Hi readers, now its time to discuss on an important topic of microbiology. The topic is Culture media(medium) or growth media(medium) and types of culture media.
- Definition of culture medium
- Types of culture medium
- Composition of an ideal culture media
Culture medium definition:
The growth media or culture media is a liquid or gel which provides essential materials for the growth of microorganisms. There are different types of media for growing different types of cells.
The most common growth media for microorganisms are nutrient broth and agar plates. Some organisms, termed fastidious organisms, required specialised environments due to complex nutritional requirements, for example viruses are obligate intracellular parasites and require a growth medium containing living cells.
the most common growth media for microbes are liquid nutrient medium or lb medium (Lysogeny Broth). Agar is often mixed with liquid media and poured into Petridishes to solidify. It provides a solid medium on which microorganisms are cultured. This medium remain solid because very few bacteria are able to decompose agar.
Nutrient medium: It is a source of amino acids and nitrogen. The amino acid source contains a variety of compounds with the exact composition being unknown that is why it is termed as undefined medium. Nutrient media is non selective and possess all the the elements that most bacteria need for their growth. Bacteria kept in laboratory culture collection are cultivated and maintained in this medium.
An undefined medium which is also known as complex medium or basal medium contains a carbon source(example glucose), water, various salts.
A defined medium which is also known as synthetic medium or chemically defined medium is a medium in which all the chemicals used are known and no yeast, animal or plant tissue is present.
A differential medium is a medium that includes some sort of added indicator that allows for the differentiation of particular chemical reactions occurring during growth.
Types of culture medium:
- Minimal medium
- Selective medium
- Differential medium
- Transport medium
- Enriched medium
- Complete medium
Minimal media: minimal media is a type of media which contain the minimum nutrients possible for colony growth generally with absence of amino acids. It is used by geneticists and microbiologists for the growth of wild type microorganisms. Minimal medium typically possess: a carbon source;it may be a sugar like glucose or less energy rich source like succinate, various salts which provide essential elements like magnesium, nitrogen, phosphorus and sulphur to allow the bacteria to synthesise protein and nucleic acid, water etc.
Selective Media: It is used for the growth of only selected microbes. For example if a microorganism is resistant to a certain an antibiotic, such as ampicillin or tetracycline, and then that antibiotic can be added to the medium in order to prevent other cells which do not possess the resistance from growing. It is also used in cell culture for the survivality of cells with certain properties,such as antibiotic resistance or the ability to synthesise a certain metabolite.
Example: Mannitol salt agar(MSA): it is selective for gram positive bacteria. Mac Conkey agar(MCA) it is selective for gram negative bacteria. Eosin methylene blue(EMB) selective for gram negative b
Differential media or indicator media is used to distinguish one microorganism type from another growing on the same medium. This type of media uses biochemical features of a microbes growing in the presence of specific nutrients or indicator such as methylene blue added to the medium to visibly indicate the definiting characteristics of a microorganism. Molecular biologist also use this type of medium for the detection of recombinant strains of bacteria and detection of microorganisms.
Example: blood agar,it contains bovine heart blood that becomes transparent in the presence of haemolytic Streptococcus. Eosin methylene blue(EMB), it is differential for lactose and sucrose fermentation. MacConkey ,it is differential for lactose fermentation. Mannitol salt agar(MSA) ,it is differential for mannitol fermentation. X-gal plates, it is differential for lac operon mutants.
Transport medium are used for temporary storage of specimen being transported to the laboratory for cultivation. It contents only buffers and salt. There is lack of carbon nitrogen and organic growth factors to prevent microbial multiplication.
Example: Thioglycolate broth,it is used for strict anaerobes.
The nutrients required for the growth of microorganisms are present in enriched medium. It is used to harvest as many different types of microbes as a present in the specimen.
Example: Blood agar is an example of Enriched media. it contain nutritionally rich whole blood supplements. Chocolate agar, it is enriched with heat treated blood which turns Brown and give the medium the colour for which it is named.
It is a type of culture medium where all the nutrients (like amino acids ,vitamins , minerals etc) required for the proper growth of microbes are present. Both wild and mutant type of microorganisms are cultured in this medium.
Composition of ideal culture medium:
1) Nutrient agar medium composition: Beef extract- 0.3gm(mineral and carbohydrate),NaCl- 0.5gm( electrolyte), Distilled water- 100ml, peptone- 0.5gm(protein and nitrogen source), Agar- 1.5gm(solidifying agent), pH- 7
2) LB broth: it is also known as Luria broth or Luria- Bertani medium or lysogeny broth. It possess peptone( the enzymatic digestion product of casein), yeast extract and NaCl. Peptone is casein digested by pepsin.
3) SOB medium: It is also known as super optimal broth or Hanahan’s broth. It contains peptone(twice as much LB broth contains). It is rich in amino acids and peptides. It also contains magnesium that is required for high density cell growth.
4) SOC medium: It is also known as super optimal broth with catabolic repressor. It contains all the nutrients that SOB has. Apart from this it contains glucose. Glucose acts as catabolic repressor.
5) TB medium: It is also known as terrific broth. It contains 20% more peptone and 380% more yeast extract than LB and 0.4% glycerol as an extra carbon source.
6) SB medium: It is also known as super broth. It has 220% more peptone and 300% more yeast extract than LB. It is super rich in peptone and yeast extract.