• 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