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Lab Exercise 14: Data Analysis & Presentation I
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Miramar College
Biology 205 Microbiology
Lab Exercise 14: Data Analysis & Presentation I
Background
In
Lab Exercise 10:
, you examined two ways of collecting data about the growth
of a bacterial population.
You measured the populations indirectly, using a spectrophotometer, and directly using serial
dilutions and spread plating.
The optical density (OD) readings you collected using the spectrophotometer can be used
to graph the growth of a population over time without any further manipulation.
The colony forming unit (CFU) data
you collected, however, need a little explanation. You collected these data by counting the CFUs on your plate and
then correcting for the dilution of the plate (
i.e
., 10
4
, 10
5
, 10
6
, 10
7
).
For instance, a 10
5
with 230 colonies on it would be
expressed as 230 × 10
5
= 2.3 ×10
7
(remember to take the reciprocal of the dilution to yield
the
dilution
factor
or
DF
).
What that notation represents is the number 23,000,000, which is large and cumbersome and so expressing these
numbers is scientific notation is preferable.
It is important to note, though, that although numbers written in scientific
notation may seem close to one another, each number change for the 10 is a tenfold change in the amount you’re
talking about.
If you need a refresher on this concept, reread the
Background
information from
Lab Exercise 2:
The
.
Scientific notation allows numbers who are magnitudes different from one
another to be compared easily, by counting the zeros separately.
For instance, if your CFU count is 230 on a plate that was diluted to 10
5
, you have 230 × 10
5
CFUs/ml, or more correctly
2.3 × 10
7
CFUs/ml.
What this means is that there were 23,000,000 CFUs/ml in the original culture.
Let’s say that at the
next 30 minute reading, you count a total of 1.1 × 10
9
CFUs/ml.
These two numbers may not seem very different from
one another, but a glance at Table 1 will show you how different they are.
As you can see, the change in the actual digit
between 10
5
and
10
5
is quite
a bit larger than it looks at first glance.
time
plate
count
dilution
CFU/ml
(scientific notation)
natural number
mantissa
90
230
10
5
230 × 10
5
= 2.3 ×10
7
23,000,000
7.36
120
115
10
7
115 × 10
7
= 1.1 × 10
9
1,100,000,000
9.04
Table 1:
A comparison of data as noted in scientific notation; natural numbers and the mantissa (result of the common log of the natural number).
If you tried to graph the two natural numbers in Table 1, your X axis would be too large to fit on a piece of paper.
In
order to graph large numbers, one of two things must be done.
Large numbers that are expressed in scientific notation
can easily be graphed using
logarithmic
or
semilogarithmic graph paper
.
Logarithmic graphs have cycles of 10 whose
digits vary in distance from one another in order to accommodate the wide data set.
Semilogarithmic graphs have one
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This note was uploaded on 12/23/2009 for the course BIO 205 taught by Professor Murphy during the Fall '09 term at Miramar College.
 Fall '09
 Murphy
 Microbiology, Bacteria

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