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Introduction to Microbial Media

Microbial-Media
  • Introduction
  • Types of microbial media
  • FAQs
  • Microbial medial selection guide

Introduction

Microbiological studies require the availability of different species of bacteria, yeast or viruses in the laboratory. Large scale processes such as fermentation, protein and vaccine production require large numbers of bacteria in physiologically active state. A suitable nutrient medium that provides appropriate biochemical environment and maintains all the characteristics of microbes is therefore required for various applications.

Any microbial medium should include sources of nutrition, energy, essential minerals, buffering agents and pH indicators. Sometimes they also include selective agents and solidifying agents. Obligate parasites such as bacteriophages may be cultivated in a nutrient medium that also contains host bacteria.

The following are the essential components of a typical microbial medium:

  • Nutrition sources – sources of proteins, vitamins, mineral and carbohydrates; mostly mixture of peptone and meat extract
  • Energy sources – sources of carbohydrates; mostly glucose
  • Essential minerals – sources of micro and macro minerals; mostly present in peptone and meat extract
  • Buffering agents – agents that maintain optimum pH; mostly specific amino acids, phosphates, citrates and zwitterions
  • pH indicators – agents that indicate variation in pH of the medium by change in color; mostly phenol red
  • Selective agents – agents that allow the growth of only specific bacteria; antibiotics, tellurites, azides and bile salts are examples
  • Solidifying agents – agents that maintain the medium in solid or semi-solid state, if required; widely used agent is agar though gelatin is sometimes used
  • Specific substances such as growth factors, enzymes may be incorporated into the medium for specific bacteria

Types of microbial media

  • Simple or basal media: Include nutrient broth and peptone water; used routinely to isolate and culture a variety of bacteria in a molecular biology research laboratory
  • Complex media: Contain mixture of a variety of nutrients; the exact composition of amino acid source is not defined. Examples include chocolate agar, MacConkey agar, Lowenstein-Jensen medium
  • Defined media: Contain known and defined sources of carbon and nitrogen as required by certain bacteria
  • Special media: Contain specific substances that encourage growth of fastidious bacteria, discourage the growth of unwanted bacteria or contain indicators that distinguish different types of bacteria
Table 1: Types of microbial media

FAQs

What are the differences among the Luria, Lennox and Miller LB formulations?

LB, (originally termed lysogeny broth) was initially composed of tryptone, yeast extract, NaCl and glucose.Soon after, the glucose was omitted (Miller's version), and later the NaCl content lowered by half (Lennox's version).For some applications, even lower salt is required (Luria's low salt version).

What is the difference among the LB - Miller products?

LB - Miller is available in many types to suit your needs.The different product formats include powder and liquid form.The powder form is also available with agar for easy LB-agar plate preparation.L2542 (LB Miller liquid)L3522 (LB Miller powder)L3147 (LB Miller powder with agar)

Which bacterial culture medium is the best choice for my application?

Each of the broths will likely grow E. coli very well, but there are still general guidelines for choosing a broth when you are working without a protocol.Generally:LB - Miller and LB - Lennox are used for E. coli growth and maintanence, DNA plasmid production and protein production.The Lennox formulation has a lower salt content required for some salt-sensitive selection antibiotics.LB - Luria low salt is used for special applications where the E. coli growth or other constraints require the lowest possible salt content.Terrific Broth is used for higher yield protein production and high yield DNA plasmid production, because of the faster growth of the E. coli in this medium.SOB is used for protein production, DNA plasmid production and the generation of high-efficiency competent cells.SOC is used for initial growth of competent cells and the transformation procedure.

Will adding magnesium to the culture medium increase cell density?

In microbial broth formulations that do not already contain magnesium, the addition of 10-20 mM MgCl2 or MgSO4 may increase cell densities. You may need to also increase the shaking speed of the incubator.

Microbial Media Selection GuideLB media
Minimal salts (M9)
Terrific Broth - modified
2X YT medium
NZCYM Broth
Yeast Media
Phage Media

References

1.
Sambrook, J, Fritsch E, Maniatis, T. 1989. Molecular cloning: a laboratory manual. New York: .
2.
Parija, SC. 2009. Textbook of Microbiology and Immunology. Elsevier India.
3.
http://www.scienceprofonline.org/microbiology/types-culture-media-for-growing-bacteria.