A culture media is typically a medium used to provide growth environment for microorganisms. It consists of everything that is crucial for the growth of microbes for example a carbon source, a nitrogen source, minerals and all essential nutrients.
A media can be solid (Agar based media) and liquid (Broth).
Here is the list of different types of media that are available based on different requirements of microbes:
On the basis of consistency:
- Solid medium: Contains 1-2% of agar or other inert solidifying agent that provides a physical structure to allow the bacteria to grow in an informative way. It is used to study the morphology of the colonies of the streaked or spread microbes.
- Semi solid medium: Contains 0.5% or less agar to facilitate the motility of microbe.
- Liquid Or Broth medium: Contains no agar. It is used when a large amount of microbes is needed to grow.
Common broadly defined media:
- Nutrient media – A source of amino acids and nitrogen (e.g., beef, yeast extract). This is an undefined medium because the amino acid source contains a variety of compounds with the exact composition being unknown. These media contain all the elements that most bacteria need for growth and are non-selective, so they are used for the general cultivation and maintenance of bacteria kept in laboratory-culture collections.
- Minimal media – Media that contains the minimum nutrients possible for colony growth, generally without the presence of amino acids, and are often used by microbiologists and geneticists to grow “wild type” microorganisms. These media can also be used to select for or against the growth of specific microbes. Usually a fair amount of information must be known about the microbe to determine its minimal media requirements.
- Selective media – Used for the growth of only selected microorganisms. For example, if a microorganism is resistant to a certain antibiotic, such as ampicillin or tetracycline, then that antibiotic can be added to the medium in order to prevent other cells, which do not possess the resistance, from growing.
- Differential media – Also known as indicator media, are used to distinguish one microorganism type from another growing on the same media. This type of media uses the biochemical characteristics of a microorganism growing in the presence of specific nutrients or indicators (such as neutral red, phenol red, eosin y, or methylene blue) added to the medium to visibly indicate the defining characteristics of a microorganism. This type of media is used for the detection and identification of microorganisms.
On the basis of Purpose:
- The Preservative Culture Media: Composed of all the basic nutrients required by the microbes to grow and preserve for a longer period of time. It lets grow the microbes safely providing an ensured environment to the microbes and protecting them against any environmental stress.
- The Enriched Culture Media: Contains additional nutrients in the form of egg yolks, blood and chocolate to grow nutritionally fastidious bacteria.
- The Selective Culture Media: It works on the principle of Differential Growth Suppression. It allows the growth of some microbes while suppressing the growth of others. For example, MacConkey’s medium allows growth of Enterobacteriaceae members only, while Mannitol Salt Agar is used to recover Staphylococcus aureus.
- The Enrichment Culture Medium: It is a broth medium that is usually used to just increase the amount of microbes. For example Alkaline Peptone Water (APW) is used to isolate pathogens from faecal specimens.
- The Differential/Indicator Medium: Contains dyes or metabolic substrates to recognise bacteria on the basis of their colony colour. For example on MacConkey’s agar medium the lactose fermenters produce pink colonies whereas the non-lactose fermenters produce colourless colonies.
On the basis of composition:
Synthetic Medium: It is a chemically defined medium that contains purified ingredients and all the chemical compositions of a synthetic medium are well known.
Non-Synthetic Medium: It is a chemically undefined medium that contains at least one of all the ingredients that is not purified or completely characterized or defined.
Media used in fermentation technology:
In fermentation technology the quality of fermentation media is crucial for the growth of microbes as it provides substrate for the product to be synthesized in the fermenter.
Fermentation media contains Major and Minor components. Major components includes Carbon and Nitrogen sources whereas Minor components includes anti-foaming agents, enzymes, buffers etc.
There are two types of fermentation media used:
- Synthetic Media: The components are well known and the composition of all the nutrients is pre-determined so the concentration can be controlled as per the requirement. It usually lacks proteins and peptides that helps reduce formation of foam that reduces the chances of contamination.
- Crude Media: It contains a roughly known composition and undefined sources of ingredients. It contains relatively high numbers of minerals, vitamins, anti-foaming agents, precursors, growth factors, nitrogen sources, carbon sources, inorganic nutrients and buffers.
Defined vs. undefined media
This is an important distinction between growth media types.
A defined medium will have known quantities of all ingredients. For microorganisms, it provides trace elements and vitamins required by the microbe and especially a defined carbon and nitrogen source. Glucose or glycerol are often used as carbon sources, and ammonium salts or nitrates as inorganic nitrogen sources.
An undefined medium has some complex ingredients, such as yeast extract, which consists of a mixture of many, many chemical species in unknown proportions. Undefined media are sometimes chosen based on price and sometimes by necessity – some microorganisms have never been cultured on defined media.
• Defined media is made from constituents that are completely understood.
• Undefined media has some part of which is not entirely defined.
• The presence of extracts from animals or other microbes makes a media undefined as the entire chemical composition of extracts are not completely known.