Growth Rate of Microorganisms (AQA A Level Biology)

• The population growth rate of microorganisms, such as bacteria or yeast, can be investigated by growing the microorganisms in a broth culture
• The turbidity of the suspension can then be used as a way of estimating the number of cells (i.e. the population size) of the microorganisms in the broth culture
  • Turbidity is simply a measure of the cloudiness of a suspension (i.e. how much light can pass through it)

• As the microorganisms in the broth culture reproduce and their population grows, the suspension becomes progressively more turbid (cloudy)
• This changing turbidity can be monitored by measuring how much light can pass through the suspension at fixed time intervals after the initial inoculation of the nutrient broth with the microorganisms
  • A turbidity meter, a light sensor or a colorimeter (connected to a datalogger) can be used to take these measurements

• The results can then be used to plot a population growth curve to show how the population of microorganism grew over time

• Bacterial colonies can grow at rapid rates when in culture with very large numbers of bacteria produced within hours
• Dealing with the experimental data relating to large numbers of bacteria can be difficult when using traditional linear scales
  • There is a wide range of very small and very large numbers
  • This makes it hard to work out a suitable scale for the axes of graphs

• Logarithmic scales can be very useful when investigating bacteria

Using logarithms to deal with orders of magnitude

• Logarithmic scales allow for a wide range of values to be displayed on a single graph
• For example, yeast cells were grown in culture over several hours. The number of cells increased very rapidly from the original number of cells present
• The results from the experiment are shown in the graph below, using a log scale
  • The number of yeast cells present at each time interval was converted to a logarithm before being plotted on the graph
  • The log scale is easily identifiable as there are not equal intervals between the numbers on the y-axis
  • The wide range of cell numbers fit easily onto the same scale

Image showing the number of yeast cells grown in culture over 10 hours, using a logarithmic scale

• The pH scale is logarithmic
  • The concentration of hydrogen ions varies massively between each pH level

Image showing the range of hydrogen ion concentrations within the pH scale