Role of Substances Added to the Media

The role of substances added to the media are described below  

Carbohydrates provide energy, act as a carbon sonrce and also indicate fermentation reactions. 

whole blood and ascitic fluid promote the growth of less vigorous organisms. 

Dyes act as selective growth inhibitors for certain bacteria and indicate pH change due to acid formation, e.g., phenol red is red in alkaline media and yellow in acidic media. Gentian violet inhibits the growth of Gram-positive bacteria. 

Agar solidifies media - it is made from red algae Gelidium (Rhodophyta). Chemically it is mostly comprised of galactose and very few microbes can degrade it. It remains liquid at 100 °C (easy to pour) and solidifies at 40 °C (incubation temperatures). 

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