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.
BASAL 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.
ENRICHED MEDIA
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.
SELECTIVE MEDIA
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.
DIFFERENTIAL (INDICATOR)MEDIA
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.
Back to Homepage
Back to Menu Page
Back to Microbiology Start Page
