2.5.6 Culture Media

2.5.6 Culture Media

Specialized media are essential in isolation and identification of microbes, although all microorganisms require sources of energy, carbon, nitrogen, phosphorus, sulfur and minerals.

Synthetic / Defined media

Medium in which all components are known chemically is called a defined medium or synthetic medium.

Complex media

Media that contain some ingredients of unknown chemical composition. These are useful as the exact nutritional requirements of many microorganisms are not known and use of rich nutrients will facilitate their growth.

For ex. Yeast extract is a good source of B vitamins and Carbon & Nitrogen compounds. Some examples of complex media are tryptic soya agar (TSA), Nutrient agar (NA) and Plate count agar (PCA). These media are also referred to as general purpose media or non selective media.

(Agar is a solidifying agent not degraded by most bacteria and it is incorporated at the level of 1.5% for solid media.

42oC – 44oC - solidifying point of agar

90oC - melting point)

Selective media

Media that favour growth of particular group of microorganisms. Specific salts / dyes are used to suppress organisms other than the target microorganisms. For eg. Bile salts or dyes like basic fuchsin and crystal violet favour the growth of G-ve bacteria by inhibiting G+ve bacteria. Some examples are Bismuth sulphate agar (BSA) – for Salmonella spp, Baird Parkar agar (BPA) – for Staphylococcus spp.

Differential media

Media that distinguish between different groups of bacteria and even permit tentative identification of microorganisms based on their biological characteristics.

Last modified: Monday, 7 November 2011, 6:17 AM