The document discusses various ways of classifying culture media used for growing microorganisms in vitro, including by physical state (solid, semi-solid, liquid), chemical contents (synthetic vs non-synthetic), and function (general purpose, selective, differential, transport, anaerobic, assay media). Culture media provide nutrients necessary for microbial growth and are used for identification, study, and production of biological products. Classification is important for selecting the proper medium for isolating and identifying microorganisms from clinical specimens.

1. Medical Microbiology Laboratory
(Culture Media Classification)
Hussein A. Abid
Medical Laboratory Scientist
Member at American Society of Microbiology
Chairman of Iraqi Medical Laboratory Association
Teacher at Middle Technical University

2. DEFINITIONS
 Culture media (single, medium): the food materials or
substances required for growing microorganisms in vitro
(outside the body).
USES OF CULTURE MEDIUM
1. To identify the cause of infection from the clinical sample,
so that the proper treatment can be given.
2. To study the characteristics or properties of
microorganisms
3. To prepare biological products like vaccines, toxoids,
antigens ..etc. 1

3. COMPOSITION OF CULTURE MEDIUM
 Water
 Energy source
 Carbon source
 Nitrogen source
 Mineral salts
 Special growth factors
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4. CULTURE MEDIA

5. 4
TYPES OF CULTURE MEDIA
 Classification based on physical state (or solidity):
1. Solid medium
2. Semi-solid medium
3. Liquid medium
 Classification based on chemical contents:
1. Synthetic media: contains organic or inorganic compounds that are
chemically defined (i.e. known molecular formula).
2. Non-synthetic (or complex) media: contains ingredients that are
not chemically defined or pure (i.e. animal extract)
 Classification based on function (or the purpose of using):
1. General purpose media 2. Enriched (complex) media 3. Selective media
4. Differential media 5. Transport media 6. Anaerobic media
7. Assay media

6. CLASSIFICATION BASED ON PHYSICAL STATE
1- Solid Medium
 Agar is the most commonly used
solidifying agent; is golden-
yellow granular powder, prepared
from seaweeds, is not affected
by the growth of bacteria. Agar
melts at 98 oC & sets at 42 oC.
 E.g. nutrient agar
 Agar percentage in solid media ≈
1.5 – 2% of total weight.
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7. CLASSIFICATION BASED ON PHYSICAL STATE
2- Semi-solid Medium (agar ≈ 0.5%)
 Such media are soft and are useful in
demonstrating bacterial motility and
separating motile from non-motile
strains.
3- Liquid Medium (agar = 0%)
 Are sometimes referred “broth”,
bacteria grow uniformly producing
general turbidity.
 e.g. nutrient broth
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8. CLASSIFICATION BASED ON CHEMICAL CONTENTS
1- Synthetic (or Defined) Media
 Specially prepared media from pure chemical substances
for research purpose and composition of every
component is well known (e.g. peptone water – 1%
peptone + 0.5% NaCl in water).
2- Complex (or Enriched) Media
 Such as blood agar, it has ingredients that exact
components are difficult to estimate.
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9. CLASSIFICATION BASED ON FUNCTION
1. General purpose media: for growth of all microorganisms (e.g.
nutrient agar, nutrient broth).
2. Enriched media: solid media rich in nutritional substances, that
enhance the growth of microorganism (e.g. blood agar).
3. Selective media: allow to specific types of microorganism to grow
while inhibit the others (e.g. MacConkey’s agar it is inhibit gram +ve
bacteria).
4. Enrichment media: liquid media inhibit commensals and increase
the concentration of pathogenic microorganism (e.g. selenite F broth).
5. Differential media: differentiate between different microorganism
due to colors resulting from metabolic reaction (e.g. MacConkey’s
agar it is differentiate between lactose fermenters and non-lactose
fermenters Enterobacteriaceae) 8

10. CLASSIFICATION BASED ON FUNCTION
6. Transport media: a media used to preserve a clinical sample until
the processing (e.g. Cary Blair transport medium).
7. Anaerobic media: used for anaerobic bacteria growing (e.g.
Robertson Cooked Meat (RCM) medium that is commonly used to
grow Clostridium spp.).
8. Assay media: used for the assay of vitamins, amino acids and
antibiotics (e.g. antibiotic assay media are used for determining
antibiotic potency by the microbiological assay technique).
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11. CLASSIFICATION BASED ON FUNCTION
1. General purpose media: Basal
media are basically simple media
that supports most non-fastidious
bacteria. Peptone water, nutrient
broth and nutrient agar are
considered as basal medium.
These media are generally used
for the primary isolation of
microorganisms.
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Nutrient agar

12. CLASSIFICATION BASED ON FUNCTION
2. Enriched medium (Added growth
factors): Addition of extra nutrients in the
form of blood, serum, egg yolk etc, to basal
medium makes them enriched media.
Enriched media are used to grow
nutritionally exacting (fastidious) bacteria.
Blood agar, chocolate agar, Loeffler’s
serum slope etc are few of the enriched
media. Blood agar is prepared by adding
5-10% (by volume) blood to a blood agar
base. Chocolate agar is also known as
heated blood agar or lysed blood agar.
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Blood agar

13. CLASSIFICATION BASED ON FUNCTION
3. Selective media: are designed to suppress the growth of
some microorganisms while allowing the growth of others.
Selective media are agar based (solid) media so that
individual colonies may be isolated.
Examples of selective media:
 Thayer Martin Agar used to recover N. gonorrhoeae contains
antibiotics; vancomycin, colistin and nystatin.
 Mannitol Salt Agar and Salt Milk Agar used to recover S.
aureus contains 10% NaCl.
 Potassium tellurite medium used to recover C.
diphtheriae contains 0.04% potassium tellurite.
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14. SELECTIVE MEDIA
 MacConkey’s Agar used for Enterobacteriaceae members contains
bile salt that inhibits most gram positive bacteria.
 Pseudosel Agar (Cetrimide Agar) used to recover P.
aeruginosa contains cetrimide (antiseptic agent).
 Crystal Violet Blood Agar used to recover S. pyogenes contains
0.0002% crystal violet.
 Lowenstein Jensen Medium used to recover M. tuberculosis is made
selective by incorporating malachite green.
 Wilson and Blair’s Agar for recovering S. typhi is rendered selective
by the addition of dye brilliant green.
 Selective media such as TCBS Agar used for isolating V.
cholerae from fecal specimens have elevated pH (8.5-8.6), which
inhibits most other bacteria. 13

15. MacConkey’s agar Thayer-Martin agar TCBS agar

16. CLASSIFICATION BASED ON FUNCTION
4. Enrichment medium is used to increase
the relative concentration of certain
microorganisms in the culture prior to
plating on solid selective medium. Unlike
selective media, enrichment culture is
typically used as broth medium.
Enrichment media are liquid media that also
serves to inhibit commensals in the clinical
specimen. Selenite F broth, tetrathionate
broth and alkaline peptone water (APW)
are used to recover pathogens from fecal
specimens.
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Selenite F broth

17. CLASSIFICATION BASED ON FUNCTION
5. Differential media: Certain media are designed in such a
way that different bacteria can be recognized on the basis
of their colony colour. Various approaches include
incorporation of dyes, metabolic substrates etc, so that
those bacteria that utilize them appear as differently
colored colonies. Such media are called differential media
or indicator media. Differential media allow the growth of
more than one microorganism of interest but with
morphologically distinguishable colonies.
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18. CLASSIFICATION BASED ON FUNCTION
Examples of differential media:
 Mannitol salts agar (mannitol fermentation = yellow).
 Blood agar (various kinds of hemolysis i.e. alpha (α), beta (β)
and gamma (γ) hemolysis).
 MacConkey agar (lactose fermenters, pink colonies whereas
non-lactose fermenter produces pale or colorless colonies.
 TCBS (Vibrio cholerae produces yellow colonies due to
fermentation of sucrose).
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19. DIFFERENTIAL MEDIA
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Blood agar and
types of hemolysis
Mannitol salt agar

20. CLASSIFICATION BASED ON FUNCTION
6. Transport media: clinical specimens must be
transported to the laboratory immediately after collection
to prevent overgrowth of contaminating organisms or
commensals. This can be achieved by using transport
media. Such media prevent drying (desiccation) of
specimen, maintain the pathogen to commensal ratio and
inhibit overgrowth of unwanted bacteria. Some of these
media (Stuart’s & Amie’s) are semi-solid in consistency.
Addition of charcoal serves to neutralize inhibitory factors.
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21. CLASSIFICATION BASED ON FUNCTION
Examples of transport media:
 Cary Blair transport medium and Venkatraman
Ramakrishnan (VR) medium are used to transport feces
from suspected cholera patients.
 Sach’s buffered glycerol saline is used to transport feces
from patients suspected to be suffering from bacillary
dysentery.
 Pike’s medium is used to transport streptococci from
throat specimens.
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22. CLASSIFICATION BASED ON FUNCTION
7. Anaerobic media: anaerobic bacteria need special media for
growth because they need low oxygen content, reduced
oxidation-reduction potential and extra nutrients.
 Media for anaerobes may have to be supplemented with
nutrients like hemin and vitamin K.
 Such media may also have to be reduced by physical or
chemical means.
 Boiling the medium serves to expel any dissolved oxygen.
Addition of 1% glucose, 0.1% thioglycollate, 0.1% ascorbic
acid, 0.05% cysteine or red hot iron filings can render a
medium reduced.
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23. ANAEROBIC MEDIA
 Before use the medium must be boiled in water bath to expel
any dissolved oxygen and then sealed with sterile liquid
paraffin.
 Robertson Cooked Meat (RCM) medium that is commonly
used to grow Clostridium spp. contains a 2.5 cm column of
bullock heart meat and 15 ml of nutrient broth. Thioglycollate
broth contains sodium thioglycollate, glucose, cystine, yeast
extract and casein hydrolysate.
 Methylene blue or resazurin is an oxidation-reduction potential
indicator that is incorporated in the medium. Under reduced
condition, methylene blue is colorless.
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24. CLASSIFICATION BASED ON FUNCTION
8. Assay media: These media are used for the assay of
vitamins, amino acids and antibiotics. E.g. antibiotic assay
media are used for determining antibiotic potency by the
microbiological assay technique.
Other types of medium includes:
 Media for enumeration of Bacteria,
 Media for characterization of Bacteria,
 Maintenance media etc.
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25. POST-TEST
 What is the difference between “culture media” and
“culture medium”, “enriched media” and “enrichment
media”?
 How to classify culture media according to solidity? Give
an example for each class.
 What is selective media?
 What is Thayer-Martin media?
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