Author: Roshni Nepal
A culture media is a special medium used in microbiological laboratories to grow different kinds of microorganisms. A growth or a culture medium is composed of different nutrients that are essential for microbial growth.
Since there are many types of microorganisms, each having unique properties and requiring specific nutrients for growth, there are many types of media, based on what nutrients they contain and what function they play in the growth of microorganisms.
Classification of bacterial culture media
a. on the basis of consistency
- Solid medium
Solid medium contains agar (solidifying agent), at a concentration of (1.5-2.0)%. It has physical structure and allows bacteria to grow in physically informative or useful ways (e.g. as colonies or in streaks). Solid medium is useful for isolating bacteria or for determining the colony characteristics of the isolate. E.g. Nutrient agar
- Semi-solid media
Semisolid media are prepared with agar at concentrations of 0.5% or less. They have soft custard like consistency and are useful for the cultivation of microaerophilic bacteria or for determination of bacterial motility. E.g. SIM media
- Liquid (Broth) medium
no agar; medium serves various purposes such as propagation of large number of organisms, and various other tests. e.g. MR-VP broth.
b. on the basis of composition
- Synthetic or chemically defined medium
A chemically defined medium is one prepared from known mixture of chemical compounds like salts and sugars and therefore its exact composition is known. E.g. Peptone water (0.5% NaCl in water)
- Non synthetic or chemically undefined medium
Non-synthetic medium contains at least one component that is neither purified nor completely characterized. Often these are partially digested proteins from various organism sources. Nutrient broth, for example, is derived from cultures of yeasts.
c. on the basis of purpose/ functional use/ application
1. General purpose media/ Basic media
These are simple media that are generally used for the primary isolation of microorganisms. E.g. Peptone water, NB, NA
2. Enriched medium (Added growth factors)
Enriched media are used to grow nutritionally exacting (fastidious) bacteria. E.g. Blood agar, chocolate agar. Blood agar is prepared by adding 5-10% blood to a blood agar base.
3. Selective and enrichment media
They are designed to inhibit (stopping the growth) unwanted commensal or contaminating bacteria and help to recover pathogen from a mixture of bacteria. While selective media are agar based, enrichment media are liquid in consistency. Both these media serve the same purpose. Any agar media can be made selective by addition of certain inhibitory agents that don’t affect the pathogen of interest. Various approaches to make a medium selective include addition of antibiotics, dyes, chemicals, alteration of pH or a combination of these.
a. Selective medium
Selective medium is designed to suppress the growth of some microorganisms while allowing the growth of others. Selective medium are agar based (solid) medium so that individual colonies may be isolated.
Examples of selective media include:
Mannitol Salt Agar used to recover S.aureus contains 10% NaCl.
b. Enrichment culture medium
Enrichment medium is used to increase the relative concentration of certain microorganisms in the culture prior to plating on solid selective medium. Selenite F broth are used to recover pathogens from fecal specimens (Salmonella spp.)
4. Differential/ indicator medium:
Certain media are designed in such a way that different bacteria can be recognized on the basis of their colony colour. Various approaches include incorporation of dyes, metabolic substrates etc, so that those bacteria that utilize them appear as differently coloured colonies.
E.g. Mannitol salt agar (yellow)
5. Transport media:
Clinical specimens must be transported to the laboratory immediately after collection to prevent overgrowth of contaminating organisms or commensals. This can be achieved by using transport media. Such media prevent drying (desiccation) of specimen, maintain the pathogen to commensal ratio and inhibit overgrowth of unwanted bacteria.
E.g. Cary Blair transport medium is used to transport feces from suspected cholera patients.
6. Anaerobic media:
Anaerobic bacteria need special media for growth because they need low oxygen content and extra nutrients. Media for anaerobes may have to be supplemented with nutrients like hemin and vitamin K. Such media may also have to be reduced by physical or chemical means. Boiling the medium serves to expel any dissolved oxygen; then sealed with sterile liquid paraffin.
E.g. Robertson Cooked Meat (RCM) medium that is commonly used to grow Clostridium spps contains a 2.5 cm column of ox heart meat and 15 ml of nutrient broth. Unsaturated fatty acids present in meat utilize oxygen for auto-oxidation.