When cultivating microorganisms in a laboratory, a growth environment called a culture medium is used. The culture medium may be pure chemical substances (medium with a determined chemical composition), or it may contain organic substances, or it may be composed of living organisms such as fertilized eggs. Microorganisms that grow in or on such media form cultures. If there is only one type of organism, the culture is regarded as a pure culture, and if there are populations of different organisms, it is regarded as a mixed culture. When used for the first time, the culture medium should be sterile, which means that there are no life forms before the inoculation of the microorganisms.
General microbial culture medium
For bacterial culture, the commonly used medium is nutrient broth, which is a liquid containing protein, salt and growth promoters, which can support many bacteria. In order to solidify the medium, add agar and other reagents. Agar is a polysaccharide, it does not add nutrients to the medium, but only solidifies it. The resulting medium is nutrient agar.
Many media for microorganisms are complex and reflect the growth requirements of microorganisms. For example, most fungi require additional carbohydrates and an acidic environment to achieve optimal growth. The medium used for these organisms is potato dextrose agar, also known as Sabouraud dextrose agar. For protozoa, liquid culture medium is generally required. For rickettsiae and virus, living tissue cells must be provided for optimal culture.
For anaerobic microorganisms, the atmosphere must be anaerobic. To eliminate oxygen, the culture medium can be placed in a container that produces carbon dioxide and hydrogen and removes oxygen from the atmosphere. Commercial products meet these conditions. Anaerobic chambers can also be used in closed compartments where technicians can manipulate the culture medium. In order to promote the formation of carbon dioxide, candles can be burned to consume oxygen and replaced with carbon dioxide.
Special microbial culture medium
Certain microorganisms are cultivated in selective media. These media prevent the growth of unwanted organisms while promoting the growth of desired organisms. For example, mannitol salt agar is selective for staphylococci because most other bacteria cannot grow in its high salt environment. Another selective medium is Bright Green Agar, which inhibits Gram-positive bacteria while allowing the growth of Gram-negative bacteria such as Salmonella.
There are other cultural media that are different media. These media provide an environment in which different bacteria can be distinguished. For example, Fuchsia Bile Agar is used to distinguish coliforms, such as Escherichia coli and non-coliforms. The coliforms are bright pink colonies in this medium, while the non-coliforms are pale pink or transparent.
Certain media are both selective and differentiated. For example, MacConkey Agar distinguishes lactose-fermenting bacteria from non-lactose-fermenting bacteria, while inhibiting the growth of gram-positive bacteria. Since lactose-fermenting bacteria are often associated with water pollution, they can be distinguished by adding a water sample to MacConkey agar and waiting for growth to appear.
In some cases, it is necessary to prepare enriched media. This medium provides specific nutrients and can promote the reproduction of specific types of microorganisms in mixed samples. For example, when trying to isolate Salmonella from a stool sample, it is helpful to place the material sample in an enriched medium to promote the propagation of Salmonella before the isolation technique begins.
In order to use microorganisms in the laboratory, they need to be obtained in pure culture. A pure culture of bacteria can be obtained by spreading the bacteria and allowing individual cells to form growing clumps called colonies. You can then pick a sample from the colony and make sure it contains only one type of bacteria. Culturing these bacteria on a separate medium will produce a pure culture.
In order to preserve microbial cultures, they can be placed in the refrigerator to slow down the rate of metabolism. The other two methods are deep freezing and freeze drying. For deep freezing, microorganisms are placed in a liquid and quickly frozen at a temperature below –50°C. Freeze-drying (lyophilization) is carried out in equipment that uses vacuum pumping after freezing the microbial suspension. The culture is similar to a powder, and the microorganisms can be stored for a long time under this condition.
microbial cultivation