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Types of culture media
categorized according to their function and use .
In diagnostic microbiology there are four general
categories of media.
1.Enrichment media
2.Supportive media
3.Selective media
4.Differential media
• Enrichment media:
it contain specific nutrients required for the
growth of particular bacterial pathogen that may be
present alone or with other bacterial species in
specimen.
. used to enhance the growth of particular bacterial
pathogen from mixture of organism by using
nutrient specificity
Eg: blood agar, chocolate agar
• Supportive media:
it contain nutrients that support growth of most
fastidious organisms with out giving any particular
organism a growth advantage.
Selective media:
it contain one or more agents that are
inhibitory to all organism except preferable
one.
• Inhibitory agents used for this purpose include
dyes,bile salt,aclcohol ,acids antibiotics
eg: phenyl ethyl alcohol agar
Differential media: this is used to identify the
colonies of one bacterial species or type to
exhibit certain metabolic or cultural
characteristics in a same agar plate
• Eg:
• Eosin methylene blue (EMB), which is
differential for lactose and sucrose
fermentation
• MacConkey (MCK), which is differential for
lactose fermentation
• Mannitol Salt Agar (MSA), which is differential
for mannitol fermentation
• Nutrient agar
• It is a microbiological growth medium
commonly used for the routine cultivation of
non-fastidious bacteria. Nutrient agar is a solid
medium. Very few microbes can degrade agar,
so the agar remains solid during microbial
growth.
• Nutrient agar typically contains (w/v
•
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0.5g peptone
0.3g beef extract
1.5g agar
100ml Distilled water
pH adjusted to neutral at 25 C.
• Blood agar
• It contains mammalian blood (usually sheep or
horse), typically at a concentration of 5–10%.
• Blood agar are an enriched, differential media used
to isolate fastidious organisms and detect hemolytic
activity.
• β-hemolytic activity will show complete lysis of red
blood cells surrounding colony.
• hemolytic activity will show complete lysis of
red blood cells surrounding colony. Examples
include Streptococcus haemolyticus. αhemolysis will only partially lyse hemoglobin
and will appear green
On the right is a positive Streptococcus culture; on the left a positive Staphylococcus cultur
• Chocolate agar:
chocolate agar is essentially the same as
blood agar except that during preparation the
red blood cells are lysed when added to
molten agar base.
This lysis release intracellular nutrient s such as
hemoglobuin,hemin and the coenzyme
,nicotinamide adenine dinucleotide into the
agar for utilization by fastidious bacteria
• Blood cell lysis gives the medium a chocolate
brown color.
• The pathogens require this medium for
growth include Neisseria gonorrhoeae,
• Haemophilus spp
• Macconkey Agar:
it is the most frequently used primary
selective and differential agar.
This medium contain crystal violet dye to inhibit
the growth of gram positive bacteria and
fungi.
It allows the growth of many gram negative
bacilli to grow.
The medium contains peptone with lactose and
neural red pH indicator.
• Bacterial fermentation of lactose results in
acid production which decreases0 medium
pH and cause the neutral red indicator to give
bacterial colonies as pink to red color.
Sabouraud`s agar
• It is used to culture fungi and has a low pH
that inhibits the growth of most bacteria; it
also contains the antibiotic gentamycin to
specifically inhibit the growth of Gram
negative bacteria.
Transport media
• Transport medium inhibits self destruction and
multiplication of microorganisms.
• Delay during the transport of specimens from the
point of collection noticeably affects the viability of
such delicate organisms as Nesseria,
haemophilus,shigella and anaerobes.
• Various media are used for transporting different
types of specimen
• Stuart transport medium:
it is used for CSF and cervical
specimens.this medium preserves both
Neisseria species and Trichomonas vaginalis..
Cary-Blair and Amies medium:
is used to transport faecal material on
rectal swab and are effective in preserving
Salmonella, shigella and vibrio