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Topic 6.pdf

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BIOL 240
Josh Neufeld

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Fundamentals of Microbiology Biology 140 Course notes Dr Josh D Neufeld Topic 6 Growth of Bacterial Populations 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 ratechange in cell timeGeneration time gtime for formation of two cells from onedoubling timeIf the generation time of a culture remains constant the growth rate will change in an exponential fashion This is called exponential growth see Figure 58The 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 59Growth cycle of populations A typical growth curve for a batch culture is shown in Figure 510Lag phase the time required for the organisms to adapt to new culture conditions or recover from injuryExponential phase exponential growthStationary phase no net change in cell numberDeath phase some populations are unable to maintain stationary phase indefinitely and there will then be a net reduction in cell numberMeasurement of growth The number of cells in a culture can be measured by either direct microscopic count Figure 514 or viable count Figures 515 and 516 One of the limitations of the direct method is that it does not distinguish living from dead cellsAnother very useful method of estimating cell numbers is to measure the culture turbidity Figure 517 In culture there is a proportional relationship between turbidity and cell number
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