There is no way to directly convert genomic copies to CFU/ml. Copy numbers are typically used for reporting sensitivity for molecular methods, CFU/ml is for culture-based approaches. You can do two parallel experiments (plating a serial dilution, then quantify those with PCR) but PCR will likely have higher sensitivity than plating so you still have to extrapolate sensitivity, not measure it directly. In short, copy numbers/ul should be sufficient to report sensitivity for a molecular method.

Thank you for your answer. My supervisor said I need to do a CFU/ml count for my thesis to compare it to blood culture methods. It's a PCR test that would detect E.coli in sepsis patients and because the gold standard is blood culture I think we want to compare it to CFU/ml for that reason. I do agree though that the copy number is the most accurate way to measure its sensitivity.

Also, if I wanted to convert DNA concentration in ng/ul to copy number, how would I do that?

Thanks for the help! Péter Gyarmati

the dead E.coli also contains the DNA, the PCR will amplify those DNAs.

you could try to spin the samples, remove the sup (there will be some naked DNA in the sup), wash several times, then serial dilute it, one for PCR, one for staining (so count both live and dead E.coli).

William Mears the average MW of a nucleotide is 330 g/mol, 650 for a base pair. You know the length of the genome of your bacteria, so just calculate a weight of one bacterial genome and divide with the total DNA weight. Or just insert your numbers here:

https://www.thermofisher.com/us/en/home/brands/thermo-scientific/molecular-biology/molecular-biology-learning-center/molecular-biology-resource-library/thermo-scientific-web-tools/dna-copy-number-calculator.html