140 - Topic 6 Notes

140 topic 6 notes

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Unformatted text preview: Topic 6: Growth of Bacterial Populations Fundamentals of Microbiology (Biology 140) Course notes Dr. Josh D. Neufeld Learning Objectives: Following this lecture, you should understand the concept of exponential growth, and how growth is measured in the laboratory. You should also realize that microorganisms have preferred conditions, and that these differ according to the specific type of microorganism. Different microorganisms have different optima for temperature, pH, water availability, and oxygen. Population growth Growth rate = change in cell # / time Generation time (g) = time for formation of two cells from one = doubling time If the generation time of a culture remains constant, the growth rate will change in an exponential fashion. This is called exponential growth (see Figure 5.8). The generation time (g) can be calculated if the number of cells in a culture at the beginning and end of a given time period are known (Figure 5.9). Growth cycle of populations A typical growth curve for a batch culture is shown in Figure 5.10. Lag phase: the time required for the organisms to adapt to new culture conditions or recover from injury. Exponential phase: exponential growth Stationary phase: no net change in cell number Death phase: some populations are unable to maintain stationary phase indefinitely, and there will then be a net reduction in cell number Measurement of growth The number of cells in a culture can be measured by either direct microscopic count (Figure 5.14) or viable count (Figures 5.15 and 5.16). One of the limitations of the direct method is that it does not distinguish living from dead cells. Another very useful method of estimating cell numbers is to measure the culture turbidity (Figure 5.17). In culture, there is a proportional rela...
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