air, so they need to get them from us.

Salt. Most bacteria like a little salt in their food. Salt ions are a part of many biological processes, and adding salt makes it easier for bacteria to maintain their internal osmotic balance: not too salty, not too watery.

Growth Factors. Some bacteria, given the basic building blocks, can make everything they need to grow. Others can’t. In this case, we have to feed them the specific things that they need. It might be vitamins, amino acids, or other micronutrients.

Temperature, pH, Oxygen, Carbon, Nitrogen, Salt, Other stuff. It seems like a long list. But actually, it is not too hard to find things that meet all these conditions. Just a glass of milk, or example, if you leave it in a warm place, has everything you need to grow a bacterium like lactobacillus.

So. How do we actually mix up a media for our microbes? How do we know if they will grow on the food that we give them? Well, in practice, the answer has usually been worked out long ago through trial and error. For most bacteria that you would want to grow, a microbiologist 50 years ago figured out a standard recipe that is cheap and reliable. The most famous of these is probably LB, Luria-Bretani medium, which is loved by E. coli and many other bacteria. In the notes for this video, we’ll present the recipe for LB and talk about why its got all the delicious nutrients that bacteria crave.

Until next time, Happy Culturing!


 Optional Links and Further Reading

Course Curriculum