Dear All,
I need your help on how to calculate CFU/g soil. I would greatly appreciate if you could check if my calculation is correct.
5 g of soil was placed into 50 mL water. The solution was serial diluted 10x (1/10). 0.1 mL of the dilution was spread onto the agar plate.
30 bacterial colonies were obtained.
My calculation:
5 g of soil was placed in 50 mL water so I have 100 mg soil in 1 mL of water.
With a dilution of 1/10, I have 10 mg of soil in 1mL of water
Next, I spread 0.1 mL of this dilution -> 1 mg of soil
I have obtained 30 CFU so I have 30 CFU on 1 mg of soil. Consequently, I have 30 000 CFU on 1 g of soil
Is this correct?
Thanks in advance.
@ Wong, CFU /g soil = Count per Plate/ dilution factor
For example, if 30 colonies are present on 10 -6 dilution plate, the calculation will be:
CFU = 30/ 10 -6 = 3x 10 7 colonies per gram soil.
@ Wong, CFU /g soil = Count per Plate/ dilution factor
For example, if 30 colonies are present on 10 -6 dilution plate, the calculation will be:
CFU = 30/ 10 -6 = 3x 10 7 colonies per gram soil.
Dear Wong,
Your calculation is correct and the steps simple. I do hope your soil sample was composite enough and your plated 0.1ml same. The idea is for representative sampling. So if all the cautions were followed, good, very good.
You have diluted it to 1/10 and then spreaded. You should have performed at least 5 dilutions. Though your calculation seems to be fair. Good luck.
I recommend you to make serial dilutions 3-5 times and express the results as log10 CFU per gram of soil.
Regards,
Youssef
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