Preparation of Selective and Differential Media –  Amrita University

Preparation of Selective and Differential Media – Amrita University

Preparation of Selective and Differential
media A culture medium is a solid or liquid preparation
used to grow, transport, and store microorganisms. To be effective, the medium must contain all
the nutrients the microorganism requires for growth. Specialized media are essential in
the isolation and identification of microorganisms, the testing of antibiotic sensitivities, water
and food analysis, industrial microbiology, and other activities. In addition to nutrients
necessary for the growth of all bacteria, special-purpose media contain one or more
chemical compounds that are essential for their functional specificity.These includes
Selective media, Differential media and Enriched media Selective media allow certain types of organisms
to grow, and inhibit the growth of other organisms. selective inhibition of some types of microorganisms
can be achieved by adding dyes, antibiotics, salts or specific inhibitors which affect
the metabolism or enzyme systems of the organisms. Examples are Eosine Methylene Blue (EMB) agar,
Mannitol salt agar, Hektoen enteric agar (HE), Phenylethyl alcohol agar
Differential media are used to differentiate closely related organisms or groups of organismsEg;
MacConkey agar, Eosine Methylene Blue (EMB) agar Materials required:
Bacterial cultures inoculating loop
differential and selective media’s which include
Mannitol salt agar Macconkey agar
Eosine methylene blue agar Hektoen enteric agar Procedure:
Arrange all the cultures and media in the laminar air flow
Flame the inoculating loop and Transfer a loopful of culture from the tube labelled
Escherichia coli on to an EMB plate and perform a continous streak.
Similarly to a second plate of EMB transfer a loopful of culture of Enterobacter aerogenes
by continous streaking. Similar procedure was repeated in all selective
and differential media for differentiating their growth characterestics of the organisms
used. All the plates were incubated aerobically
at 370c for 24 hours . Results:Mannitol salt agar is used for the
isolation of pathogenic Staphylococci from mixed cultures.
Staphylococcus aureus was grown in one plate of MSA and into a second plate Staphylococcus
epidermidis was grown. Staphylococcus aureus ferments mannitol, thereby changing the colour
of the medium from red to yellow. Staphylococcus epidermidis- produce colonies with red or
purple zones without mannitol fermentation. Eosin methylene blue agar is both a selective
and differential medium used for the detection and isolation of Gram-negative intestinal
pathogens. In Eosine methylene blue agar , Escherichia
coli- produce dark blue-black colonies with metallic green sheen indicating vigorous fermentation
of lactose and acid production. Enterobacter aerogenes- produce blue, dark-centered,
mucoid colonies indicating lactose fermentation and acid production. Macconkey agar, is selective for Gram negative
bacteria and can differentiate those bacteria that are able to ferment lactose .
Escherichia coli are Lactose fermenters and thereby appear pink
while Pseudomonas aeruginosa are Non lactose fermenters and appear colourless or Golden
colored. Hektoen enteric agar: Hektoen Enteric (HE)
Agar is a moderatively selective medium used for the isolation and cultivation of gram-negative
enteric microorganisms, especially Shigella and Salmonella from a variety of clinical
and nonclinical specimens. On HEA agar Shigella species develop into
green-colored colonies with darker blue-green centers. Salmonella species appear as blue-green
colonies with or without black centers.


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