Lab 4: Using The Spectrophotometer To Measure The Rate Of
Bacterial Cell Division: A Bacterial Population Growth Curve
Introduction
In order to understand
bacterial growth behavior, one needs to recognize the different stages of
growth. Through the use of spectrophotometer, one can measure the turbidity of
a sample for a certain time frame and graph the results to form a growth curve.
In this lab, students are observing the stages of growth form previously
cultured E.coli samples.
Bacteria cells growth are
dependent upon nutrient source and amount along with temperature. These growth factors allow for cells to divide
at a steady, rapid growth through the process of binary fission. This progression has four phases of growth. Lag-phase,
the initial phase, is the time that it takes for bacteria to acclimate to its
surroundings. There is not population growth during this phase as it is just
adapting to the environment in preparation for the growth process. Once the
bacteria is well adjusted, the cells begin to divide and devour their nutrient
source transiting into the log phase. In this phase, the bacteria population surges
exponentially. After some time, the bacterial population becomes crowded and cell
division recedes entering in to the stationary phase. When the bacterial food
source becomes scarce and space it too limited the cells enter the final phase
of death.
Methods and Materials
There were ten time
samples of E.coli, in which students were to measure the turbidity. This was
done through the use of a spectrophotometer set at a wavelength of 600nm. Due to the time, lab groups were to only
measure four samples and collaborate with the rest of the class for remaining
samples. Students were to take a 1 mL sample, with our awesome pipette skills, and
place in a clean cuvette. Depending on which samples your group choose, student
may have had to dilute the E.coli in order to obtain an accurate reading. With
the guidance of our lab manager, readings for each sample were taken. All
readings are located in table1. Once we
had our reading, a growth curve for the E.coli cells could be extrapolated,
shown in figure 1.
Results
Table 1 – Time and Absorbance
|
Time (hr)
|
Absorbance (nm)
|
Average
|
Standard Deviation
|
||
|
1
|
-0.007
|
-0.054
|
0.01
|
-0.017
|
0.033151169
|
|
2
|
0.134
|
0.117
|
|
0.1255
|
0.012020815
|
|
3
|
0.132
|
0.132
|
0.378
|
0.214
|
0.142028166
|
|
4
|
0.82
|
0.462
|
0.775
|
0.685667
|
0.195003419
|
|
5
|
0.456
|
0.488
|
0.476
|
0.473333
|
0.016165808
|
|
6
|
0.598
|
0.386
|
|
0.492
|
0.149906638
|
|
7
|
0.718
|
0.744
|
|
0.731
|
0.018384776
|
|
8
|
0.72
|
0.86
|
|
0.79
|
0.098994949
|
|
9
|
1.006
|
1.126
|
|
1.066
|
0.084852814
|
|
18
|
1.054
|
1.154
|
|
1.104
|
0.070710678
|
Figure
1 – Growth Curve of E.coli cells
Discussion
As the time increased, the absorbance
increased as well, because the amount of light that can pass through changes as
the turbidity changes. This is also due to the dilution factor that many of the
samples partook in. As indicated in
figure 1, the lag phase did last very long, hours 0-1. On the other hand the log
phase was current in hours 2-9. Unfortunately,
I do not believe that our data is very good. The growth curve is very hard to
relate in terms of
