Introduction to Microbial Media

• Introduction
• Types of Microbial Media
• FAQs
• Microbial Medial Selection Guide

Introduction to Microbial Media

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