Back To CourseMicrobiology 101: Intro to Microbiology
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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
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.
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 above 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 below that all of the 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?
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.
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.
Parameters like pH, salt concentration, and available nutrients are best set by adjusting the media. You need to include the appropriate nutrients, add only the appropriate amount of salt, and use buffers to maintain the appropriate pH for your bacteria. Temperature and oxygen must be controlled from the outside. Many bacterial cultures are placed in incubators or warm water baths that ensure a constant temperature. Some bacteria like cool temperatures, while others like warmer temperatures.
Oxygen concentration is often a bit trickier to control. Solid media in plates or slants will be able to get enough oxygen from the air to support bacterial growth. If you're growing bacteria in a liquid, the bacteria will quickly use up all the oxygen and die. Liquid tubes need to be shaken during growth to allow oxygen to reach the entire volume, from top to bottom. But, not all bacteria can survive in an oxygen-rich environment. These bacteria need to be sealed in containers that have had the oxygen removed.
Keep in mind that it is very difficult to cookie-cutter bacterial growth conditions. There are so many different bacterial species, media, and experimental goals that you really need to do your research and customize your growth conditions for your needs. And, don't get frustrated if you fail. Despite being everywhere and exploiting a huge range of conditions, when you actually want to grow bacteria, it can be surprisingly difficult. And, of course, contamination is everywhere. Remember to always be precise, take your time, and good things will grow eventually.
Every microbiologist eventually has to grow bacteria in the lab. Bacterial culturing is the process of growing bacteria. Cultures can be mixed, containing multiple species, or pure, made up of only one species.
Bacteria must be grown in a medium, which is a liquid or gel designed to support the growth of a bacterial culture. The media must contain everything the bacteria need to survive and can be liquid or solid. Agar is added to hot liquid media to make a gel used for culturing in plates, tubes, slants, and stabs.
Bacterial growth also requires tightly controlled environmental conditions. Parameters such as pH, temperature, oxygen concentration, salt concentration, and available nutrients must be specifically tailored to the bacterial species you are growing.
Remember, it is up to you to determine exactly what type of culture and the ideal environmental conditions you will need. Every species of bacteria is unique, and what works well for one could be lethal to many others.
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Back To CourseMicrobiology 101: Intro to Microbiology
20 chapters | 207 lessons