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Practical skills assessed in a written examination

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Question 30

Scientists wanted to estimate the number of bacterial cells in a 45 cm345\text{ cm}^345 cm3 suspension of Lactobacillus acidophilus. They carried out the following two serial dilutions:

  • 1 cm31\text{ cm}^31 cm3 of the original suspension was mixed with 9 cm39\text{ cm}^39 cm3 of sterile nutrient broth to make Solution 2.
  • 1 cm31\text{ cm}^31 cm3 of Solution 2 was mixed with 9 cm39\text{ cm}^39 cm3 of sterile nutrient broth to make Solution 3.

The scientists transferred 0.2 cm30.2\text{ cm}^30.2 cm3 of Solution 3 onto an agar plate. After incubation, 333333 separate bacterial colonies grew on the plate.

Calculate the number of bacterial cells in the original 45 cm345\text{ cm}^345 cm3 suspension.

Express your answer in standard form to three significant figures. Show your working.

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Practical skills assessed in a written examination Questions

  1. A Level
  2. /Biology
  3. /Practical skills assessed in a written examination