Culturing microorganisms

Question 1

how do you prepare an uncontaminated culture using aseptic technique

Answer 1

1. Sterislise petri dish, nutrient broth and agar
2. Pass an inoculating loop through a Bunsen burner to sterilize it
3. After transferring the bacteria, the lid of the ptri dish should be lightly taped on
4. Store the petri dish upside down
5. Store it at 25 degrees or lower

Question 2

why must petri dishes and culture be sterilised before use

Answer 2

• to kill any unwanted microorganisms
• to prevent contamination

Question 3

what temperature should culture be incubated at

Answer 3

25 degrees

Question 4

why should the lid of the petri dish be taped on

Answer 4

• to stop lid falling off
• to stop microorganisms from the air getting in

Question 5

why should the petri dish be placed upside down

Answer 5

to stop drops of condensation falling into the agar surface and disrupting the colonies

Question 6

why is bacteria incubated at a certain temperature

Answer 6

to reduce the chance that harmful bacteria will grow

Question 7

how do we use agar plates to investigate the effect of antibiotics on bacterial growth

Answer 7

1. Place paper discs soaked in different types of antibiotics on an agar plate that has even coverings of bacteria. leave some space between the discs
2. The antibiotic should soak into the agar jelly. antibiotic resistant bacteria will carry on growing on the agar, but non resistant strands will die. a clear area will be left where the bacteria have died, this is called the zone of inhibition
3. as a control, use a paper disc that has not been soaked in an antibiotic, instead soak it in water
4. Leave the plate at 25 degrees and leave it for 48 hrs
5. The more effective the antibiotic is, the larger its zone of inhibition

Question 8

why should you open a sterile agar plate next to a Bunsen flame

Answer 8

the flame kills bacteria in the air

Question 9

what is the zone of inhibition

Answer 9

the region where bacteria had not grown

Question 10

why is it important to use uncontaminated cultures of bacteria

Answer 10

unwanted microorganisms may have affected the results of the experiment

Question 11

why should we use a control disc of a paper disc that has been soaked in water and not an antibiotic

Answer 11

so that we can be sure that any difference between the growth of the bacteria around the control disc and around one of the antibiotic disc is due to the effect of the antibiotic alone, not something weird in the paper for example

Question 12

why do we open the petri dish away from ourselves

Answer 12

to prevent us from contaminating the dish by breathing
- to prevent the bacteria infecting u

Question 13

why is the petri dish sealed using tape

Answer 13

• so that the lid doesnt fall off
• only partially sealed to prevent anaerobic atmosphere
  (bacteria grown in anaerobic environments are more likely to be harmful)

Question 14

why do we need to label the plate on the bottom of the agar dish

Answer 14

• so that lids can be moved
• correct labels might not be associated with correct dish or parts of dish