Monai Elnaasani
Describe how to investigate the antimicrobial properties of plants.
Past paper - January 2010, June 2014, October 2017
1. Culture of one species of bacteria is mixed into melted agar
2. Mixture is poured into a sterile petri dish and the lid is replaced on the dish and the mixture
is allowed to cool so the agar can set
3. Extracts made using fruit (by crushing it with 5cm3 of a solvent) of the same mass and age
and extracts has the same concentration of plant
4. Extracts made using same solvent
5. Same size of paper discs is dipped in same volume of extracts and placed evenly in agar
plate
6. Same range of species of bacteria used
7. Incubate different sample in same temperature and length of time 24hrs, 1-7 days
8. 25ºC-30ºC to prevent growth of pathogenic bacteria
9. Measure zone of inhibition
10. Repeat for each sample and calculate mean
Suitably-prepared petri dish - bacteria is distributed uniformly in an agar plate prepared with
aseptic techniques like; using a sterile plate and sterile pipette to spread the bacteria.
Discs should be sterilized before being soaked in the extract to prevent contamination by
microbes that could be pathogenic and compete with growth of bacteria in disc affecting its
growth
Antimicrobial properties - ability to kill or slow down the growth of microorganisms
Bacteria are mixed with agar so that the bacteria is distributed throughout evenly and because
agar is a source of nutrient
Using sterile conditions makes the investigation valid
A control disc is used that doesn’t have any antibiotics or an inactive chemical to allow
comparison with the other discs, showing that any differences between the discs is due to the
treatment given to those discs
Clear zones;
• they show that the antibiotic has diffused out of the disc and killed the bacteria, inhibits
bacterial growth
• To assess effectiveness of extracts by comparing sizes of zones since they indicate degree of
inhibition of bacterial growth
• The bigger the diameter the better the effect
• To calculate inhibition zone you record several measurements of the diameter of inhibition
zone around the disc and divide by the number of measurements to obtain a mean
• More Accurate method - place graph paper over lid and trace squares, count them and
subtract from total squares covering the well
To get the minimum strength of a certain antibiotic test several diluted samples of it, the minimum
strength is the one which is same as original strength with 100% concentration
Incubate bacteria at 25ºC rather than 37ºC because 37 is human body temperature thus it allows
growth of pathogenic bacteria and encourages their reproduction, pathogenic bacteria are really
harmful to humans, it could infect the person and compete against the bacteria we’re testing on.
It could denature the enzymes since it is not the optimum temperature for growth of bacteria
Secure a lid to the base of the petri dish and tape it from two sides but not all the way through,
this reduces contamination of culture and allows aerobic conditions by the addition of air
therefore it reduces growth of anaerobic bacteria being cultured
Pearson Edexcel Advanced Level Biology Unit 3 - notes for success
Describe how to investigate the antimicrobial properties of plants.
Past paper - January 2010, June 2014, October 2017
1. Culture of one species of bacteria is mixed into melted agar
2. Mixture is poured into a sterile petri dish and the lid is replaced on the dish and the mixture
is allowed to cool so the agar can set
3. Extracts made using fruit (by crushing it with 5cm3 of a solvent) of the same mass and age
and extracts has the same concentration of plant
4. Extracts made using same solvent
5. Same size of paper discs is dipped in same volume of extracts and placed evenly in agar
plate
6. Same range of species of bacteria used
7. Incubate different sample in same temperature and length of time 24hrs, 1-7 days
8. 25ºC-30ºC to prevent growth of pathogenic bacteria
9. Measure zone of inhibition
10. Repeat for each sample and calculate mean
Suitably-prepared petri dish - bacteria is distributed uniformly in an agar plate prepared with
aseptic techniques like; using a sterile plate and sterile pipette to spread the bacteria.
Discs should be sterilized before being soaked in the extract to prevent contamination by
microbes that could be pathogenic and compete with growth of bacteria in disc affecting its
growth
Antimicrobial properties - ability to kill or slow down the growth of microorganisms
Bacteria are mixed with agar so that the bacteria is distributed throughout evenly and because
agar is a source of nutrient
Using sterile conditions makes the investigation valid
A control disc is used that doesn’t have any antibiotics or an inactive chemical to allow
comparison with the other discs, showing that any differences between the discs is due to the
treatment given to those discs
Clear zones;
• they show that the antibiotic has diffused out of the disc and killed the bacteria, inhibits
bacterial growth
• To assess effectiveness of extracts by comparing sizes of zones since they indicate degree of
inhibition of bacterial growth
• The bigger the diameter the better the effect
• To calculate inhibition zone you record several measurements of the diameter of inhibition
zone around the disc and divide by the number of measurements to obtain a mean
• More Accurate method - place graph paper over lid and trace squares, count them and
subtract from total squares covering the well
To get the minimum strength of a certain antibiotic test several diluted samples of it, the minimum
strength is the one which is same as original strength with 100% concentration
Incubate bacteria at 25ºC rather than 37ºC because 37 is human body temperature thus it allows
growth of pathogenic bacteria and encourages their reproduction, pathogenic bacteria are really
harmful to humans, it could infect the person and compete against the bacteria we’re testing on.
It could denature the enzymes since it is not the optimum temperature for growth of bacteria
Secure a lid to the base of the petri dish and tape it from two sides but not all the way through,
this reduces contamination of culture and allows aerobic conditions by the addition of air
therefore it reduces growth of anaerobic bacteria being cultured
Pearson Edexcel Advanced Level Biology Unit 3 - notes for success
