# Calculations for culture media preparation

Pseudomonas aeruginosa growing on cetrimide selective agar

In this section, you will learn how to calculate for culture media preparation. Calculations for culture media preparation should not be a stressful activity. However, beginners might find some hurdles along the way. And this section is geared towards helping beginners in the filed of microbiology understand the simple math involved in properly calculating for their culture media preparation.

QUESTION: I want to prepare 26 plates of nutrient agar (NA); but I do not know the actual volume of distilled water to use and the actual amount of powdered NA base to weigh or measure out for my experiment on preparation of 26 plates of NA.

To prepare 26 plates of NA culture media plates; you must first read the manufacturers instruction on the side of the NA bottle or container. Every culture media comes with a manufacturers instruction; and you can find that on the side of the bottle or container containing the powdery form of the agar base. Once you read and understand the manufacturer’s instruction, then your problem is half solved. And this applies to all culture media as aforementioned. The calculation here can guide you to prepare other culture media of your choice, no matter the number of plates you may wish to prepare.

After reading manufacturers instruction, the next thing you need to know is that: each Petri dish plate should contain approximately 20 ml of molten NA

According to the manufacturers instruction for nutrient agar (NA), 28 g of NA = 1000 ml (1 Liter) of distilled water.

This implies that, 28 g of NA should be dissolved in 1000 ml of distilled water; and this is usually for 50 plates of NA.

To get the actual volume of water to prepare 26 plates of NA; multiply 26 by 20

i.e., 20 x 26 = 520 ml

NOTE: ’20’ is the volume of molten NA to be poured into each plate; and this volume helps you to get the actual volume of distilled water to use; and you do this by multiplying the total number of plates you want to prepare (in this case, 26 plates) by ‘20‘ as aforementioned.

The actual amount of powdered NA base to weigh or measure out = A (still unknown)

Since, 28 g of NA = 1000 ml; and we need 520 ml of distilled water for this purpose,

therefore, A   = 520 ml

By cross multiplying;

A = 28 x 520 / 1000

Therefore A = 14.56 g

Finally, to prepare 26 plates of NA; you are required to weigh or measure out 14.56 g of NA powder/base, and dissolve this amount in 520 ml of distilled water.

You are now on your way to prepare your 26 plates of NA without STRESS!

Remember; this also applies to other bacteriological and/or mycological media used in the microbiology laboratory. All you need to always have at the back of your mind is that:

1. Always read and understand the manufacturers instruction first.
2. Follow the simple math/calculation above.
3. Ask question where you still did not understand.
4. And you are on your way to greatness and success in your quest.

MicroDok!