Three grams of soil were added to 27 ml of sterile water and shaken vigorously. After the soil particles settled, 0.1 ml of this was added to 9.9 ml of sterile water. This was further diluted by 4 successive 1/10 dilutions. One ml from the last dilution was used to prepare a pour plate. After incubation, 289 colonies were present on this plate.
What was the number of colony-forming units/gram of the soil?

289 × 10 ⁷ colony-forming units/gram

Explanation:

Given that;

Three grams of soil were added to 27 ml of sterile water;  ----- Let that be A

We would notice from the above statement that 3 grams of soil that was added was equivalent to 1:10 dilution of the sample

After settling of soil particles;

0.1 ml of this was added to 9.9 ml of sterile water.------------- Let that be B

Furthermore,

4 successive 1/10 (1:10) dilutions takes place ------------- Let that be C

NOW, the total dilution we have = 10 ⁻⁷

The question proceeds by saying; One ml from the last dilution was used to prepare a pour plate and After incubation, 289 colonies were present on this plate.

∴ the  number of colony-forming units/gram of the soil =

=

=

= 289 × 10 ⁷ colony-forming units/gram