# Thread: How do we calculate bacterial colony forming units (cfu/ml)?

1. Hi
I am trying to calculate colony forming units calculation. I cultured bacteria overnight (20ml) and did O.D reading at 0.4 and did serial dilution (10*1 to 10*9) and I got 250 colonies at 10*6 dilution. So I got 2.5 × 10*9 cfu/ml. But I need 10*6 or 10*7 CFU/ml (for my experiment). How can I reduce it down to it. Please give example calculation.

Thank you

2.

3. incorrect, you don't have 2,5*10^9 cfu/ml. it's 2,5*10^8 cfu/ml..

250*10^6 --> 25*10^7 --> 2,5*10^8 cfu/ml

If you need 10^6

250*10^6/1*10^6 = 250/1 = 250 times dilluted

for 10^7, you need to dillute 10 times more. so 2500..

Keep in mind not to confuse the dillute factor with the number of cfu/ml.

If you count 100 bacteria at a 10^7th dillution, you probably have somewhere between 10-1000*10^7 cfu/ml. Depending on your method, skill and buffer homogenysation. Depending on the used buffer, you can predict more, or less bacteria. With pfz medium as a buffer, waiting 20 minuts in between experiments, can almost double your actual number. Using a selective 3% ethanole, salt buffer, your actual number can be slightly higher then the value you measured

Mixing with FZ (0,9% NaCl solution) usually provides the best results, though using formalin, ethanol, methanol, chlorine, hydroxide or the opposite peptone, can change the results. It depends on the used bacteria for what is the best solution to dillute.

Though i wonder if this wasn't a homework question... On the other hand, this gave me the opportunity to provide a more depth answer to a simple question.

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