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Simple Media
Nutrient broth
Nutrient agar
Complex medium
Synthetic media
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The culture media so far discussed are all made up of components of variable composition such as meat infusion, liver extract, beef, serum, other body fluids etc.
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They are called non synthetic media. Synthetic media with defined composition of chemicals are used primarily in research.
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These media have the advantage that whenever they are prepared, their composition is the same.
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Therefore the results of microbial action of these media in one laboratory are strictly comparable with the results of other laboratories thousands of miles away.
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Simple synthetic media contain a carbon energy source such as glucose or lactose and inorganic source of nitrogen usually in the form of NH4 salts and various others required inorganic salts dissolved in buffered aqueous solution.
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Complex synthetic media incorporate in addition certain aminoacids, purines, pyrimidines etc., serving as special growth factors.
Enriched medium
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By definition an enriched medium is the one containing the basic nutrient medium to which serum, blood or any one of a number of growth factors may be added.
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The addition of nutritive supplement enhances the growth of the desired microorganisms, which will not grow otherwise.
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Example. Blood broth, serum agar, egg yolk agar, Mueller Hinton agar etc.
Enrichment medium
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The isolation of some pathogenic bacteria that may be overshadowed by presence of commensals of the normal intestinal tract required the use of enrichment medium.
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This medium chemically inhibits or even kills the undesired normal flora but encourages a profuse growth of the desired pathogens.
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Eg. Tetrathionate broth, selenite broth – both are used primarily in isolation of Salmonella.
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Though by function enrichment media are similar to selective media, enrichment media are generally liquid and selective media.
Selective media
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Those media by virtue of their complexity of composition promote the growth of one organism and retard the growth of others are called selective media.
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Selective media may also differentiate among certain genera and microorganisms.
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Ex. Desoxycholate citrate agar and Lowenstein – Jensen medium.
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Mannitol Salt agar with 7.5% NaCl for Staphylococcus sps.
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Desoxycholate citrate agar inhibits the growth of most coliforms alongwith many strains of Proteus, while favouring the selective isolation of intestinal pathogens especially those of Salmonella and Shigella sp.
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The coliform colonies that are sometimes appearing on the medium are readily separated from the non-fermentation of lactose by their colour.
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Lowenstein-Jensen medium promote the growth of tubercle bacilli and retards the growth of any other organisms present.
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Although MacConkey and Eosin Methylene Blue (EMB) agar are primarily differential media, they display a selective function that they allow the growth of certain bacteria but retard the growth of other bacteria.
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Selective and differential media are of great value in the diagnosis of infections involving intestinal and respiratory tracts, whereas normally, a variety of organisms reside and the presence of pathogenic bacteria may not disturb the normal behavioral pattern of the normal bacteria of the area.
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In test specimens taken from the area, the pathogens tend to be mixed with many other microorganisms.
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Only with the use of selective & differential media, it is possible to isolate and identify the disease producers.
Differential Media
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Medium, that by virtue of their ingredients distinguishes organisms growing together eg. MacConkey agar, Eosin Methylene blue agar (EMB) used in the differentiation of entero bacteria from the intestinal tract.
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In MacConkey agar, incorporation of lactose makes possible a sharp separation in colony characteristics between the organisms those ferment lactose and those do not ferment lactose.
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The addition of neutral red (pH indicator) helps to give colour to the colonies of bacteria, which are lactose fermenters.
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Hence colonies of lactose fermenters are deep red or pink in colour and the colonies of lactose non-fomenters are colourless.
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The inhibitory action of crystal violet on the growth of Gram positive organisms allows for the isolation of Gram negative organisms.
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This is important because generally pathogens in intestinal tract such as Salmonella & Shigella sp. do not ferment lactose whereas certain of the normal inhabitants like coliforms do ferment lactose.
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In Eosin-methylene blue agar, the Lactose and the dyes eosin and Methylene blue permit differentiation between enteric lactose fomenters and non fermentors as well as identification of the colon bacillus E.coli.
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The E.coli colonies are blue-black with a metallic green sheen caused by the large quantity of acid that is produced and that precipitates out the dyes onto the growth’s surface.
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Other coliform bacteria such as Enterobactor aerogenes produce thick, mucoid, pink colonies on this medium.
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Enteric bacteria that do not ferment lactose produce colourless colonies, which, because of their transparency, appear to take on the purple colour of the medium.
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This medium is also partially inhibitory to the growth of Gram-positive organisms, and thus Gram-negative growth is more abundant.
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The medium that contains indicators or dyes are generally called indicator media. MacConkey and EMB are also called as indicator media.
Indicator media
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This media contain an indicator which changes colour when bacteria grow with them.
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Eg. Incorporation of sulphide in “Wilson and Blairs medium”.
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Salamonella typhi reduces sulphide to sulphate in the production of glucose and Salmonella typhi appear as black metallic sheen.
Sugar Media
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Usually sugar media consists of sugar concentrated in peptone water along with appropriate indicator.
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The sugar denotes any fermentable substance.
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A small tube called, “Durham’s tube” is kept inverted in sugar tube to identify gas production.
Transport Media
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A transport medium often a liquid, maintains the viability and infectivity of microorganisms while the specimens in it are carried to the laboratory where it can be processed.
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Such a medium must be just sufficent that the specimen in it must not be allowed to dry.
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Its chemical composition must prevent deterioration of any pathogen present.
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If anaerobes are to be sent through transport medium, they should be sheltered from atmospheric oxygen.
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A number of transport media have been deviced for anaerobes, delicate organisms and viruses.
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Eg. Stuart’s transport medium.
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Hanks Balanced salt solution for anaerobe, double stoppered tubes or vials that have been gassed out with oxygen or the medium should contain a layer of liquid paraffin.
Anaerobic Media
Example:
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