One gram of soil was suspended in 99 ml saline and thoroughl

One gram of soil was suspended in 99 ml saline and thoroughly mixed. From this suspension, 5 serial, 1/10 dilutions were made. 100 hundred microliters of each of the last four serial dilutions were used for spread plating on TY media, resulting in 1245 colonies, 598 colonies, 69 colonies and 12 colonies. What do you calculate as the bacterial concentration in the soil? (For your information 1 gram = 1 ml)

Please help! And explain! Struggling! Thank you.

Solution

Answer:

Dilutions:

Dilution 1 (1g soil into 99 mL water): 1/100

Serial Dilution 1 : 1/10^3

Serial Dilution 2: 1/10^4

Serial Dilution 3: 1/10^5

Serial Dilution 4: 1/10^6

Serial Dilution 5: 1/10^7

Given:

Let’s take tube number 4.

Dilution factor= 1/10^6

Number of colonies= 69

Volume of sample = 0.1 mL to the plate

CFU/ml = ?

cfu/ml = (no. of colonies)/ dilution factor x volume of culture plate

cfu/ml = (69)/ (1/10^6 x 0.1)

cfu/ml = 69 x 10^7