Microbiology

Bacteriological Media

As we know there are microorganisms all around us. Majority of them is of bacteria. To study them properly and to understand them it is required to cultivate them artificially in the lab. Obviously many bacteria have some certain growth requirements towards their growth. So, it become important to keep different cultural media are required for different bacteria.

Chemically defined media are needed for the cultivation of autotrophs and also useful for defining the nutritional requirements of heterotrophs.

However, for routine cultivation of heterotrophs, chemically defined media are not generally used. Instead, certain complex raw material such as peptones, meat extracted, and yeast extract are used. And resulting media support the growth of a wide variety of heterotrophic bacteria. Agar is used as a non nutritional component to make solid/ semisolid media. Nutrient broth and nutrient agar is the most commonly used media to grow a wide range of heterotrophic bacteria in liquid and solid media respectively. Addition of yeast extract to the above media can further improves it’s nutritional value as yeast extract contain several of the B vitamin and other growth promoting substances. 

Other complex supplements such as bovine rumen fluid, animal blood, blood serum, or extract of plant and animal tissue may be required for the cultivation of certain fastidious heterotrophs bacteria.

Types Of Media

Once it come to grow only certain strain of bacteria for further examination or studying it’s properties many special purpose media are needed. By keeping it in mind microbiologist have developed many types of bacteria those are required or used to meet the nutritional requirements of different types of bacteria. On the basis of their application and function, culture media can be classified as follow:

Selective Media

This type of media enhance the growth of certain type of bacteria and do not enhance (and may even inhibit) other type of organisms that may be present. For example, a media in which cellulose is only carbon source enhance the growth of cellulose consuming bacterial and not other.

For instance if an inoculum is inoculated from soil to the media, it will selectively promote the growth of cellulose eating bacteria. Further selection can be done by indusing some type of antibiotics to the media, which will kill all other type of bacteria but enhance or ineffective against some bacteria.

Differential Media

To get a differential media certain reagents or supplements, were incorporated into culture media, this help in differentiation of various kind of bacteria in the same culture.

For example, if a mixture of bacteria inoculated on to a blood agar medium, some of the bacteria may hemolyze (destroy) the red blood cells. Other do not do so. This we can distinguish between hemolytic and nonhemolytic bacteria on the same medium.

Assay Media

Media of prescribed composition are used for the assay of vitamin, amino acids, and antibiotics. Media of special composition are also used for the testing disinfectant.

Media For Enumeration Of Bacteria

Specific kind of media are used for determining the bacterial content of such materials as kill and water.

Media For Characterization Of Bacteria

A wide variety of media are conventionally used to determine the type of growth produced by bacteria, as well as to determine their ability to produce certain chemical changes.

Maintenance Media

Satisfactory maintenance of the viability and physiological characteristics of a culture over time may require a medium different from that which is optimum for growth. Prolific, rapid growth may also be associated with rapid death of the cells at the time of growth phase. For example, glucose in a medium frequently enhance growth, but acid harmful to the cells is likely to be produced. Therefor, omission of the glucose is preferable in a maintenance medium.

Solid And Semisolid Media

In addition to liquid medium, solid and semisolid media are also frequently used for the cultivation of bacteria. Solid media is ideal if we want to isolate the bacteria or studying it’s colony characteristics. 

Widely used solidifying agent in media is agar. Agar is nonreactive and can easily solidify at room temperature so it is more widely used in concentration of 1.5 to 2.0 percent form.

Addition to agar Silica gel is some times used as an inorganic solidifying agent for autotrophic bacteria.

Semisolid media, prepared with agar at concentration of 0.5 percent or less, have a soft, custardlike consistency and are useful for the cultivation of microaerophilic bacteria or for determination of bacterial mortality. 

Reference: Microbiology Palzar

Nihal Sharma

Assistant Editor

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