Cell Biology: Culturing Microorganisms

Question 1

How do bacteria multiply?

Answer 1

binary fission

Question 2

How often do bacteria multiply?

Answer 2

Once every 20 minutes in optimum conditions (sufficient nutrient availablability and suitable temperature)

Question 3

State 2 ways in which bacteria can be grown

Answer 3

• nutrient broth solution
• agar plate

Question 4

What nutrients make up a nutrient broth solution?

Answer 4

All nutrients required for bacteria to grow including:
- nitrogen for protein synthesis
- carbohydrates for energy and other mineral

Question 5

What are uncontaminated culture of microorganisms needed for?

Answer 5

Investigating disinfectant and antibiotic action.

Question 6

Describe the preparation of an uncontaminated culture using aseptic technique.

Answer 6

1. Use pre-sterilised plastic Peri dishes or sterilise glass Petri dishes and agar gel before using with an autoclave.
2. Pour the sterile agar gel into the Petri dish and allow time to set.
3. Sterilise the inoculating loop by passing it through a Bunsen burner flame.
4. Dup the inoculating loop into the solution of microorganisms and make streaks with the loop on the surface of the agar.
5. Put the lid on the Petri dish and secure it with tape. Label accordingly and then store upside down.
6. Incubate culture at 25°C (in school laboratories).

Question 7

Why must Petri dishes and culture media be sterilised before use?

Answer 7

To kill any bacteria already present.

Question 8

Why must inoculating loops be sterilised by passing them through a Bunsen burner flame?

Answer 8

To kill any bacteria present on the inoculating loop.

Question 9

Why must the Petri dish lid be secured with tape?

Answer 9

• stops bacteria in air contaminating culture
• lid not full sealed to prevent growth of anaerobic bacteria in a lack of oxygen

Question 10

Why must the Petri dish be stored upside down?

Answer 10

Prevents condensation from forming and dripping down onto the colonies.

Question 11

Why are cultures incubated at 25° in school laboratories?

Answer 11

Harmful pathogens are less likely to grow at this temperature.

Question 12

What the formula to calculate the number of bacteria after certain time using the number of divisions in a time period?

Answer 12

no. of bacteria in population a end of time period = no. of bacteria at the beginning of the time period x 2^{number of divisions in the time period}

Question 13

Calculate the number of bacteria that will present after 3 hours for a population that divides every 15 minutes and have 5 bacteria present now.

Answer 13

15 min = 0.25 hours
3/0.25 hours = 12 divisions
5 x 2¹² =
20480 or 2.048 x 10⁴