What are Culture Media: Classification – Overview

What is Culture Media

Culture is the technique
employed to grow bacteria on inanimate culture media, to obtain stock, and to
prepare medicines and vaccines. The field through which the necessary nutrition
supplied to the bacterial cells for their good growth is called culture medium.

Bacteria need to be
grown for their identification and study as they are found mixed at almost all
its habitats and mixed ones are hard to be studied. So to study properly, to
maintain stock, to prepare antibiotics and vaccines, it becomes necessary to
cultivate bacteria and isolate them on an inanimate culture medium, containing
the required nutrition for the microbes, particularly bacteria.
Numerous culture media
have been devised. The original media used by Louis Par were liquid such as
urine or meat broth. The liquid media have many disadvantages as the may not
exhibit specific characteristics for their identification and their isolation
from sud liquid cultures.
Liquid media can only be
used to maintain a stock culture of specific bacteria. But isolation from such
cultures is always impossible until it is not solidified. The earliest the solidifying agent used by Dr. Robert Koch was cooked potato chips, later
gelatin was introduced to solidify the liquid media but it could not give
satisfactory results as gelatin liquefied at 24┬░Cand by many proteolytic
bacteria.
 Agar-agar was used
by Robert Koch on the suggestion of his co-worker Frau Hesse whose mother used
to make jellies in the kitchen with agar-agar powder. Agar now is being used at
a large scale in the world to solidify the liquid culture media as its melting
point is around 100 C and colonies grown on such media do not lyse even under
fluctuating temperature. 



Culture media gives an artificial environment
necessary for the growth of bacteria. It is quite necessary to study and
identify microorganisms. 
Culture media provide
bacteria energy, carbon, nitrogen, minerals, and salts like phosphates,
sulphates, and carbonates, etc.

Composition of Media

1. Water, Peptone, Meat
extract, Minerals, and NaCl.
2. Agar used as a
solidifying agent.
3. Blood, serum, egg,
and chocolate are used to enrich the media
4. Nutrient broth-It
contains peptone 1% meat extract 1% and Naa05%.
5. For special media
certain ingredients are added to basal media to obtain specific growth of
bacteria.

Classification of
Media 

A. Based on Consistency 

1. Liquid media 
2. Solid media 
3. Semi-solid media


 B. Based on the Constituents 

1. Synthetic or
semi-synthetic media
2. Basic media/ special
media


C. Based on the aerobic
condition in which bacteria grow 
1. Aerobic media 
2. Anaerobic media 
D. Based on Nutrition 
  1. Simple media 
  2. Complex mediA
  3. Enriched media 
  4. Enrichment
    media 
  5. Indicator media 
  6. Selective Media 
  7. Differential Media
  8. Sugar Media
  9. Transport Media

Based on
Consistency 

1. Liquid Media: The
media is in liquid form and has no solidifying agent. They are distributed in a
test tube with a cotton/wood stopper. Bacteria grow well in 24-48 hrs

 Advantage: When bacteria
are present in a small number, they will grow only in liquid media.
 Disadvantage:
Isolation of bacteria in pure culture is not possible. The culture tube is
inoculated by touching with a loop or by adding the inoculum with forceps.
Types of liquid media
broth: It is a clear transparent color and prepared from meat extract.

  • Infusion Broth:
    fat-free minced beef meat added to water 
  • Meatextract: filtrate
    of meat boiled and filtered.
  • Digest broth: itis
    prepared from meat by enzymatic action.

2. Solid media: The
media which are solid in composition due to the addition of any solidifying
agent in appropriate concentration. They are essential for the isolation of
organisms in pure form. When an organism grows on solid media they grow and
multiply at the site of inoculation and form usable colonies. example Blood
agar, nutrient agar.

3. Semi-solid Media: The
consistency of this media is like a gel. It is used to show the motility of
bacteria and is prepared by the addition of a few contents of agar-agar to the
liquid media.


Based on the
Constituents
1.Synthetic media: Those
media in which the concentration of each ingredient is known.
There is no variation in
composition like peptone water. These media are prepared by using
natural products like
meat extract. They are easy to prepare and relatively cheaper like nutrient
agar, enriched media.
2. Simple/Basic Media:
These media are in general use in the laboratory.
Based on the
Constituents
1. Enriched Media: When
some special nutrients such as blood or serum are added to solid
basal media. These media
are employed for the cultivation of organism e.g. Blood agar, serum agar, 
chocolate agar, etc.
2. Enrichment Media:
Some substances are added to liquid media with the result wanted
the organism, to grow more
in number than the unwanted organisms. e.g. Peptone Robertson’s 
cooked meat medium.
3. Selective Media: In
addition to basal media they contain substances such as bile salt
which inhibit or poison
all bacteria except those of a particular type of group of wanted
organisms. e.g.
Lowenstein’s Jensen medium for Mycobacterium tuberculosis.
4. Indicator Media: When
certain indicator (neutral red) is incorporated in the culture
medium it is called an
indicator medium. The color of the indicator in the medium changes with 
bacterial growth.
example Diphtheria bacillus produces black colonies on McLeod’s medium.
5. Differential Media:
When a culture medium containing certain substances helps to
distinguish differing
properties of different bacteria it is called differential media e.g
MacConkey’s medium
6. Transport Media: When
specimens e.g. feces are to be sent to the laboratory. at a distant place, the 
pathogenic bacteria may
not survive there or maybe overgrown by non-pathogenic bacteria, 
For transporting such
specimen, special media called transport media. e.g. Stuart’s transport medium
for Gonococci.
7.Sugar Media: The
standard sugar media used for biochemical test contain 2% sugar in
peptone water. A small
tube (Durham’s tube) kept in inverted in the larger tube with
the production of acid
by bacteria, the colorless medium turns pink and gas production is
indicated by the
accumulation of gas bubbles on the top of the inverted tube. e.g. Lactose broth
medium.

STANDARDIZATION OF
CULTURE MEDIA

Microbes easily grow
under a wider range of nutrition and the environment. They require
growth factors, optimum
temperature and optimum range of pH of media and optimum
incubation period.
Bacteria require CO, N, O, mineral salts, bacterial vitamins and other
growth factors for
growth and multiplication.
Meat Extract: It is used
in medium either as meat infusion or lab Kemco.
Mineral Salts: Inorganic
substances i.e. chlorides, phosphates, and sulphtes are important
for nutrient media.
Growth Factor: Magnesium
activates phosphatase, copper is required for polyphenol
Oxidase, zinc for carbonic
anhydrase and calcium for the action of lecithinase enzymes. Para amino benzoic
acid 5% is added to basal culture media as an important growth factor.
pH Estimation: pH is
maintained to 7.2-8.00 by adding phenol red, thymol blue (alkaline) or litmus and
observed through coloration as 6.8-8.4, 8.0-9.6 and 6.8-8.3, giving colors
yellow/pink, yellow to blue and red to blue respectively.

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