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GROWTH MEDIUM AND THEIR SELECTION CRITERIA���Ms. Devika Tripathi�Assistant Professor�PSIT-Pranveer Singh Institute of Technology (Pharmacy), Kanpur

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Topic: Growth medium and their selection criteria

Aim and Objectives

To understand the nutrient requirement of micro-organism growth

To select the right growth medium to sustain the bacterial growth

To understand the nutrient consumption by micro-organism

Outline of Course

Students will be able to discover the antibiotics based on nutrient requirement.

Student will get job as microbiologist

By understanding the concept can prevent microbial contamination.

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Bacterial Culture Media�

  • Depending on their composition or use, culture media can be categorized into several groups includes:
  • In a defined medium, the exact chemical composition is known.
  • These types of media are usually composed of pure biochemicals, and are often used to study the minimal nutrient requirement of a microorganism.

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Complex medium the exact chemical composition is not known.

Complex media usually provide a large range of growth factors that assist in the cultivation of unknown and fastidious bacterial species.

Media may also be formulated as selective or enriched.

Selective

Consist of additional selective reagents, such as high salt concentration.

Enrichment medium also allows for the growth of specific bacterial species; however, enrichment media are supplemented with a reagent that permits, rather than inhibits, the growth of a particular species.

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Selecting ingredients for Media

  • Bacterial culture media are mixtures of proteins, salts, trace elements, amino acids, and carbohydrates.
  • Presence and volume of these components can vary significantly among bacterial species depending on the macro- and micro-nutrient requirements of each strain. 
  • Growth and propagation of pure batch cultures: Liquid media
  •  Isolation of pure cultures:  Solid Agar media

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Additional requirements

Liquid medium can assist in the maintenance of pH: 3-(N-morpholino)propanesulfonic acid (MOPS) or potassium phosphate.

Osmotic environment can be maintained through the addition of salts, such as sodium chloride.

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Media Ingredients

Peptone

  • Peptone is a water-soluble protein derivative used in bacteriology culture media.
  • Peptone is used in bacterial media as an organic source of nitrogen and is often used in serum-free medium.
  • The starting material for peptone can range from animal to plant; these can include meat, soybean, casein, and whey.

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Yeast Extract

Yeast extract is prepared as a water-soluble extract of autolyzed Saccharomyces cerevisiae yeast cells.

Yeast extract provides an essential source of water-soluble B-complex vitamins, carbohydrates, and free glutamic acid.

Sodium Pyruvate

Pyruvate is an intermediary organic acid metabolite produced as a product of glycolysis.

Provides both an energy source and a carbon skeleton for anabolic processes.

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Dextrose

  • Dextrose is a highly nutritious supplement used for the cultivation of fastidious organisms, older cultures, and cultures with small inoculums.
  • Dextrose provides bacteria with a carbon source, and can be supplemented with 5% blood to provide additional growth factors for fastidious micro-organisms.

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Media Supplements

  • The growth media recommended for some bacterial species may require the addition of components not already available in the base medium.
  • These components may include antibiotics, serum, blood, or chemical supplements.

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Chemical supplements

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  • Trace Elements�
  • Trace elements are micronutrients required by microorganisms in small amounts.
  • These nutrients are metal ions required by most microorganisms for survival as they usually function as cofactors for essential enzymatic reactions.

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Salts�Various salts, such as magnesium, calcium, iron, and potassium salts, are required for bacterial growth as they provide major elements that can function as cofactors for certain enzymatic reactions.

Eg: Calcium and iron form major components of endospores and cytochromes.

  • Sodium chloride, within a medium assists in the maintenance of osmolality.

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Glucose�Microbial media are often supplemented with glucose as a source of carbohydrates. 

Eg: Escherichia coli, this inhibition requires the cells to use glucose as the primary energy source.

Blood�Blood is often added to growth medium to enhance the cultivation of highly fastidious microbial species

Eg: Tryptic Soy Agar and Dextrose Agar, both of which are often supplemented with 5% defibrinated sheep blood ------------------------Haemolytic reactions and pigmentation.

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Others

Antibiotics

  • Antibiotics can be added to bacterial culture media to select for the growth of a specific variant. 
  • Used for the selection of strains that have inherent antibiotic resistance, harbour plasmids, or are genetically engineered.
  • The addition of antibiotics often decreases the shelf life of a medium as compared to the base medium.
  • To extend the shelf life, media containing antibiotics should be stored at 2°C to 8°C in the dark.

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Serum

  • Sera can serve as a source of growth factors, proteins, vitamins, hormones, carbohydrates, lipids, amino acids, minerals, and trace elements.
  • Serum can function as a pH buffer and can inactivate proteolytic enzymes.
  • Sera from rabbit, horse, foetal bovine, and calf bovine sources are used to support the growth of some bacterial species in culture.

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  • Many pathogens, such as members of the genera Corynebacterium, Borrelia, Leptospira, and Mycoplasma, require the addition of serum to support growth.
  • ATCC offers the following three types of animal sera:
  • Fetal Bovine Serum (also known as fetal calf)
  • Fetal Bovine Serum qualified for embryonic stem cells
  • Iron-supplemented Calf Bovine Serum

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