Dear all,

I am confused regarding the calculation of dilution factor. As far as I know, dilution factor = volume of sample/total volume and CFU = (#colony x dilution factor)/volume plated in mL.

For example, if I added 1g of sample into 9mL of broth - 10^-1, and transfer 1mL from 10^-1 to second tube (9mL as well). Then this will be 10^-2.

Thus, if 1000mL of sample in tube 11 plated onto the agar and I obtained 30 colonies in this plate, CFU = (30 x 10^-11)/1000mL

Well, now came to my confusion part,

What if I added 1g of sample directly to 99mL of broth, what is the dilution factor here? According to formula it will be 1/100mL or 10^-2? But 100mL is equivalent to 11 tubes of 9mL broth, so if look at this way, it will be 10^-11 instead of 10^-2?

Please advice.

Thank you.