lab_10_bacterial_growth_curve

lab_10_bacterial_growth_curve - Miramar College Biology 205...

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Lab Exercise 10: Bacterial Growth Curve & Serial Dilutions Page 1 of 4 Miramar College Biology 205 Microbiology Lab Exercise 10: Bacterial Growth Curve & Serial Dilutions Background 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 1) 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. 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. Death phase : Because of the continuing depletion of nutrients and buildup of metabolic wastes, the microorganisms die at a rapid and uniform rate. This decrease in population closely parallels its increase during the log phase. Theoretically, the entire population should die during a time interval equal to that of the log phase. This does not occur, however, since a small number of highly resistant organisms persist for an indeterminate length of time. Figure 1: Hypothetical growth curve for an hypothetical population. Note the four phases of growth: lag; exponential; stationary and death. Construction of a complete
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lab_10_bacterial_growth_curve - Miramar College Biology 205...

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