I have constructed an OD600 vs cfu/ml regression line plot to find the line of best fit allowing a conversion from optical density to cell number. I'd like to know if this conversion for one species in one media would be approximately representative of the growth of that species in all media if exponential growth was observed?
It really depends on the bacteria but at best it would be a very rough approximation. A standard curve should really be produced for each medium. Media which induce a change to the outer surface of the bacteria will cause a different light scattering effect, and so, a different OD. A new standard curve should really be produced even when dealing with different mutants of the same species.
"..approximately representative.." ? Yes.
If you are going to use the OD600 (in different media) as a qualitative estimate, then you should be fine. If you want to use the OD600 values to quantify small differences in growth across different media, better to use a standard curve specific for each medium.
Good luck!
Thanks, Michael & Nitesh, you have both confirmed what I was thinking really. I mean even when these conversion charts are generated for one species in a particular medium, it really is only an estimate to begin with. Granted it is a very best estimate, but one that would lose its crude accuracy if using a different media and assuming the same growth profile. I just needed a quick look see and was not going to be using it quantitatively. Thanks for the quick response guys, much appreciated !!
Your OD600/cfu curve is, unfortunately, only valid for the strain and conditions under which it was determined. OD 600 is more closely related to biomass than cfu, and as cells get bigger under conditions of rapid growth, you get less cfu per OD unit.
Nonetheless, the OD still remains useful as a rule of thumb. It will rarely be wrong by more than a factor of 10.
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