Population growth

Microbial population growth can be studied using the colorimeter to measure transmittance or absorbance.


Yeast is easy to use and gives good results. We have found the doubling time for a culture of yeast grown at 30°C to be about 4.5 hours. The actual number of cells can be measured using a haemocytometer.

Growth medium

The results shown below were obtained using the following recipe at 35 °C with aeration.

  • Sucrose 5g
  • Yeast extract* 1g
  • Peptone* 1g
  • Water to 100cm3

    *Yeast extract and peptone can be replaced with 2g Marmite or Vegemite


Within the range 0.1 to 0.7 absorbance units the number of cells per cm3 can be approximated using this formula.

    number of cells (millions per cm3) = 50 × (absorbance – 0.05)

Go back to the top of the page

E. coli

The results shown below were obtained by growing a culture of E. coli B in nutrient broth at 25°C in a 4cm3 cuvette. The colorimeter containing the cuvette was connected to a computer then placed in an incubator to maintain the temperature. The absorbance of blue light was recorded every 10 minutes for 8 hours.

    E. coli
Go back to the top of the page