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DIFFERENT TYPES OF CULTURE MEDIA
For a culture medium to be successful in growing
the pathogen sought it must provide all essential
nutrients, ions, and moisture, maintain the correct
pH and osmotic pressure, and neutralize any toxic
materials produced. It is also essential to incubate
the inoculated medium in the correct atmosphere, at
the optimum temperature and for an adequate
period.
The main types of culture media are:
● Basic
● Enriched
● Selective
● Indicator
● Transport
● Identification
Basic media: These are simple media such as nutrient agar
and nutrient broth that will support the growth of microorganisms
that do not have special nutritional requirements.
They are often used in the preparation of enriched media, to
maintain stock cultures of control strains of bacteria, and for
subculturing pathogens from differential or selective media
prior to performing biochemical and serological identification
tests.
Enriched media: Enriched media are required for the growth
of organisms with exacting growth requirements such as
H. influenzae, Neisseria species, and some Streptococcus
species. Basic media may be enriched with whole or lyzed
blood, serum, peptones, yeast extract, vitamins and other
growth factors. An enriched medium increases the numbers of
a pathogen by containing all the necessary ingredients to
promote its growth. Such a medium is often used for specimens
collected from sites which are normally sterile to ensure
the rapid multiplication of a pathogen which may be present
only in small numbers.
Enrichment media: This term is usually applied to fluid selective
media which contain substances that inhibit the growth of
unwanted organisms, e.g. Rappaport-Vassiliadis broth which
is often used as an enrichment medium for Salmonella
serovars in faeces.
Selective media: These are solid media which contain substances
(e.g. bile salts or other chemicals, dyes, antibiotics)
which inhibit the growth of one organism to allow the growth

of another to be more clearly demonstrated. A selective
medium is used when culturing a specimen from a site having

a normal microbial flora to prevent unwanted contaminants
overgrowing a pathogen. Media made selective by incorporating
antibiotics are usually expensive.
Other ways to select organisms
Incubation conditions may be used to select organisms, e.g.
P. aeruginosa is inhibited by anaerobic conditions. Also the
pH of a medium may make it selective for a particular
organism, e.g. V. cholerae can be isolated on an alkaline
medium such as TCBS agar. Temperature may also help to
select an organism e.g. Listeria monocytogenes can grow at
4 C whereas other organisms are inhibited. Growth, however,
is slow.
Indicator (differential) media: These are media to which dyes
or other substances are added to differentiate microorganisms.
Many differential media distinguish between
bacteria by incorporating an indicator which changes colour
when acid is produced following fermentation of a specific
carbohydrate e.g. MacConkey agar.
Note: Many media used to isolate pathogens are both selective
and enrichment or both selective and differential.
Transport media: These are mostly semisolid media that
contain ingredients to prevent the overgrowth of commensals
and ensure the survival of aerobic and anaerobic pathogens
when specimens cannot be cultured immediately after collection.
Their use is particularly important when transporting
microbiological specimens from health centres to the district
microbiology laboratory or specimens to the Regional Public
Health Laboratory. Examples of transport media include
Cary-Blair medium for preserving enteric pathogens and
Amies transport medium for ensuring the viability of gonococci.
Identification media: These include media to which substrates
or chemicals are added to help identify bacteria isolated on
primary cultures. Examples include peptone water sugars,
urea broth, and Kligler iron agar. Organisms are mainly identified
by a change in the colour of the medium and or the
production of gas. Organisms used to inoculate identification
media must be first isolated in pure culture.
Choice of culture media
The choice of culture media to use in microbiology
laboratories will depend on:
– The major pathogens to be isolated, their growth
requirements, and the features by which they are
recognized.
– Whether the specimens being cultured are from
sterile sites or from sites having a normal microbial
flora. Although a selective medium is usually
more expensive than a non-selective one, it often
avoids subculturing, isolates a pathogen more
quickly, and makes it easier to differentiate and
interpret bacterial growth.
– Cost, availability, and stability of different media
in tropical countries.
– Training and experience of laboratory staff in
preparing, using, and quality controlling culture
media.

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