1. Starter
Decide whether the following statements relate to optical methods or dilution plating:
Takes a very long time
Uses a colorimeter
Requires very simple apparatus
Need to use a calibration curve
Counts only live cells
Counts dead and live cells

2. L2 – Bacterial Growth
Understand the different phases of a bacterial growth curve (lag phase, log phase, stationary phase and death phase) and calculate exponential growth rate constants

3. Independent Study
Create a one page revision summary to describe what is meant by aseptic technqiues and how we can measure bacterial growth?

4. Bacterial Growth media
Selective Bacterial Growth Media
Broth Cultures
Nutrient Agar

5. Selective growth media
For example MacConkey’s Agar (MAC) is used to cultivate Gram-negative bacteria, by discouraging the growth of Gram- positive bacteria through the use of crystal violet dyes and bile salts.

6. Broth cultures
Nutrient broth is typically made of a powdered beef extract. The powder is dissolved in water, put in test tubes, and sterilized.

7. Nutrient Agar
Bacteria cannot digest agar, and the broth-agar contains water, nutrients and often buffers to help regulate pH, all while providing a solid surface for bacteria to grow on.
Colony morphology - their characteristic appearance when growing in a group of bacteria that arose from one parent cell.

8. Questions Describe how to produce a culture of microorganisms:
In a flask of broth
On a plate of nutrient agar
2. Describe 3 safety precautions that should always be taken when culturing microorganisms and explain their importance.
3. Explain how different media may be used to produce a pure sample of microorganisms for culturing.

9. Answers Describe how to produce a culture of microorganisms:
In a flask of broth – Add water and powdered beef extract, sterilise work surfaces, use a sterilised pipette to add required volume of bacteria to nutrient broth. Make sure neck of bacteria stock is flamed. Incubate and leave to grow.
On a plate of nutrient agar – Use sterilised inoculation loop, flame inoculation loop over a Bunsen flame. Flame neck of bacteria stock. Streak loop over pre made agar plate. Seal using tape but not completely.

10. Answers
2. Describe 3 safety precautions that should always be taken when culturing microorganisms and explain their importance.
Do not completely seal up agar plates/petri dishes as this can promote the growth of harmful anaerobic bacteria.
Agar plates must be sterilised in an autoclave and bacteria destroyed before disposed of.
3. Explain how different media may be used to produce a pure sample of microorganisms for culturing.
Selective growth media
Nutrient broth
Nutrient agar

11. Bacterial Population growth curve
Spend 5 minutes in your own words describing what you think is happening at stages A-D. H h

12. What is happening at each stage?

13. Lag phase Organisms adjusting to their surroundings
(taking in water, growing, switching genes on and off, making enzymes)
Active cells but not reproducing
Numbers do not build up very quickly
Length of this phase depends on conditions

14. Growth Phase

15. Growth phase Organisms multiply rapidly
Size of population doubles every generation
(plenty of space and nutrients: not much waste)
If bacteria generation is 30 minutes
Calculate how many new bacteria an individual can produce in ten hours:
Answer: 1,048,576

16. Stationary Phase Organisms stop reproducing and start to die
Lack of food
Waste or metabolites build up
Births = Deaths
In an open system this represents the CARRYING CAPACITY of the environment

18. Death Phase Deaths outnumber births Food has run out
Waste or metabolites at toxic levels
In a closed system this would lead to extinction

19. Exponential Growth rate constant

20. Complete exam questions

21. Using the textbooks…

22. https://www.youtube.com/watch?v=c6pcRR5Uy6w Exponential growth rate

23. [3]

24. Answer (c)(i)96 hours ; + 1.5 hours1
(ii)X includes, rapid / exponential / main, growth phase ; ora
when primary, metabolites / products are made ;
penicillin is a secondary, metabolite / product ;
(A) antibiotics (for penicillin) excess of nutrients in X or penicillin produced when nutrients, limited / depleted ;

25. [4]