Overview
BacterialGrowthin LiquidMedia
LEARNING OBJECTIVES Overall: Examine growth of bacteria in liquid broth both qualitative and quantitatively Detailed: 1. Understand how turbidity measurements can be correlated to growth of bacteria. 2. Draw a bacterial growth curve and describe what is happening in the cells in each phase. 3. Calculate generation time 4. Predict the effect of different growth conditions on a growth curve 5. Predict expected results of an experiment that studies bacterial growth. 6. Analyze the results of a simple experiment that studies bacterial growth. BACKGROUND Bacteria multiply by the process of binary fission. Binary Fission is a type of cell division wherein number of cells are doubled after each division (1-> 2-> 4->8->16->32.....). The time it takes for a population to double in number is called the generation time. This time varies among different species and is influenced by the conditions in which the cells are grown, such as temperature, oxygen, and acidity. Each species of bacterium has an optimum for its growth conditions. Growth depends upon several environmental factors surrounding the bacteria such as pH, temperature, oxygen content and presence of nutrients in the medium. When the supply of nutrients is limiting, bacteria will deplete the nutrients in their environment and produce waste products which will eventually inhibit their growth Bacterial growth can be followed by determining the number of bacteria in a sample over a period of time. An increase in the number of bacteria over time indicates growth. Two common methods used to determine growth are (i) standard plate count, and (ii) measurement of absorbance. In the standard plate count method, a dilute bacterial sample is spread on an agar plate and the number of colonies that are formed are counted after 24-48 hours of incubation. This method assumes that a single bacterium will divide to produce a colony on the plate. Therefore, the number of colonies multiplied by the dilution factor (in cases where the sample is diluted before plating) will give the number of bacteria in the sample. The counts determined by the standard plate count is reported as colony forming units (CFU). This method counts only viable cells (live cells capable of forming colonies). The second method follows the growth of bacterial cells by measuring turbidity or cloudiness of the liquid culture. Turbidity of a culture increases as cells divide and increase in numbers. Turbidity is measured using a spectrophotometer. FIG 1 shows culture tubes containing increasing bacterial concentration (number of bacterial per mL of broth). As number of cells increase (from tube left to right), the culture becomes more turbid. This is an indirect