Question

1. You are given a 1 gram soil sample of unknown bacterial load. After doing 10-fold serial dilutions of the soil in sterile water, 100 uL volumes are taken from each dilution for preparation of pour plates. Following incubation, each half of the 10-8 plate has 46 colonies.
a) What was the dilution factor?
b) How many bacteria were present in the soil?

2. Staphylococcus aureus divides every 20 minutes. A culture begins with 10 bacterial cells.
a) After 5 hours, how many generations have occurred
b) After 5 hours, how many bacteria are present?

3. How many milliliters would you need to prepare a 10-2 dilution from a 10ml starting culture?

Answer 1

Answer:

1a. 1:10

1b. 4.6×10¹³

2a. 15generations

2b. 327,680

Explanation:

10 fold serial dilution means 1ml of stock is mixed in 9ml distilled water. This is called as 10⁻¹ solution.

In this way, take 1ml each of every dilution and mix in 9ml distilled water to make 10⁻², 10⁻²-------10⁻⁸ dilutions.

Now, CFU/ml = no. of colonies × dilution factor/ volume of sample plated.