E. Coli Growth Curves

Paul N Hengen pnh at ncifcrf.gov
Fri Jun 13 12:03:57 EST 1997


Joe Boutell (boutell at cf.ac.uk) wrote:

>> I want to do a growth curve on my E. coli to see how expression of a
>> variety of plasmids affects them; what's the best way?  Can you spin down
>> the cells and weigh them, or is taking an OD more accurate (and at what
>> wavelength)?

| The OD will not tell you if the plasmid is stable in E. coli. For
| studying that you'll have to plate dilutions on selective media.

> But how about if you're growing the E. coli up in selective liquid media 
> and measuring the OD 600?? They must have the plasmid to grow in the 
> media, and if most of the bugs are chucking out a particular plasmid 
> because its toxic, that line of bugs will take longer to grow.
> 
> Joe Boutell.

I don't think this is absolutlely true. If you grow a population of bacteria
in selective liquid media, a certain proportion of the cells will not carry the
plasmid and they will not be resistant to the antibiotic, yet grow in liquid.
This is particularly a problem with secreted lactamases like Amp resistance.
You can prove this if you want, but my feeling is that growing up a culture in
liquid and determining density is different from determining colony forming
units (CFU) on solid selective agar plates. You can prove this by growing a
culture in selective liquid media and plating dilutions onto non-selective
plate media, then toothpick the colonies onto selective plates. Calculate the
ratio of those that have the marker to those not having it. I doubt it will be
100%. I think that if there are slight differences in the growth rate with the
plasmid resident, you won't see it unless you plate. My argument is that using
OD for determining growth rate is _not_ as accurate as plating for viability.

-Paul.

--
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* Paul N. Hengen, Ph.D.                           /--------------------------/*
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