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