Bacterial growth (plotting data)

John W. Patching 0001705s at Bodkin.UCG.IE
Thu Sep 21 11:01:03 EST 1995

>Is it neccessary to plot growth (aborbance vs. time) on a semi-log scale?
>I agree that plotted data on semi-log gives a better representation of
>growth rate.If one were to describe and compare the amount of growth per
>set time in the growth cycle would plotting the data on a normal scale be
>adequate? I would like to compare the amount of growth using two
>different substrates but at the same set time in growth. Plotting the
>data on semi-log does not give a very good comparison. If anyone out
>there can help please e-mail me.
>                                        Thanking you in advance,
>                                         Frank Pietrantonio
>                                          xpii at musicb.mcgill.ca

Make sure that you know what you are doing before you abandon semi-log
plots. Someone has already pointed out their importance for calculating
exponential growth rates/doubling times and working out if they are
constant or varying.  If you are going to work out the biomass accumuated
in a set time (dont forget the inoculum!) all well and good, but why then
do a graph in the first place? Further, what would such a result tell you?
Is the culture in the stationary phase? In this case you certainly not
measuring anything to do with growth (how long has the stationary phase
been going on, was there a lag phase or not etc.?)  At  best, if you  are
assuming that growth has stopped because of nutrient lack, what  you have
is something vagely related to the yield coefficient for that nutrient.  I
often find that students who are producing growth curves in projects etc.
plead to be allowed to use linear plots because they look like the "right"
shape for a batch culture curve that you see in the textbooks.  This is an
illusion.  What they call the exponential phase is actually a decelleration
phase of growth.  Using a semi-log plot shows the depressing truth. As so
often happens, the exponential phase of growth was pretty short and was
followed by a long decelleration phase.  Further, that  break you took for
sleep/tea/a couple of beers etc. coincided with the  exponential phase, and
you missed it.  Look at the plot and go away and repeat the experiment with
a different sampling schedule.
In case you think this is unfairly directed at students, I would point out
that I have refereed several papers where the same mistake was made by
people who should know better!

John W. Patching
The Martin Ryan Marine Science Institute
University College Galway, Ireland

Email: John.Patching at UCG.IE
Phone: +353-91-24411 Ext 2398
Fax:       +353-91-750456 or +353-91-525005

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