Micro Biology > Lab Report > Lab Report 1: Bacterial Closed System Growth Curve Experiment MCB 3020L Microbiology Lab (All)

Lab Report 1: Bacterial Closed System Growth Curve Experiment MCB 3020L Microbiology Lab

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Abstract The purpose of this experiment is to create a growth curve from indirect measurements of bacterial growth using a spectrophotometer. For a closed system growth curve, bacteria go through four phases which depends on the time. From this we can determine the mean growth constant and generation time. The bacteria used in this growth curve is Escherichia coli or E. coli. A sample of the E. coli is placed into the spectrophotometer and the optical density id determined along with the time taken. A direct measurement is also taken by counting the number of colonies on a spread plate and calculating the cell density. At an absorbance of 0.544 the cell density was calculated to be 5x10-8. Using the graph in figure 1. the growth rate constant was calculated to be 1.02 and the generation time was 58.8. Introduction A growth curve is when bacteria are placed into a favorable growth environment, they will follow a characteristic pattern of increasing in number followed by a decrease in number. This can be plotted as the number of cells present over a period of time. The variables that can increase or decrease a microbe’s growth curve are temperature (minimum, maximum and optimum), pH, pressure, oxygen concentration and solute and water activity. There are four phases of a growth curve. The Lag Phase does not show a significant increase in the number of microbes present. This is a period of time where the cells are adjusting to their new environment and are preparing for cell division. The Exponential Growth Phase or Log Phase represents a period where the population is continuously doubling. This will continue until conditions become less favorable. This is also the phase where generation time can be calculated. Fast growers will have a steep slope. A first look at the Stationary Phase may give the impression that there is nothing happening. It is a flat portion of the curve that shows there is no net increase in the number of cells. That does not mean new cells are never produced. At some point the population stabilizes, that is, the number of new cells is equal to the number of cells dying off. There is cell division occurring it's just that cells that die are replaced in equal numbers. Since we have a closed system in a single test tube, there is no way to add nutrients or remove the waste products that begin to build up. Eventually this leads to unfavorable conditions and a decrease in the number of living cells in the population. This is called the Decline or Death Phase

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