Question

I have done these problems but am unsure if the answers I got were correct, Please show all steps so I can fully understand!

1.) How many microliters of original sample are required to produce a final dilution of 10^-3 in a total volume of 1 mL?       INFO: 1 microliter is 10^-6 L or 10^-3 mL.

2.) If you suspect your culture of bacteria has 240 x 10⁶ cells per mL, what would you want the final dilution to be in order to end up with 30 cfu on a plate?

Express your answer as an exponent rounded to two decimal places. Eg. 10.01E-3, not 0.010011.

Answer 1

1)

Dilution = 1/1000

1 part in 1000, so volume (parts) in 1 mL,

= 1/1000 mL

= 10-3 mL

= 1 microliter

2)

you can do serial dilutions :

1st tube : 1/100 dilution

Concentration of cells will be : 240 × 104 cell/mL

2nd tube : 1/100 dilution

Concentration of cells : 240 × 102 cells/mL

3rd tube : 1/100 dilution

Concentration of cells would be : 240 cells/mL

4th tube : 1/2 dilution

Concentration of cells would be 120 cells/mL

5th tube : 1/2 dilution

Concentration of cells would be 60 cells/mL

6th tube : 1/2 dilution

Concentration of cells would be 30 cells/mL

Total dilution of tube 6 :

= 1/100 × 1/100 × 1/100 × 1/2 × 1/2 × 1/2

= 1/8000000