Culture media

A culture medium or a growth medium is a solid, liquid or semi-solid preparation that is used to support the growth, transport and storage of microorganisms. A medium must includes all the nutrients that are essential for the growth of microorganism like source of energy, nitrogen, carbon, phosphorus, sulfur and other minerals in precise composition. Different type of organisms requires different type of media. Knowledge of the normal habitat of microorganism will be an advantage in selecting culture media as its natural surrounding reflects its nutrient necessity. Basically, a medium is used to grow and maintain a particular microorganism and to study them.

Culture media are generally divided into two groups on the basis of the character of the compounds that are building up their composition:

Synthetic media

Synthetic media or Defined media are one in which all the components are exactly known. They are generally made up of inorganic salts with some organic compounds. This kind of media is used to know the nutritional requirement of organism i.e. to know which organic or inorganic compound is required for the growth or to study the effect of any particular compound. For example, cyanobacteria can be grown on a synthetic media containing CO₂ as a carbon source, nitrate or ammonia as nitrogen source, phosphate, sulfate and other variety of minerals.

Nonsynthetic media

Non synthetic media or Complex media are one in which exact composition of the compounds are not known. They contain some of the ingredients of unknown chemical composition. They may contains compounds like beef extract, yeast extract, peptone, blood etc. Complex media are very useful as because a single media can meet the nutritional requirement of many different microorganism. Also, it to difficult to construct a defined media if the nutritional requirement of a particular microorganism is unknown. In such case, complex media are very useful. One disadvantage is that it is impossible to construct two similar lots of same media from different batches of ingredients. Some commonly used complex media are nutrient broth, tryptic soy broth and MacConkey agar.

On the basis of physical type, culture media can be liquid, semi-solid or solid:

Liquid media:

Liquid media or nutrient broth contains all the necessary nutrients in dissolved form. They are used to grow large number of organisms and to grow pure batch culture for fermentation study.

Semi-solid media:

They are prepared by addition of agar (a complex polysaccharide from red algae) to nutrient broth. They contains agar at a concentration of 0.5% or less than that. They are useful for the growth of microaerophilic bacteria. This media is also used to study bacterial motility.

Solid media:

Liquid media containing high concentration of agar (1.5 to 2.0 %) forms solid media. They have physical structure and allows bacteria to grow as colony or in streaks. They are useful in isolating bacteria.

Media are also classified on the basis of functional use:

General purpose media or basic media

Media that support the growth of many microorganisms are known as general purpose media or basic media. For instance: tryptic soy broth, nutrient agar. These type of media helps in the primary isolation of organisms.

Enriched media

When special nutrients are added in general purpose media to support the growth of particular microorganism, they forms enriched media. For example, blood is added to basic media to support the growth of fastidious organisms. Blood agar, chocolate agar etc. are few of the enriched media.

Selective media

This type of media favors the growth of particular microorganism and inhibits the growth of unwanted organism. Bile salts or dyes like basic fuchsin supports the growth of Gram negative bacteria and suppresses the growth of Gram positive ones.

Organisms may also be selected by adding that nutrient in the medium, which only they can use. For instance, a medium containing only cellulose as a carbon source will allow only cellulose-digesting bacteria to grow.

Some examples of selective media are Eosin Methylene Blue (EMB) agar, Mannitol Salt agar, MacConkey agar and Phenylethyl Alcohol (PEA) agar. Mannitol salt agar is selective because it contains 7.5 % sodium chloride that promotes the growth of some microbes while inhibiting the others.

Differential media

This media distinguishes between different groups of bacteria. Blood agar is a differential media as it distinguishes between hemolytic and nonhemolytic bacteria. Clear zones are produced around the colonies of hemolytic bacteria due to the destruction of red blood cells. Another example is MacConkey agar that distinguishes between lactose fermenting and nonfermenting bacteria.

Note: Blood agar is both differential and enriched media. MacConkey agar, EMB agar, Mannitol Salt agar are both differential and selective media.