OD and cfu/ml - (Feb/13/2009 )

I always have the problem in adjusting OD to obtain certain cfu/ml. Is there anyway to do that?

How does OD correlate to CFU? optical density to particles only bij calibration

Nrelo on Feb 13 2009, 04:11 AM said:

If I remember correctly than the relationship between OD and CFU is that OD measures live as well dead cells (optical density of the solution which includes cell debris) where as CFU relates to organisms that can grow on agar plates (the portion that is alive in your solution). I am not sure if there is a way to say if you have a certain OD that you will get a related number of CFU.....maybe when you take the measurement in the log-phase where they are happily growing and where there aren't too many dead cells....

VERY roughly, 1 OD is 1e9 cells/ml

phage434 on Feb 13 2009, 11:44 AM said:

yeah that's the most frequent value i heard about

calibrate your curve to the microorganism you're studying and control the conditions of application (even at same OD , cfu/ml of a suspension sitting in the frig for a week is not going to be the same of a fresh culture.

GeorgeWolff on Feb 14 2009, 05:23 AM said:

OD = dead and viable cell

while

CFU = only viable cells

exactly the point

When we do infections we plate the infection medium, inoculum or what it is you use to infecct.
This way you can check if your OD and the bact/ml (1 OD is roughly 1e9 bact/ml) is live/dead bacteria and you'll know the exact CFU that you've used to infect.

exactly, we used to do it just that way as well. and it's different for every bug; most of the approximations you see will be based on E coli. you could do a couple validations for your bug, to get an idea; but it's still wise to plate every time and infect with dilutions to be certain.