Media Types

Lecture 07 [Notes]
Types of Media

1. Synthetic (Chemically Defined) ­ - contains every essential nutrient as pure chemicals of known constitution in known amounts ­ usually of synthetic origin
2. Complex (Chemically Undefined) ­ - extracts of plants, animals, or microbes; an ill defined mixture containing the essential nutrients but in unknown amounts; malt, yeast, meat, potato (see Difco Manual)
3. Liquid (broth)
4. Solid ­ - addition of agar (dehydrated red algae), non­nutritive merely solidifies media; can be used for preparation of slants or deeps
5. Semi­solid ­ - used for motility studies, e.g. SIM media; prepared by varying concentration of agar

The growth of microbiology can be attributed in large part to a single development ­ advances in the quantitative and qualitative production of culture media. The following is a chronological breakdown of the development of various culture media.


The basal medium is, as the name implies, the most basic in content and universal in use. The first media were composed of chemically undefined meat and plant extracts mixed with agar and sugars. The introduction of synthetic, defined media was an important developmental step.
Defined media are solutions of simple chemical substances and the composition and amount of materials constituting such media is known exactly. These are particularly useful for the cultivation of autotrophic bacteria but are not in common use in the clinical bacteriology laboratory. Simple, semi­defined media such as peptone waters, are routinely used. The chemical composi- tion of semi­defined media is known approximately but is not exactly defined and minor variations in composition may occur.


These are simple, defined or semi­defined media to which other substances have been added to enhance bacterial growth. In most cases the additive is a natural product such as yeast extract, sterile blood or serum, or milk, potato, and tomato extracts. These additives cannot be chemically defined and the term "natural media" is sometimes used to describe media containing them.


Many clinical specimens contain a mixture of two or more microorganisms. The isolation and identification of any organism from a mixture of organisms may be carried out by using a medium which permits the growth of one type of organism only, inhibiting the growth of others. Such media are termed selective media and are simple or enriched media to which has been added a selective factor such as dyes or high salt concentration. Selective media may contain substances such as antibiotics, potassium tellurite, bile salts, or malachite green, all of which selectively inhibit some bacteria but allow the growth of others. Hoyle's tellurite medium is an example of a selective media. It contains potassium tellurite at a concentration that inhibits most bacterial species other than Corynebacterium. This medium is used in the isolation of diphtheria organisms.


The defined basal medium may be combined with biochemically reactive substrates and pH indicators. The action of the organism on this substrate allows the differentiation of the cultural characteristics of closely related organisms.
These media enable the study of biochemical properties of bacteria and thus aid in bacterial identification. The simplest type of indicator media are those used in the study of bacterial fermentation reactions. Such media usually consist of peptone waters, enriched if necessary with other essential nutrients, in which is incorporated an appropriate carbohydrate substrate and a Ph indicator. Fermentation of the carbohydrate source results in acid production which is indicated by a color change in the pH indicator. It is important to differentiate organisms which ferment carbohydrates to form acid only and those which form acid and gas (e.g. carbon dioxide and hydrogen). For this purpose a small Durham tube is inverted in the peptone water and gas, formed during the fermentation reaction, collects in the Durham tube displacing the medium. Further examples of indicator media are Christiansen's urea medium, which is used to study the ability of an organism to decompose urea, and litmus milk medium, which undergoes a variety of changes brought about by the ability of bacteria to ferment milk sugar (lactose) and digest milk proteins (casein).
Some media, such as EMB (Eosin Methylene Blue) and Endo Agar, are selective and differential. They inhibit the growth of Gram positive organisms and exhibit color differences among the Gram negative species which have grown on the media.

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