# Plotting growth data

Fri Sep 22 11:06:29 EST 1995

```In article <199509211747.NAA18701 at andromeda.rutgers.edu> kafkwtz at ANDROMEDA.RUTGERS.EDU (David Kafkewitz) writes:

>Subject: Plotting growth data
>Date: 21 Sep 1995 10:50:01 -0700

>There is another reason to use semi-log paper: it reminds you that the
>correct way of determining whether an organism is growing exponentially is
>by plotting Log (cell no.) vs. time. All too often students fall into the
>logical trap of saying that the A value is already a log function (due to
>the Beer-Lambert law) and therefore they can use an arithmetic plot.  They
>forget that they are not really measuring A, but using it as a surrogate
>measure of light scattering.  They also forget that if your eye cannot see
>turbidity,neither can the spectrophotometer. I have seen many a growth curve
>plottted with A values of 0.001, 0.002, etc. I don't know what is measured,
>pobably scratches in the glass.  The rule I learned was that A values are
>linearly proportional to cell mass(or volume) between A=0.1  and A =0.8.
>Thus the log of a value in this range is equivilant to (Log cell #l) as far
>as plotting is concerned.
>David Kafkewitz, Department of Biological Sciences,
> Rutgers University, Newark N.J. 07102, U.S.A.
> 201 648 5865; fax: 201 648 1007
>kafkwtz at andromeda.rutgers.edu

It isn't just students who can fall into the logical trap over using
arithmetic plots for absorbance data in microbial growth experiments.  Back in
the mid 1970's there was a fairly prolonged exchange of letters in the
American Society for Microbiology News over this very topic.  It was
surprising how many microbiologists thought that the spectrophotometer was
doing the log plot for them.
----------------------------------- * ----------------------------------
Austin Reade, Microbiologist      Phone:    (902) 424-8670
Nova Scotia Research Foundation   Fax:      (902) 424-4679
101 Research Drive, PO Box 790    Internet: areade at nsrfc.ns.ca