3Thurs_Growth Curve

3Thurs_Growth Curve - Lab Exercise 13: Growth Curve...

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Lab Exercise 13: Growth Curve OBJECTIVES 1. Know the different phases of a standard growth curve. 2. Understand and perform direct measurement of bacterial growth through serial dilutions and standard plate counts. 3. Understand and perform indirect measurement of bacterial growth through spectrophotometer readings and optical density measurements. INTRODUCTION Bacterial population growth studies require inoculation of viable cells into a sterile broth medium and incubation of the culture under optimum temperature, pH, and gaseous conditions. Under these conditions, the cells will reproduce rapidly and the dynamics of the microbial growth can be charted by means of a population growth curve , which is constructed by plotting the increase in cell numbers versus time of incubation and can be used to delineate stages of the growth cycle. It also facilitates measurement of cell numbers and the rate of growth of a particular organism under standardized conditions as expressed by its generation time , the time required for a microbial population to double. The stages of a typical growth curve (figure below) are: 1. Lag phase : When the cells are adjusting to their new environment. During this phase, cellular metabolism is accelerated, resulting in rapid biosynthesis of cellular macromolecules, primarily enzymes, in preparation for the next phase of the cycle. Although the cells are increasing in size, there is no cell division and therefore no increase in numbers. 2. Logarithmic (log)/Exponential phase : Under optimum nutritional and physical conditions, the physiologically robust cells reproduce at a uniform and rapid rate by binary fission. Thus there is a rapid exponential increase in population, which doubles regularly until a maximum number of cells is reached. The length of the log phase varies, depending on the organisms and the composition of the medium, although the average may be estimated to last 6 to 12 hours. 3. Stationary phase : During this stage, the number of cells undergoing division is equal to the number of cells that are dying. There is no further increase in cell number and the population is maintained at its maximum level for a period of time. The primary factors responsible for this phase are the depletion of some essential metabolites and the accumulation of toxic acidic or alkaline end- products in the medium. 4.
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This note was uploaded on 12/23/2009 for the course BIO 205 taught by Professor Murphy during the Fall '09 term at Miramar College.

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3Thurs_Growth Curve - Lab Exercise 13: Growth Curve...

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