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