Lab.3
Culture media
Culture
media: Can be defined as an artificially prepared substance containing various
microbial nutritional requirements of optimum pH which will allow the growth of
most microorganisms.
Classification of culture media:
* According to consistency (hradness),
culture media can be classified into liquid media, semi solid media and solid
media.
(A) Liquid media:
Are
used for particular biochemical tests, sensitivity tests. It has some
disadvantages:
Growth
usually do not exhibit special characteristic appearance.
When
there is more than one type of organisms they can not be separated by growing
in liquid media.
(B) Semi solid media:
Its
use to separate a mixture of a motile and non motile organisms (contain
0.2-0.5% agar).
(C) Solid media: Contain 2.-3% agar.
Agar-ager:
it is long chain polysaccharide containing inorganic salts and a small amount
of proteins. It act as a solidifying agent and solidify only when cooled to 42°C.
Solid
media has some special advantages:
Growth
usually shows special characteristic appearance that helps in identification of
the organisms.
Microorganisms
can be separated with certainly from mixtures by growth fall in a solid media.
*
According to composition, nutritional media divided into two types:
Basic
or simple media: This type of media contains only the basic requirement for the
growth of bacteria like sources of carbon, nitrogen, energy, e.g. nutrient
agar, nutrient broth and peptone water.
Special
media: Which provide specific requirements that may include inorganic salts.
Enriched
media: The media is enriched by addition of extra growth factors like egg,
serum, blood, e.g. blood agar (use to detect hemolytic activity of bacteria),
chocolate agar (when blood agar heated to 80°C for 10 min., the media change to a chocolate brown color).
Selective
media: They are solid media in which some special substances are incorporated
which will allow the growth of some bacteria and prevent the others.
Examples of selective media:
MacConkey
agar: Used for selective isolation of intestinal G-ve rods and to detect
lactose fermentation, lactose-fermenting bacteria appear in red pink colony
while non-lactose fermenting appear as pale colony.
T.C.B.S.
(Thiosulphate Citrate Bromothymole blue Sucrose) medium, for selective isolation
of vibrios.
Lowenstein
Jensens medium: For selective isolation of mycobacteria.
S-S
agar (Salmonella-Shigella agar): For isolation of Salmonella and Shigella.
These two species of bacteria grow on S-S agar in pale colonies with (black
center) in Salmonella spp. growth.
Differential
media: They are incorporated with some substances that help in differentiation
one type of microorganism that is able to grow in this media. The colonies of
some microorganism on these media possess culture characters that are
morphologically distinguished from other M.O. e.g. MacConkey agar, S-S agar,
blood agar.
According
to the hemolytic activity of the bacteria, there are three types of hemolysis:
b-hemolytic (complete hemolysis): A clear zone around the colony.
a-hemolytic (partial hemolysis): A greenished coloration of the medium.
g-hemolytic (no hemolysis).
Indicator
media: Contain pH indicator that change the color of the medium according to
the pH e.g. mannitol salt agar, which uses for selective isolation of Staphylococcus
aureus. The indicator is phenol red.
Preparation of culture media:
The
media are generally prepare according to direction manufactured as in the following
steps:
Dissolve
the powder of media in D.W. on heat stirrer or water bath.
Sterilized
by autoclave.
Cool
sterilized media to 45-50°C.
Power
the media into a sterilized Petri dishes or tubes.
Allow
to solidify.