Culture media may be classified in different ways:
Types of Culture Media
(A) Depending on the physical state (media consistency):
- Solid media (1.5 to 2.5% agar) e.g. Nutrient agar.
- Semisolid media (0.2 to 0.5% agar) e.g. Nutrient broth containing 0.5% agar.
- Liquid media (absence of agar) e.g. Fluid thioglycollate broth.
(B) Depending on oxygen requirement:
- Aerobic media e.g. MacConkey’s broth.
- Anaerobic media e.g. Robertson’s cooked meat medium.
(C) Depending on chemical composition:
- Simple or basal media.
- Synthetic or defined media.
- Non-synthetic or undefined or complex media.
(D) Depending on functional type:
- Enriched media
- Enrichment media
- Selective media
- Indicator media
- Differential media
- Sugar media
- Transport media
- Assay media
- Storage media
Simple or basal media: These media includes peptone water and nutrient broths which form the basis of most media used in the study of common bacteria. The addition of 2% agar to nutrient broth forms a nutrient agar medium which is a solid basal medium.
Complex or non-synthetic or undefined media: Complex media usually contains complex materials of biological origin such as blood or milk or yeast extract or beef extract. The exact chemical composition of this media is not known but it provides all the growth factors for the cultivation of unknown bacteria. These media may be added ingredients for special purposes for the growth of specific microorganisms.
Synthetic or defined media: These media are prepared from pure chemical substances and the exact composition of the medium is known. These media are used for research purposes as well as metabolic studies of different microorganisms.
(i) Enriched media: These enriched media are prepared for fastidious microorganisms by the addition of substances such as blood, serum and egg to basal medium. e.g. blood agar (Streptococcus), chocolate agar (Neisseria, Haemophilus), Loeffler serum slope (Corynebacterium diphtheriae).
(ii) Enrichment media: When a specific substance is added in a liquid medium which inhibits the growth of unwanted bacteria and favours the growth of wanted bacteria, such a media is called enrichment media e.g. tetrathionate broth, selenite F broth. These media inhibits coliforms (e.g. Escherichia coli) by allowing the growth of pathogenic cultures (e.g. Salmonella species, Shigella species) from faeces.
(iii) Selective media: This is like enrichment media but is in a solid form. When a substance is added to a solid medium which inhibits the growth of unwanted bacteria but permits the growth of required bacteria in the form of colonies is known as a selective medium. Physical conditions (temperature, pH, gaseous) of a culture media may be adjusted and made selective for the growth of specific microorganisms. e.g. MacConkey’s media (Escherichia coli). It contains sodium taurocholate (bile salt) which selectively allows the growth of Gram-negative bacteria by inhibiting Gram-positive bacteria, Deoxycholate citrate agar, DCA (Salmonella and Shigella species), Lowenstein and Jensen medium (Mycobacterium tuberculosis).
(iv) Indicator media: These media contain an indicator that changes colour when a bacterial species grows in them. e.g. Wilson and Blair medium, Salmonella typhi reduces sulphite to sulphide in the presence of glucose and the colonies have a black metallic shine.
(v) Differential media: A differential medium is used to distinguish between different types of bacteria based on some observable characteristics. e.g. MacConkey’s medium (contains peptone, lactose, neutral red and taurocholate). It shows lactose fermenter as red colonies while non-lactose fermenter as white or pale colonies. MacConkey’s medium is also called an indicator medium or selective medium. Blood agar medium is an enriched medium but bacteria lysing red cells (haemolytic) show a clearing around their colonies. Thus, it is an indicator medium or differential medium.
(vi) Sugar media: In a sugar medium, 1% sugar is added to peptone water with a suitable indicator. Durham’s tube (a small tube) is kept inverted in the tube containing this medium to detect gas production.
(vii) Transport media: There are delicate micro-organisms like Neisseria gonorrhoeae which may not survive the time taken for transporting the specimen to the laboratory or may be overgrown by non-pathogens (Escherichia coli) and pathogens (Shigella and Salmonella species). Special media are devised for such types of delicate microorganisms which are called transport media e.g. Stuart’s transport medium, and Amies transport medium.
(viii) Assay media: These media have specific compositions and are used for the assay of antibiotics, amino acids and vitamins. Media containing specific components are also used for testing disinfectants.
(ix) Storage media: These media help in the preservation and storage of bacteria for long periods e.g. Dorset’s egg medium, nutrient agar stabs, blood agar slants Robertson’s cooked meat medium etc.
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