When you want to study the features of bacteria, it is important that you should know how to make culture media. You cannot propagate bacteria without it, which means there is no way around it.
A culture media is either an organic or a synthetic substance that provides both the biophysical and the biochemical factors necessary for the growth of bacteria.
Researchers have developed a variety of culture media to serve different needs/purposes. Each type of media serve the needs of a particular bacteria and/or the special requirements of the investigator.Support #bacteria; they're the only #culture some people have. Steven Wright Click To Tweet
One of the purposes of a culture media is to isolate and maintain pure bacterial strains. That is why the media is crucial in the identification of different bacteria.
The differentiating factors depend on the biophysical and biochemical properties of each bacteria.
You can use liquid media to grow pure batch cultures and to estimate the bacterial populations. A gelling agent (usually nutrient agar), makes the media either solid or semi solid. Nutrient agar is a hydrocolloid of red algae.
Agar imparts unique physical characteristics to the culture media, which makes it suitable for its purpose.
For instance, agar enables the media to melt at 100ºC and then cool back to less than 40ºC before re-solidifying.
Additionally, very few organisms are capable of degrading nutrient agar.
Solid media are instrumental in isolation of pure cultures and determination of the number of viable bacterial populations.
Basically, you can classify culture media into various categories depending on:
- Their chemical composition, and
- The intended use of the media.
The classes of culture media include:
- Chemically defined (synthetic) media: – have well defined chemical components and well known proportions
- Complex (undefined) media: – their exact chemical compositions are not known
- Selective media: – contains components that will inhibit the growth of certain bacteria spp. but promote the growth of the desired species
- Differential media: – allows the investigator to distinguish between different types of bacteria based on some observable patterns of growth in the medium. For instance, you can eliminate Staphylococcus aureus from a culture by increasing the salt concentration in the media
- Enrichment media: – contains some components that permit the growth of specific bacteria spp. This is usually because only such species can utilize the nutrients from the media
Procedure for the preparation of the liquid media
To begin with, you will need a functional autoclave. You will use it to sterilize the media at high temperatures under pressure.
After tightly securing the machine, heat it to a pressure of 15psi and temperature of 121ºC for 15 minutes. Read more about the operating pressures of an autoclave here.
You will also need the following:
- Distilled water,
- Clean 500 ml measuring cylinder,
- Clean conical flask,
- Electronic weighing scale,
- Sterile media, and
- Clean test tubes.
How to make culture media: 8-step-process
- Weigh 6.5 grams of the sterile nutrient broth and transfer into the clean conical flask. The manufacturer recommends a dilution of 13 g/l but we need to make only 500 ml of the media.
- Add 500 ml of distilled water into the measuring cylinder and transfer into the conical flask to dilute the media.
- Put the conical flask with the media solution from step 2 into an autoclave basket. Ensure you properly secure the mouth of the flask with cotton wool before lowering it into the autoclave. Secure the autoclave and start the sterilization. It will take some time to attain sterilization temperatures. However, once you achieve the recommended temperature/pressure combinations, hold it there for the recommended 15 minutes. That time is adequate under these conditions to sterilize the media.
- Allow the autoclave to cool down.
- Put the petri dishes into a hot air oven at 80ºC for one hour to sterilize them.
- Remove the conical flask containing the now sterile media from the autoclave. Pour 15 ml into each petri dish and seal.
- Make sure your working bench is not only clean but also always sterile. Wipe the surfaces with 7% alcohol and keep the UV lamp on.
- Label the petri dishes and store in a refrigerator for later use.