Culture media are used to grow microorganisms outside the body. There are many types of culture media based on their physical state (solid, semi-solid, liquid), chemical composition (synthetic/defined, non-synthetic/undefined), and purpose (general, enriched, selective, differential). Solid media contain agar and allow microbes to grow as colonies. Selective media inhibit unwanted bacteria to recover pathogens. Differential media distinguish bacteria based on colony color. Culture media allow identification and study of microorganisms.

1. Culture Media

2. Culture Media
• A culture media is a special medium used in
microbiological laboratories to grow different kinds of
microorganisms.
• A growth or a culture medium is composed of different
nutrients that are essential for microbial growth.
• There are many types of microorganisms, each having
unique properties and requiring specific nutrients for
growth, there are many types based on what nutrients
they contain and what function they play in the
growth of microorganisms.
• Culture media contains nutrients and physical growth
parameters necessary for microbial growth. All
microorganisms cannot grow in a single culture
medium.

3. Uses of culture medium
• It is important to grow microorganisms outside
the body for the following purposes:
• to identify the cause of infection from the clinical
sample, so that proper treatment can be given.
• to study the characteristics or properties of
microorganisms.
• Differentiate among different kind of bacteria.
• Can be used to enrich the number of bacteria.
Select for certain bacteria and
suppress others.
to prepare biological products like
vaccines, toxoids, antigens…etc.

4. Composition of culture media
• Water
• Energy source
• Carbon source
• Nitrogen source
• Mineral salts
• Special growth factors

5. Classification of culture media
on the basis of consistency
• Classification based on physical state
• a) solid medium
• b) semi solid medium
• c) liquid medium

6. Solid medium
• agar is the most commonly used
solidifying agent.
• solid medium contains agar at a
concentration of 1.5-2.0% or some
other, mostly inert solidifying agent.
• Solid medium has physical structure
and allows bacteria to grow in
physically informative or useful
ways (e.g. as colonies or in streaks).
• Solid medium is useful for isolating
bacteria or for determining the
colony characteristics of the isolate.
What is agar
Golden –yellow
granular powder
Prepared from
seaweeds.
Not affected by the
growth of the bacteria.
Melts at 98oC & sets at
42oC

7. Semi-solid media
• Such media are soft and are useful
in demonstrating bacterial motility
and separating motile from non-
motile strains.
• They are prepared with agar at
concentrations of 0.5% or less.
They have soft custard like
consistency and are useful for the
cultivation of microaerophilic
bacteria or for determination of
bacterial motility.

8. Liquid media
• Sometimes referred as “ broth “.
• bacteria grow uniformly producing
general turbidity e.g. Nutrient broth
• These media contains specific amounts
of nutrients but don’t have trace of
gelling agents such as gelatin or agar.
• Broth medium serves various purposes
such as propagation of large number of
organisms, fermentation studies, and
various other tests. e.g. sugar
fermentation tests.

9. Classification
based on the basis
of composition

10. Classification based on the
basis of composition
• Synthetic or chemically defined
medium
• Non synthetic or chemically undefined
medium

11. Synthetic or chemically defined
medium
• A chemically defined medium is one prepared from
purified ingredients and therefore whose exact
composition is known.
• Synthetic medium may be simple or complex
depending up on the supplement incorporated in it.
• specially prepared media from pure chemical
substances for research purpose and composition of
every component is well known
• The use of defined minimal media requires the
investigator to know the exact nutritional requirements
of the organisms in question.

12. Non synthetic or chemically
undefined medium
• Non-synthetic medium contains at least one component
that is neither purified nor completely characterized .
• Often these are partially digested proteins from various
organism sources.
Nutrient broth, for example, is derived from
cultures of yeasts, blood agar.
It has ingredients that exact components are
difficult to estimate.
complex non-synthetic medium support the
growth of more fastidious microorganisms
(Neisseria gonorrhoeae).

13. Classification of Bacterial
Culture Media based on
the basis of purpose/
functional use/
application

14. General purpose media/ Basic 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.

15. 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 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.

16. Selective media
• Selective and enrichment media are
designed to inhibit unwanted commensal
or contaminating bacteria and help to
recover pathogen from a mixture of
bacteria.
• While selective media are agar based,
enrichment media are liquid in
consistency.
• Any agar media can be made selective by
addition of certain inhibitory agents that

17. Selective media
• Thayer Martin Agar used to recover Neisseria.
gonorrhoeae contains antibiotics;
• Vancomycin, which is able to kill most gram-positive bacteria
• Colistin: which kill gram-negative bacteria except neisseria.
and
• Nystatin: which can kill fungi
Examples of selective media include:

18. Selective media
• Eosin Methylene Blue
• Selective for gram negative bacteria
• The dye methylene blue in the medium
inhibit the growth of gram positive bacteria
Lowenstein Jensen Medium
 Used to recover Mycobacterium tuberculosis is
made selective by incorporating penicillin,
nalidixic acid and malachite green.

19. Differential medium
• 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 coloured 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.

20. Differential medium
• Mac Conkey agar
• Differentiate between lactose fermenters and non lactose
fermenters. (lactose fermenters produce pink colonies
whereas
• non- lactose fermenter produces pale or colorless colonies.
 Examples of differential media include:
 Used for the recovery of salmonelle and shigella
species.
Xylose Lysine Deoxycholate Agar (XLD)