From Wikipedia,
the free encyclopedia.
Growth media are used to grow
microorganisms or
cells in
culture. There are different
sorts of media for growing
different sorts of cells. The
biggest difference in growth media
are between those used for growing
cells in culture (cell
culture uses specific cell
types derived from plants or
animals) and those used for
growing microorganisms (usually
bacteria or
yeast). These differences
arise due to the fact that cells
derived from whole organisms and
grown in culture are often
incapable of growth without the
provision of certain requirements,
such as
hormones or
growth factors which usually
occur
in vivo. In the case of
animal cells these requirements
are often provided by the addition
of
blood serum to the medium.
These media are often red or pink
due to the inclusion of
pH indicators. In the case of
microorganisms, there are no such
limitations as they are often
single cell organisms. One other
major difference is that animal
cells in culture are often grown
on a flat surface to which they
attach, and the medium is provided
in a liquid form, which covers the
cells. Bacteria such as
Escherichia coli (E.
coli, the most commonly used
microbe in laboratories) may be
grown on solid media or in liquid
media, liquid nutrient medium is
commonly called nutrient broth.
The most common growth media for
microorganisms are nutrient broth
or
Luria-Bertani medium (L-B
medium). Bacteria grown in liquid
cultures often form
colloidal
suspensions. When
agar (a substance which sets
into a
gel) is added to a liquid
medium it can be poured into
petri dishes where it will
solidify (these are called
agar plates) and provide a
solid medium on which microbes may
be cultured.
Another important distinction
between different growth media is
that of a defined and an undefined
medium. A defined medium will have
known quantities of all
ingredients, for microorganisms
they consist of providing trace
elements, any vitamins required by
the microbe and especially a
defined
carbon source and
nitrogen source (for example
glucose or
glycerol are often used as
carbon sources,
ammonium
salts or
nitrates as
inorganic nitrogen sources),
minimal media are defined
media. Nutrient media are not
defined media as they contain
ingredients such as
yeast extract which vary in
composition depending on the
source. This definition applies to
cell culture media as well, where
any medium containing, for
example, animal blood serum is
undefined, as the composition of
the serum will vary from supplier
to supplier and
batch to batch.
A good example of a growth
medium is the
wort used to make
beer. The wort contains all
the nutrients required for yeast
growth, and under
anaerobic conditions alcohol
is produced. When the
fermentation process is
complete the microbe is removed
from the medium which is now beer
and ready for consumption.
Some organisms require
specialized environments, for
example, since viruses are
obligatory intracellular
parasites, they require a
growth medium composed of living
cells. Often special media are
required for microorganism and
cell culture growth.
Selective media are used to
grow cells which possess a desired
selectable trait. For example if a
microorganism is resistant to a
certain
antibiotic, then that
antibiotic can be added to the
medium in order to prevent other
cells, which do not possess the
resistance, from growing.
See also
