Culture media, also known as growth media, are specific mixtures of nutrients and other substances that support the growth of microorganisms such as bacteria and fungi (yeasts and molds).
A growth medium or culture medium is a solid, liquid or semi-solid designed to support the growth of microorganisms or cells, or small plants like the moss Physcomitrella patens. Different types of media are used for growing different types of cells.
- Solid Media:
- With agar (1-2% agar).
- Solid media can be:
- Slant: a tube containing solid media that was left to solidify at an angle. Used to keep the bacteria for long period of time (3 months)
- Deep agar: agar solidified at bottom of tube. Used to keep the bacteria for long time (6 months or more).
- Plate: used mostly to culture organisms, and to get pure culture of bacteria (isolated colony).
- Semi-solid media:
- Contains less agar than solid media (0.5% agar).
- Used as transport media, and for motility and biochemical tests.
- Liquid media:
- Called: broth.
- Without agar (solidifying agent).
- Used to grow bacteria in large quantity.
- Growth of bacteria—–> turbidity
- No growth —-> clear
On Chemical Composition:
- Routine Laboratory Media: These are classified into six types:
- Basic media:
Basac media are those that may be used for growth (culture) of bacteria that do not need enrichment of the media. Examples: Nutrient broth, nutrient agar and peptone water. Staphylococcus and Enterobacteriaceae grow in these media.
- Enriched media:
The media are enriched usually by adding blood, serum or egg. Examples: Enriched media are blood agar and Lowenstein-Jensen media. Streptococci grow in blood agar media.
- Selective media:
These media favour the growth of a particular bacterium by inhibiting the growth of undesired bacteria and allowing growth of desirable bacteria. Examples: MacConkey agar, Lowenstein-Jensen media, tellurite media (Tellurite inhibits the growth of most of the throat organisms except diphtheria bacilli). Antibiotic may be added to a medium for inhibition.
- Differential media:
An differential is included in the medium. A particular organism causes change in the indicator, e.g. blood, neutral red, tellurite. Examples: Blood agar and MacConkey agar are differential media.
- Transport media:
These media are used when specie-men cannot be cultured soon after collection. Examples: Cary-Blair medium, Amies medium, Stuart medium.
- Storage media:
Storage Media used for storing the bacteria for a long period of time. Examples: Egg saline medium, chalk cooked meat broth
- Basic media:
- Synthetic Media: These are chemically defined media prepared from pure chemical substances. It is used in research work.
COMMON MEDIA IN ROUTINE USE:
- Nutrient Broth:
500 g meat, e.g. ox heart is minced and mixed with 1 litre water. 10 g peptone and 5 g sodium chloride are added, pH is adjusted to 7.3. Uses:
- As a basal media for the preparation of other media
- To study soluble products of bacteria.
- Nutrient Agar:
It is solid at 37°C. 2.5% agar is added in nutrient broth. It is heated at 100°C to melt the agar and then cooled
- Peptone Water Agar:
Peptone 1% and sodium chloride 0.5%. It is used as base for sugar media and to test indole formation.
- Blood Agar:
Most commonly used medium. 5- 10% defibrinated sheep or horse blood is added to melted agar at 45-50°C. Blood acts as an enrichment material and also as an indicator. Certain bacteria when grown in blood agar produce haemolysis around their colonies. Certain bacteria produce no haemolysis. Types of changes :
- beta (β) haemolysis. The colony is surrounded by a clear zone of complete haemolysis, e.g. Streptococcus pyogenes is a beta haemolytic streptococci.
- Alpha (α) haemolysis. The colony is surrounded by a zone of greenish discolouration due to formation of biliverdin, e.g. Viridans streptococci.
- Gamma (γ) haemolysis, or, No haemolysis. There is no change in the medium surrounding the colony.
- Chocolate Agar or Heated Blood agar:
Prepared by heating blood agar. It is used for culture of pneumococcus, gonococcus, meningo- coccus and Haemophilus. Heating the blood inactivates inhibitor of growths.
- MacConkey Agar:
Most commonly used for enterobac-teriaceae. It contains agar, peptone, sodium chloride, bile salt, lactose and neutral red. It is a selective and indicator medium :
- Selective as bile salt does not inhibit the growth of enterobactericeae but inhibits growth of many other bacteria.
- Indicator medium as the colonies of bacteria that ferment lactose take a pink colour due to production of acid. Acid turns the indicator neutral red to pink. These bacteria are called ‘lactose fermenter’, e.g. Escherichia coll. Colourless colony indicates that lactose is not fermented, i.e. the bacterium is non-lactose fermenter, e.g. Salmonella. Shigella, Vibrio.
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