You dilute a sample, for instant 1mL of orange juice in 9mL sterilized Ringer's solution. After that, you make serial dilutions, taking one mL of the primary sample and put it into 9mL of Ringer's solution. Then, for example, you take 100μL from the first tube (not from the diluted primary sample) and you overlay it on a petri with a medium and the final colonies are 36. The right logcfu/mL is log(36*1000)?
I agree with the back calculation from 36 colonies to 3600 colonies per ml in the original juice BUT when you ask for a 'log number' I assume you mean log to the base 10. That being the case log to the base 10 of 3600 is NOT 36 - look this up in log tables or on your calculator and it is 3.556
Dear Michael,
The first dilution (1 mL orange juice in 9 mL solution) = 10 times dilution = 10-1
1 mL from 10-1 to 9 mL solution = 10-2
Did you diluted it more? if not, then 100 μL of 10-2 dilution on petriplate gives you x1000 cfu (100 for your dilution and 10 for converting 100 μL to 1 mL).
In short your calculation is right.
Good Luck.
Hello,
While pouring 1 mL juice into 9 mL Ringer's solution, you will make 10 fold dilution. At this scenario, total dilution would be 1/10, i.e. dilution factor being 10 . If you overlay 100μL (which is equivalent to 0.1 mL) this solution onto agar plate and got 36 colonies, the number of colony forming unit per mL is calculated as:
CFU/mL = (No. of colonies * dilution factor) / (volume of sample plated in mL unit)
i.e. CFU/mL= 36*10/0.1
= 360/0.1 = 3600.
Therefore, number of colony forming unit per mL of concerned solution is 3600. And, the log number is 36.
you can calculate the number of bacteria by CFU/ ml= (numbers of show bacteria grow * dilution factor) / (volume of sample plated in ml unit)
You dilute a sample, for instant 1mL of orange juice in 9mL sterilized Ringer's solution = this makes 10 fold dilution
After that, you make serial dilutions, taking one mL of the primary sample and put it into 9mL of Ringer's solution = this makes 10 fold dilution, so you made 100 dilution as final dilution (*)
. Then, for example, you take 100μL from the first tube (not from the diluted primary sample) and you overlay it on a petri with a medium and the final colonies are 36 = you took 10% of total vol (100ul from 1ml), so there must be 10x more cells in 1ml because you counted 10% (100ul).
. The right logcfu/mL is log(36*1000)? ? ? = you counted 36 colonies from 100ul from the first tube, so there must be 36 x 10 in 1ml. You diluted 100 fold from the original (*), so your original sample must have 36 x 10 x 100. You are correct!!!~~~
Thank all of you for your answers! I have been conducting microbiological analyses for 4 years, but there was a conflict with a colleague of mine and I wanted to ask you about my calculations!!! Thank you very much!!!
I agree with the back calculation from 36 colonies to 3600 colonies per ml in the original juice BUT when you ask for a 'log number' I assume you mean log to the base 10. That being the case log to the base 10 of 3600 is NOT 36 - look this up in log tables or on your calculator and it is 3.556
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