Culture Media: A Comprehensive Guide for Microbiology Students, Summaries of Nursing

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Culture media

Lab 3

Done by

Assist.prof. Dr. Taisir Abdulelah

Nihaya Awad

Fatima Natheer

Culture media

Collection of certain basic nutrienal requirement that provide

cultivation of bacteria and support their growth and multiplication.

Any culture medium must contains:

• A source of energy
• Sources of carbon, nitrogen, phosphorus
• Minerals, e.g., Ca 2 +, Mg 2 +, Na+
• Vitamins and growth factors
• Water

Note : Media are solidified by the addition of solidifying agents such as agar.

Agar : A complex carbohydrate obtain from Alage used as solidification agent for

media.

Pure culture:

Culture consist of medium containing the growth of single species of

Microorganism (M.O) which involve in most microbiological studies.

Mixed culture:

Culture consist of medium containing the growth of two or more species of M.O.

There are several ways to ensure purity of a culture:

1. Colony should only contain a single morphological cell type when examine

under microscope.

2. Isolated M.O from streak plate should yield only a single type of isolated

colony which contain the same morphological type of the cell.

Pure Culture Presrvation:

1 .Cooling 2. Freezing 3. Lyophilizating (freeze drying)

A. Classification of culture media according to physical state

1 - Broth media :

➢ used as transport media and for cultivation of bacteria.

➢ Contain zero% of agar.

➢ Organisms grown in broth cultures are apparent through the turbidity

Growth of bacteria-----> turbidity

No growth ----> clear

2 - Semi solid media:

➢ used for study the motility of bacteria.

➢ Contain 0.2 – 0.5 % agar.

3 - Solid media:

➢used to study the all culture characteristics of colony

➢contain 1 - 2 % agar to solidify the media.

➢the bacterial cells form masses called colonies after about

18 – 24 hours of growth.

B. Classification of culture media according to chemical composition

1. Natural medium: Culture media of which, the exact chemical composition is

not known is called natural or empirical culture media. Examples- Milk, urine,

diluted blood, vegetable juices, meat extracts, beef and tomato juice, blood.

2. Semi-synthetic: Culture media, the chemical components of which are

partially known and partially obscure are termed as semisynthetic culture

media. Examples- Potato dextrose agar (PDA) , corn meal agar (CMA) and

nutrient agar.

3. Synthetic medium: Such media are composed of the substances that are

chemically known. These media are very useful in studying the physiology,

metabolic nature and nutritional requirements of microbes. Both autotrophs and

heterotrophs can be grown in these media. Example- Mineral glucose medium.

3 ) Selective media

This is a special type of media which allows the growth of certain M.O while

inhibits the growth of the others. Various approaches to make a medium selective

include addition of antibiotics, dyes, chemicals, alteration of pH.

Examples of selective media include:

Thayer Martin Agar used to recover N.gonorrhoeae contains antibiotics;

vancomycin, colistin and nystatin.

Mannitol Salt Agar used to recover S.aureus contains 10 % NaCl.

MacConkey’s Agar used for Enterobacteriaceae members contains bile salt that

inhibits most gram positive bacteria.

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.

5 )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 of specimen, maintain the

pathogen to commensal ratio and inhibit overgrowth of unwanted

bacteria. Some of these media (Stuart transport media) are semi-

solid in consistency.

Preparation of culture media:

1 - Dissolving powder media in distilled water and mixing of

heater to boiling.

2 - Sterilization of media in autoclave at 121 °C and pressure

15 pound for 15 - 20 min

3 - Cooling the media and adding the media to petri-dishes or

tubes ,wait until solidify, plates are kept inverted.