Competent cells at OD = 0.94

D.K. dk at
Fri Mar 15 22:35:43 EST 2002

Peter Ianakiev <ianakiev at> wrote:
>Hi guys,
>Has someone tried to make competent cells at OD600=0.94?
>Original paper has been published by Tang, X. et al. 1994 Nucleic acids
>Res. 22, 2857-2858.  I wonder if someone reproduced the results.

Whenever OD600 is concerned, reproducibility is pretty much 
_impossible_. This value is spec-dependent and since nobody 
ever mentions what spectrophotometer they used, it is safe
to consider it free of useful  information. 

Here is why. When you measure OD600, you measure 
a degree of scattering, i.e. a cone of light. Hence, the amount 
of light that gets into photomultiplier will depend on 
1) cuvette to window distance, 2) window area. These things
vary widely and I've even seen case two different specs gave
1.0 and 0.3 for the same sample. (So much fort the endless 
things like "it is absolutely crucial to induce at OD 0.8-1.0" or 
"the best competent cells are obtained from cultures in the OD600
range of 0.35 to 0.45"). 

Just grow some cells and wash them for electroporation. It is
pretty difficult to go wrong there, and results are almost always
superior to all other methods.


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