## Answers

**32)** The **answer** will be **6.3 x 10 ^{6} (Option a)**.

**Explanation:** The countable range for colony is 25-250 CFU/plate. Thus, during calculation we will consider the plate that contains 63 colonies.

Now, dilution factor = Volume of stock solution taken/ Final volume

Now, 1 mL of stock solution is taken from flask to the first tube which contains 9 mL of diluent.

So, dilution factor of first tube = 1/ (1+9) = 1/10 = 10^{-1} dilution

Similarly, in each case there will be 10^{-1} dilution as similar process is followed in each case.

So, in fifth tube (From which 63 colonies grown on the plate) total dilution factor will be 10^{-5}.

Now, concentration of fifth tube is 63 CFU/mL (As 63 colonies are grown on the plate by taking 1 mL. Please note that there is no mention that 1 mL or other values of solution taken from this tube for plating. We assume 1 mL & this is only possible from the given answer options).

Now, cell concentration in fifth tube = Initial cell concentration in flask x total dilution factor (TDF) of fifth tube

So, Initial cell concentration in flask = Cell concentration in fifth tube / Total dilution factor (TDF) of fifth tube = (63 / 10^{-5}) CFU/mL = 63 x 10^{5} CFU/mL = 6.3 x 10^{6} CFU/mL