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
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 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.
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.
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