RP6 - Use of aseptic techniques to investigate the effect of antimicrobial substances on microbial growth

Question 1

Explain examples of aseptic techniques that could be used

Answer 1

• wash hands with soap / disinfect surfaces => kill microbes / prevent contamination
• Sterilise pipette / spreader / boil agar growth medium => kill microbes / prevent contamination
• flame neck of bottle of bacteria => kill microbes / prevent contamination
• bunsen burner close => upward current of air draws airborne microbes away to prevent contamination
• lift lid of petri dish slightly / minimise opening => prevent entry of microbes / contamination

Question 2

describe a method to investigate the effect of antimicrobial substances on microbial growth

Answer 2

• prepare area using aseptic techniques
• use a sterile pipette to transfer bacteria from broth to agar plate using aseptic technique
• use a sterile spreader to evenly spread bacteria over agar plate
• use sterile forces to place same size discs that have been soaked in different types / concentrations of antimicrobials for same length of time , onto agar plate
• lightly tape lid onto plate , invert and incubate at 25 degrees for 48 hours
• measure diameter of inhibition zone around each disc and calculate area using pi2

Question 3

EQ : explain why it’s important to maintain a pure culture of bacteria (1)

Answer 3

• bacteria may outcompete bacteria being investigated
• or could be harmful to humans

Question 4

EQ : explain why the lid is held with 2 pieces of tape and not sealed completely (2)

Answer 4

• allows oxygen in preventing growth of anaerobic bacteria
• more likely to be pathogenic / harmful to humans

Question 5

EQ : explain why a paper disc with water / no antimicrobial agent is used (2)

Answer 5

• act as a control
• ensuring antimicrobial preventer growth , not paper disc

Question 6

EQ : explain why petri dishes are incubated upside down (1)

Answer 6

• condensation drips onto lid rather than surface of agar

Question 7

EQ : explain how comes if inhibitions can be measured if irregular (1)

Answer 7

repeat readings in different positions , calculate mewn

Question 8

EQ : explain why a higher antimicrobial concentration isn’t used (1)

Answer 8

• more bacteria killed so clear zones may overlap

Question 9

EQ : Exolain why bacteria should be incubated at 25 degrees or less in a school laboratory (1)

Answer 9

• below human body temp to prevent growth of pathogens

Question 10

explain the presence and absence of clear zones

Answer 10

clear zones :
antimicrobial diffuses out of disc into agar , killing / inhibiting growth of bacteria
• larger clear zones => more bacteria killed => more effective antimicrobial
No clear zones :
• if antibiotic used , bacteria may be resistant or antibiotic may not be effective against that specific bacteria

Question 11

describe how data about the effect of antimicrobial substances can be presented as a graph

Answer 11

• categorical data => bar chart
  • x axis is style of antimicrobial
  • y axis area of zone of inhibition / mm3
• continuous data => line graph joined by line of best fit
  • x axis conc of antibiotic
  • y axis area of zone of inhibition