Culturing microorganisms and measuring growthEdexcel International A Level Biology: Flashcards
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What is aseptic technique?
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- What is aseptic technique?
- Procedures that prevent contamination of a culture and stop the culture escaping into the environment.
- Why is an inoculating loop flamed until red hot?
- To kill any microorganisms on it so contaminants are not introduced and the culture is not spread.
- Why is the neck of a culture bottle flamed?
- To sterilise it and create an upward current of air that carries airborne microorganisms away.
- Why are school cultures incubated at 25 °C or below?
- To reduce the chance of growing human pathogens, which grow best near 37 °C.
- What is a viable count?
- A count of living cells that can divide to form colonies.
- How do you calculate cells per cm³ from a plate count?
- Colonies ÷ volume plated (cm³) × dilution factor.
- What is a serial dilution?
- Repeated transfer of a fixed volume of culture into a fixed volume of sterile diluent, such as 1 cm³ into 9 cm³ for 10-fold dilutions.
- What is the disadvantage of a haemocytometer count?
- It counts dead as well as living cells unless a stain is used.
- How does turbidity measure growth?
- More cells scatter or absorb more light, so absorbance in a colorimeter increases, compared with a blank of sterile broth.
- Name the four phases of the bacterial growth curve.
- Lag, exponential (log), stationary and death.
- What happens in the lag phase?
- Cells adapt to the medium and synthesise enzymes, with little division.
- Why does the stationary phase occur?
- Nutrients are limiting and toxic waste has accumulated, so the rate of division equals the rate of death.
- What is the equation for the number of generations?
- n = (log Nt − log N0) ÷ 0.301.
- How is the growth rate constant k calculated?
- k = n ÷ t, the number of generations per unit time; generation time = 1 ÷ k.
- Why is dry mass a disadvantage for monitoring a culture?
- It is destructive, needs a large sample and cannot distinguish living from dead cells.
Exam questions on Culturing microorganisms and measuring growth
- 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
- 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
- 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
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).