The formula of CFU/ml is:
CFU/ml = (no. of colonies x dilution factor) / volume of culture plate
For example,
5 g of soil was placed into 10 ml water. The solution was serial diluted 10x (1/10). then 0.1 ml of the dilution was spread onto the agar plate.
the number of fungi colonies that I obtained are 30.
CFU/ml = 30 x 10 / 0.1 = 3000 CFU/ml
For this case I directly used 5 g of moist soil for the dilution.
so I have to get the dry weight of the soil (dry soil).
dry weight of 5 g soil is 4.8 g
then I wonder how to calculate the next steps (CFU/ g)?
3000 CFU/ml / 4.8 g = 625 CFU/g?
Is it correct?
can anyone help me to clarify it?
hi
If i understand your example :
4,8g of dry soil is place in 10mL water so you have 480 mg ofsoil for 1 mL ofwater.
with a dilution to 1/10 you have 48 of mg soil for 1mL of water
and next you spread 0,1 mL of this dilution -> 4,8 mg of soil
you get 30 CFU so you have 30 CFU on 4,8 mg of soil. Consequently you have 30 000 CFU on 4,8g of soil
The correct number of CFU in 1 g of dry weight is 625. Your calculation is correct.
Best regards
Vit
Hello Dear Kuan Kin Pong . You must put 1 gm of soil ( depending on dry weight ) in 9 ml of distilled water to obtain a dilution of 1/10 , then you must make series of dilutions by taking 1 ml from the last dilution and adding it to 9 ml distilled water , then you calculate the CFU as following :
CFU/1 gr soil = (no. of colonies x dilution factor x dilution revers )
Now you obtain CFU per 1 gr , if you want 5 gr , you must multiply the obtained result with 5 .
Best regards
Dear Yehya A. Salih,
What is dilution revers?
Should the dry weight of soil should be calculated?
I found from a book, the formula is
CFU/g = (no. colonies x dilution factor) / dry weight of 1g of moist soil
If I already done the experiment (as explained in my ques), but I still want to know the result of cfu/g. In which I get 30 fungal colonies.
can I use the calculation of CFU/g as described by Hani Antoun?
and if I get only 1 colonies (out of 3 rep) in which I only get 0.33 (mean) colonies. Should it be considered as 0 , or just 0.33?
but if 0.33 CFU divide by 0.0048g , I still can obtain the ans (69 cfu/g) or It is not appropriate to count it?
I know that if only one replicate obtain 1 colonies, it is not a reliable data.
Dear Kuan Kin Pong ,
dilution revers means that if you use a dilution of 1/10000 , you must multiply with 10000 and so, while the dilution factor is a constant factor.
Best regards
Dear Kuang Kin Pong:
I very appologize for my first incorrect answer. The right number is really6250. But I agree with correction made by Yehya A. Salih. It is true that his notice is right.
Sorry again for my incorrect first answer.
Regards
Vit
Dear all,
Thanks for answered my question.
I already understand the principle of soil dilution & the formula cfu/g
Thanks guys.
hi Nidhi Kakkar, CFU stand for colony forming unit.
colony mean number of bacteria that form on the plate
If we pipetted liquid on the plate then we have to count the number of bacteria / the volume of liquid that we pipette to know the corrrect number of bacteria form from the liquid.
Kevin Mok, ya that is my mistake for putting 5g in 10ml. It's a wrong serial dilution. I already corrected it.
But why is it a mistake? It is possible to calculate from your initial soil suspension with the DW and the final volume. Its simply a different way of making the initial suspension.
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