Any nutrient substrate prepared in the laboratory that is suitable for the growth, reproduction, or accumulation of metabolic products of microorganisms is called a culture medium (Media). Due to the different nutritional requirements of various microorganisms and various cultivation purposes and testing needs, there are many types of culture media. We can classify the numerous culture media into several types based on certain standards.
Natural Culture Medium
Natural culture media refer to those utilizing raw materials from various animals, plants, or microorganisms, whose components are difficult to determine accurately. The main raw materials used for such media include beef extract, malt extract, peptone, yeast extract, cornmeal, bran, various meal powders, potatoes, milk, serum, etc. Although the chemical composition of the media made from these substances is not exactly known, the nutrient content is generally rich, promoting vigorous microbial growth. They are widely available and easy to prepare, thus commonly used, especially for preparing standard laboratory media. The stability of this type of medium is often affected by manufacturers or batch numbers.
Synthetic Culture Medium
Synthetic culture media are those whose chemical composition and quantities are fully known. They are prepared using chemical substances of known chemical constituents. These media have precise chemical compositions and strong repeatability but are expensive and support slow microbial growth, making them suitable only for certain scientific research, such as studies on nutrition and metabolism.
By adding one or several natural ingredients to a synthetic culture medium, or by adding one or several known chemical substances to a natural culture medium, semi-synthetic culture media are created. Examples include potato sucrose agar. These media are most commonly used in production practices and laboratories.
Liquid Medium for Bacterial Growth
The prepared culture media are in a liquid state, with most components dissolved in water and no significant solids present. Liquid medium for bacterial growth distributes nutrients evenly and is easy to control for microbial growth and metabolism.
Solid Culture Media
By adding an appropriate amount of solidifying agents to liquid medium for bacterial growth, solid culture media are formed. Commonly used solidifying agents include agar, gelatin, silica gel, etc., with agar being the most common. Solid culture media are extensively used in practice. In the lab, they are used for microbial isolation, identification, contamination inspection, counting, preservation, bioassay, etc.
Semi-Solid Culture Media
By adding a small amount of solidifying agent to liquid medium for bacterial growth, semi-solid culture media are produced. For instance, with agar, the quantity ranges between 0.2-1%. These media can sometimes be used to observe microbial motility or to preserve microbial strains.
Some culture media have both selective and differentiation functions. For example, MacConkey agar, commonly used in food inspection, contains bile salts, lactose, and neutral red. Bile salts inhibit bacteria other than enteric bacteria (selective), and lactose and neutral red (indicators) help distinguish lactose-fermenting enteric bacteria (like E. coli) from lactose non-fermenting enteric pathogens (like Salmonella and Shigella).
In addition, based on whether the nutrient components of the culture medium are "complete," it can be divided into basic culture media, complete culture media, and supplementary culture media. These terms are mainly used in microbial genetics. Based on the purpose of production, culture media can be differentiated into seed culture media and fermentation culture media. There are also specialized media for culturing parasitic microorganisms, like viruses, such as live tissue culture media (e.g., chick embryos); and inorganic salt culture media used for culturing autotrophic microorganisms.
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