The following points highlight the top three types of Culture Media. The types are: 1. Physical Type of Culture Media 2. Chemical Types of Culture Media 3. Function Types if Culture Media!
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Culture Media: Type # 1. Physical Type of Culture Media:
Liquid, semisolid and solid media are routinely used for growth of micro-organisms.
(i) Liquid Media:
It is also called as broth and contains only dissolved nutrients in water. Liquid media are used for growth of pure batch cultures for fermentation studies and various other purposes while solidified media can be used widely for isolation of pure cultures for estimating viable microbial population and a variety of other purposes.
The usual gelling agent for solid or semisolid medium is agar, a hydro colloid derived from red algae. Agar is used because of its unique physical properties. One is that it melts at about 96°C and remains liquid until cooled to 40 to 45°C. Thus after being melted in boiling water, it can be cooled to a temperature that is tolerated by human hands as well as microbes.
Finally agar is excellent hardening agent because most microorganisms cannot metabolize it. It can be aided in light medial at consequence 1.5 to 2.0 permuted. Silca gel sometimes can also be substituted for agar as solidifying agent. Silca gel is made from silic acid and gel formed from this acid is completely resistant to microbial breakdown.
Culture Media: Type # 2. Chemical Type of Culture Media:
A medium in which the exact chemical composition is known is called chemically defined (synthetic) medium. It can be in liquid form (broth) or solidified by agents such as agar. Chemically defined media are often used for cultivation of autotrophs and are also useful.
Simple peptone water, medium, 1% peptone with 0.5% NaCI in water may be considered semisynthetic medium since its composition is approximately known.
For defining the nutritional requirement of heterotrophs. Typically they contain simple sugar as the carbon and energy source, an inorganic Nitrogen source, various mineral salts and if necessary growth factors (purified amino acids vitamins, purine and pyrimidines).
A media in which exact chemical composition are not known is called complex (non-synthetic medium).
It usually contains complex material of biological origin etc. such as blood or milk or peptone yeast extract and beef extract. Complex media provide full range of growth factor that may be required by an organism so they may be used to cultivate unknown microorganisms or whose nutritional requirement is complex (i.e., organism that requires lot of growth factor).
This is the situation with fastidious organisms that have complex nutrition requirement; they may even require a medium containing blood or serum. Potato extract agar, Sail extract agar, Oatmeal agar, nutrient broth and tryptic soya broth are commonly used complex media after cultivation of heterothophic microorganism (table).
Culture Media: Type # 3. Functional Type of Culture Media:
No single medium or set of conditions can support the growth of all the different types of organisms that occur in nature. To cultivate, recognize, enumerate and isolate certain types of microorganism many special purpose media are needed.
On the basis of their application and functions these media are classified into following types:
Selective media provide nutrients that enhance growth and predominance of particular type of microorganism and suppresses all other microorganisms that are present in culture. This medium is best used for isolating specific organism from a mixed natural population.
For instance, cellulose utilizing micro-organism alone will grow in medium containing only cellulose as a carbon and energy source. Endo agar, eosin methylene blue agar and
Mac Conkey agars are three media widely used for detection of E. coli and related bacteria in water supply.
These media contain dyes that inhibit the growth of gram +ve bacteria but allow gram negative bacteria to grow. Physical condition such as pH and temperature also used to render its selectivity for growth of microorganism in media. As an example, gonorrhea causing Nosseria gononhoeae, can be grown and isolated from medium containing certain antibiotics. Thus antibiotics exhibit growth of contaminating bacteria.
(i) Differential Media:
Differential media is used for the differentiation of various kinds of microorganisms on the basis of appearance on the medium and even permit tentative identification of microorganism. These media allow certain microorganisms to produce macroscopically distinct colonies or characteristic zone around colonies, which are differentiating these organisms from others in the samples.
This medium contains certain indicator or reagent or supplement which may allow such differentiation. For example, if a mixture of bacteria is inoculated on to a blood-containing agar mediums (blood agar), some of bacteria may hemolyze (destroy) the red blood cells, others do not. Thus we can distinguish between hemolytic and non-hemolytic bacteria on the same medium.
Mac Conkey’s agar, contains lactose and a dye which turns when pH drops below. Any microorganism that can ferment lactose produces an acid end product that Lowers the pH and causes colony to turn red. Microorganism that fails to ferment lactose produces colour-less colonies. Dye in Mac Conkye’s agar also inhibits the growth of gram +ve bacteria. This medium is, therefore, both selective and differential.
(ii) Enrichment Medium:
Enrichment medium is that in which nutritional environment is adjusted in such a manner as to enhance selectively the growth of certain bacterial type with in a gives mixed inoculum. For instance addition of extract of plant and animal tissue to nutrient broth and nutrient agar media provides additional nutrient and media starts favouring the growth of fasticious herotropic bacteria.
In the case of delicate micro-organisms like goner which may not survive the time taken for transporting the specimen to the laboratory or may be overgrown by non-pathogens (such as dysentery or cholera organism in feces), special media are devised for transporation of the specimens.
These are termed transport media for example, Stuart’s medium a non-nutrient soft agar gel containing a reducing agent to prevent oxidation, and charcoal to neutralize acetone bacterial inhibitors for gonococci and buffer acid glycol saline for enteric bacilli.