# Bacterial Calculation

• Author
Posts
• #17040
chandch
Participant

Hi
I am trying to calculate colony forming units calculation. I cultured bacteria overnight (15ml) and did O.D reading at 0.4 and did serial dilution (10*1 to 10*9) and I got 55 colonies at 10*4 dilution. So I got 5.5 × 10*6 cfu/ml (original, if I am right). But I need 1 ×10*7 CFU/ml (for my experiment). How can I Increase it it. Please give example calculation.

2) I also cultured bacteria overnight (15ml) and did O.D reading at 0.4 and did serial dilution (10*1 to 10*9) and I got 85 colonies at 10*5 dilution. So I got 8.5 × 10*7 cfu/ml (original, if I am right). But I need 1 ×10*7 CFU/ml (for my experiment). How I can I reduce it down?

Thank you

• #112988
JackBean
Participant

if you get 55 colonies at 10^4 dilution, shouldn’t you have 5.5 . 10^5 CFU?

• #113373
vanwa71
Participant

Check out this serial dilution calculator for the future. Automated and flexible. [link]

• #113375
EnricoPallazzo
Participant

Hallo,
You don`t need a serial dilution calculator … seriously.
If you get 55 colonies at a 10^4 dilution, then you have 5.5*10^5 cfu/mL (in case you used an aliquot of 1 mL). If you count 85 colonies at a 10^5 dilution then it`s 8.5*10^6 cfu/mL, which is already close to the concentration you need. To get more precise values, you could grow the cells to a higher density than you actually need and dilute them to the desired concentration.