Growing Bacteria in a Lab: Experiments & Conditions

  • 0:05 Bacterial Coworkers
  • 0:48 Culturing Vocabulary
  • 2:02 Growth Media
  • 3:42 Growth Conditions
  • 6:09 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Despite the fact that bacteria are growing all around you, they can be surprisingly difficult to grow in a lab. This lesson will explore some of the basics involved in culturing bacteria for experimentation.

Bacterial Coworkers

Mathematicians have to work with numbers. Marine biologists all get wet. When you decide to pursue a career as a microbiologist, eventually you have to work with microorganisms. But performing experiments on living things far too small to see can be difficult. It is not as easy as snorkeling to observe reef fish or putting white mice in cages. You have to plan and control every aspect of your new pet bacteria's lives. At the same time, you have to diligently protect your specimens from being contaminated by all the rogue bacteria that are all around you, covering every surface, and colonizing your own body. In this lesson, we will hit on some of the basic aspects of growing bacteria in a lab.

Culturing Vocabulary

One skill that every microbiologist has to master is how to properly grow bacteria without letting all those environmental bacteria contaminate your work. But, let's get a few vocab words out of the way first. Bacterial culturing is a process of growing bacteria. Oftentimes, if you are trying to identify a microbe causing a disease or investigating the populations in an environmental sample, you begin with a mixed culture. A mixed culture is a bacterial culture made up of more than one species of bacteria. You could have as few as two or as many as hundreds of species. It is your job to take this mixed culture and isolate the one species of interest.

Notice how many of the colonies look different? These are all different species. What you want is to make that mixed culture into a pure culture. A pure culture is a bacterial culture made up of only one species. Notice that all of this growth looks the same. You want to have a pure culture so you know with certainty that any observations you make are a result of that one species of interest and not some other unknown contaminant species. There are many different techniques you can use to isolate a species, but most involve growing the bacteria on or in media. So what is media?

Growth Media

A medium (plural media) is a liquid or gel designed to support the growth of a bacterial culture. The liquid or gel has to contain everything that the bacteria will need for growth and cell division. Leaving out even one, seemingly insignificant, ingredient from your medium can prevent all growth. This can be a very frustrating problem, especially if you spent hours painstakingly setting up your culture, only to see absolutely nothing growing. Today, most labs buy commercially prepared dry media that contains all needed nutrients. All you have to do is add sterile water!

In addition to the nutrients and growth factors needed for life, there are many additional compounds you can add to your media, for more information, see the lesson titled Differential and Selective Media in Microbiology.

So, you want to make some media? First, you need to pour your dry media into a flask and add sterile water. For some experiments, you can grow your cultures in this flask of liquid or use it to fill several smaller culture tubes. If your experiment requires solid media, all you have to do is add agar to the flask. Agar is a carbohydrate derived from marine algae that turns the liquid media into a gel. As long as the agar/media mixture is hot, it remains a liquid. Once it cools, it hardens into a suitable growth surface for bacteria. The hot agar can be poured into plates or into culture tubes. Plates are one of the most common methods for growing bacteria. Solid culture tubes are sometimes used for stab or slant cultures. Bacteria can be spread on the surface of the slant or stabbed down into the gel, depending on what you need your culture to accomplish.

Growth Conditions

So, you have your bacteria and you have put it on your medium of choice, but that is not all you need to know. Like you, bacteria require very specific environmental conditions to survive. You wouldn't survive very long in the freezer, sitting in stomach acid, or with no oxygen or food, and neither will your bacteria. There are several key environmental parameters you need to set in order to get the best growth rate for your bacteria.

Most bacteria require very moderate conditions. Anything too far on one extreme or the other will be lethal. Some of the more common conditions you need to set include temperature, salt concentration, pH, nutrients, and oxygen concentration. E. coli, one of the most commonly cultured bacteria, is a normal inhabitant of your gastrointestinal tract. E. coli has evolved for thousands of years to grow best inside your body, so it makes sense that you want to try and replicate these conditions so your E. coli will grow at peak speed and efficiency.

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