Let’s get to the Rationale of Today’s Exercise. We’re trying to maintain the
Logarithmic Phase of a Bacterial Growth Curve. Each Lab will receive a Flask of
E. coli
in Logarithmic Phase. And we’ll give each Clan a Flask of Nutrient Broth
(which happens to be a Rich Medium). You’ll inoculate your Medium with our
Exponential E. coli
and then monitor their (continued) Exponential Growth with a
Spectrophotometer.
You’ll plot your Data and generate the characteristic “Straight Curve” of
Exponential Growth on SemiLog Graph Paper. This will probably take 5 or 6
Spectrophotometer Readings (so about an Hour and a Half once you’ve setup).
Then you’ll want to use your Data from these Curves to determine the Generation
Time of your Bacterium. There should be no Lag Phase. Bacteria growing in a
Rich Medium like Nutrient Broth will have all the necessary Nutrients for
Growth, plus they’ll have a Pantry full of PreMade Amino Acids, Carbohydrates,
Lipids, Nucleotides, and Vitamins. Bacteria have learned from the School of Hard
Knocks  Natural Selection  that when a Growth Component is already present,
use it and don’t waste any Time or Energy synthesizing it for yourself. These
lucky Bacteria will be growing at their Maximum Rate from your first Time Point
and will have a short Generation Time.
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Determining Generation Time Graphically (i.e. The Really Easy Way)
Well, yes. It’s kind of like how after being molested by an Apple Tree, kidnapped
by Flying Monkeys, and traumatized by The Wicked Witch of the West, Dorothy
learns from The Witch of the North that she really didn’t have to experience any
of these stressful Events. “You always had the Ruby Slippers.” If you were
Dorothy wouldn’t you want to hauloff and punch Glinda’s Lights out?
But I digress. Remember how an Arithmetic JShaped Curve was deemed Useless
but the Logarithmic Straight Line was implied to be eversoUseful? That Straight
Line represents your Ruby Slippers.
All you have to do is pick two Points on the Linear Portion of your Logarithmic
Straight Line that represent a Doubling of the Optical Density (say, 0.1 and 0.2).
Then plot straight down onto the Horizontal Arithmetic Time Scale and read off
the Time this took. And that’s all there is to it!
And finally, a Word about Logarithms:
Today’s Exercise was made possible by the generous Contributions of
Logarithms, which were developed in the Late 16th Century by the Scotsman
John Napier, who realized that any Number  regardless of Size  can be
expressed by the Power to which a “Base” Number must be raised. So the
Number 100 can be expressed as 10
2
. You can multiply any two Numbers by
adding their Logs and you can divide any Number by another Number by
subtracting the Log of one from the other. That how we did it BC
(before Calculators).
Logarithms provided the Means for all the Calculations required to design The
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 Spring '09
 MANN
 Bacteria, Agar plate, Bacterial growth, semilog graph paper, Microtiter Pipettes

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