Population growth is inevitable as long as the birth rate of a population is higher than the mortality rate (Trzepacz, 13-2). There are two types of growth that are modeled, exponential and logistic. Exponential growth is when "members of a population double in size throughout each successive generation," (Milam, 2) and is represented by a j-shaped curve. Most populations follow this pattern, as long as the amount of resources outweigh the population and there is little-to-no competition (Trzepacz, 13-2). Most often these exponential growth curves will eventually begin to form a more s-shaped curve. This is logistic growth, which occurs when a population increases, but the resources start to become limited (Milam, 2) and it consequently reaches the carrying capacity. In this experiment Escherichia coli bacteria is used to represent population growth.
E. coli is defined as "a rod-shaped bacterium commonly found in the lower intestine of warm-blooded organisms," (Milam, 8). This means that it is readily available and easy to access, which is part of the reason why it was the best choice in this experiment. It also has a high growth rate. In a study in the journal Talanta they are testing the growth of E. coli on the surface of GaAs (001) at 37 degrees Celsius over a 24-hour time period and it was shown that the number of bacteria doubled about every thirty to forty-five minutes (Nazemi, 73). This helps substantially in this experiment, because it allows for a good representation of a logistic growth curve due to the fact that the population can increase and reach its carrying capacity in the span of one day.
The objective of the E. coli growth experiment was to provide insight into how the population of E. coli grows in different research environments like the four varying temperatures they were subjected to (Trzepacz, 13-8). We used this to compare and better visualize what it is like in other populations. From this information it can be hypothesized that the higher the growing environment temperature, the more rapid the reproduction of the E. coli, meaning that it will reach the carrying capacity sooner.
In this experiment the materials used include a plastic pipette, a cuvette, a spectrophotometer, flasks, and an incubator. This experiment began by dispersing the E. coli into flasks labeled A through T. each of these flasks were then separated into four different shaking incubators which were set to four different temperatures. Flasks A-E were placed into an incubator set to 25 degrees Celsius, F-J were set into another incubator at 30 degrees Celsius, K-O were set into the third at 37 degrees Celsius, and P-T were incubated in the last machine at 47 degrees Celsius. Each incubator was then set to rotate at the same speed, 125 rpm. Then, a plastic pipette was used to transfer 1.0 ml of a designated culture (in lettered beakers) of the bacteria into the cuvette, which is the plastic box-like vessel. Precautions were taken to be sure that the clear portions are not touched, as it would have interfered with the results given by the spectrophotometer. The spectrophotometer is a device used to measure the amount of light being absorbed, therefore showing the population growth as the amount of light being absorbed increases. The spectrophotometer was transmitting light at OD600. The machine was calibrated by inserting a cuvette containing only 1.0 ml of sterile growth media into the machine and pressing the button labeled "Blank." This allowed for the next steps to be completed, the cuvette was then filled with a ml of the E. coli and placed in with the arrow on the side facing the left. The "Sample" button was pressed, and the value and time of incubation were recorded. The cuvette was then cleaned, with every 20-minute interval throughout the day a new sample was collected, and the absorbance measurement and time was recorded accordingly. Finally, an average was calculated and recorded for each group.
To determine the temperature that was best fit for the E. coli to grow in, the bacteria samples were placed into four different incubators with four different temperatures. Within the first 20-minute interval most of the bacteria populations had each more than doubled. At first the populations multiplied at about the same rate, until approximately the 289-minute mark (Figure 1). From this point the populations appear to grow at a higher rate and begin to exhibit the two different types of growth curves.
Figure 1. The absorbance of light through the E. coli populations over time. At the 32-minute mark students began maintaining readings from the four environments (25C, 30C, 37C, 42C) using the spectrophotometer and the absorbance (OD 600) was recorded at 20-minute intervals. Each point represents the average light absorbance of a sample from a specific temperature at a specific interval compiled from 20, 5 in each temperature setting, different beakers of the E. coli.
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