Question

Compare the recipes of Nutrient Agar and XLD Agar.If an organism can grow on both media, on whick would you expect it to grow better? Why? XLD is a selective mediuam. Is it also a defined or an undefined medium? Why is that formulation desirabl 5 What would be the likely consequence of a. incubating the medium for 48 hours? b. not adding desoxycholate? c. not adding sucrose and lactose? d. not adding lysine? e. not adding ferric ammonium citrate? 6. XLD was designed to separate Salmonella from Shigella, both who do not ferment xylose, sucrose, or lactose. Why does this medium contain these sugars if the organisms it was designed to differentiate do not ferment them?

Answer 1

3).

Recipe of nutrient agar:

Peptone - 5g

Meat extract - 3g

Agar - 15g

Water - 1000 ml (final volume)

Ph = 7

Recipe of XLD agar:

Lactose	7.5 gm
Sucrose	7.5 gm
Sodium Thiosulfate	6.8 gm
L-Lysine	5.0 gm
Sodium Chloride	5.0 gm
Xylose	3.75 gm
Yeast Extract	3.0 gm
Sodium Deoxycholate	2.5 gm
Ferric Ammonium Citrate	0.8 gm
Phenol Red	0.08 gm
Agar	15.0 gm

So, when an organism that can grow on both the media is grown separately on these media, it can better grow on XLD agar because the nutrient medium provides the appropriate nutrients necessary for growth.