Culturing microorganisms and measuring growthEdexcel International A Level Biology: Mind map

Exam questions on Culturing microorganisms and measuring growth

1. A technician is preparing nutrient agar plates and inoculating them with a culture of a non-pathogenic strain of Escherichia coli for a practical class. She works near a lit Bunsen burner and uses a metal inoculating loop. The plates will be incubated for 48 hours. The technician must prevent the culture from being contaminated by other microorganisms and must prevent the culture from escaping into the laboratory.
   Explain two aseptic techniques that the technician should use when inoculating the plates.2 marks
2. A food scientist wants to find the number of viable bacteria in a sample of milk. She makes a serial dilution by transferring 1 cm³ of the milk into 9 cm³ of sterile water, mixing, and repeating the transfer with 1 cm³ of each dilution to give dilutions of 10⁻¹, 10⁻², 10⁻³ and 10⁻⁴. She spreads 0.1 cm³ of the 10⁻³ and of the 10⁻⁴ dilutions onto separate nutrient agar plates and incubates them. The 10⁻³ plate has too many colonies to count and the 10⁻⁴ plate has 42 colonies.
   Calculate the number of viable bacteria in 1 cm³ of the original milk.2 marks
3. A flask of nutrient broth was inoculated with bacteria and incubated at 37 °C. Samples were taken at intervals and the viable count was found by dilution plating. The count was 5.0 × 10⁴ cells cm⁻³ at 2 hours, when the culture had entered its exponential phase, and 3.2 × 10⁶ cells cm⁻³ at 8 hours, when it was still in this phase. The count then stayed at about 5 × 10⁸ cells cm⁻³ between 14 and 20 hours and fell steadily after 20 hours.
   Explain the changes in the viable count between 14 and 20 hours and after 20 hours.3 marks