Required Practical: Growth (AQA GCSE Biology)

Microbiology required practical

• Aim:

  • To investigate the effect of antiseptics or antibiotics on bacterial growth using agar plates and measuring zones of inhibition

• Procedure:

  • Use an aseptic technique to place filter paper discs soaked in different antiseptics/antibiotics onto uncontaminated agar plates containing bacteria

  • Measure the zone of inhibition around the growing colonies of bacteria to compare the effect of different antiseptics/antibiotics

  • Calculate the area of each zone

• In this practical, prepared Petri dish plates should be provided which are used to investigate bacterial growth

• It is important to be aware of good microbial aseptic techniques (Culturing Microorganisms)

  • Preventing contamination is vital in any microbiology investigation to ensure that only the effect of any antiseptic or antibiotic on the bacterial species is investigated

  • It is also important to provide the ideal temperature for bacterial growth, in school this will be 25 °C 

    • This is important to reduce the chances of growing harmful pathogens which can lead to various diseases.

• You will most likely use safe strains of coli or Micrococcus luteus bacterial cultures in your practical

• A control group should be used

  • It is vital that one of the paper discs placed on the bacterial agar plate is not soaked in antiseptic or antibiotic but sterile water instead

  • This is to ensure that any differences in bacterial growth observed can be attributed to the presence of the antiseptic or antibiotic used and not some other factor (such as the paper discs for example)

Whilst carrying out this practical it is important to reduce the risk of contaminating the Petri dish with other sources of bacterial

• Commercially produced antibiotic discs are available rather than soaking discs in disinfectants

• Incubating the plates allows the bacteria in the agar to multiply by binary fission, this may be visible by the agar darkening or by colonies appearing

• The antiseptics present in the discs will diffuse into the agar, with the concentration decreasing with distance from the disc

  • Where the concentration is sufficient to prevent bacterial growth or kill bacteria, the agar will remain clear

• It is possible to judge which antiseptic or antibiotic is the most effective by the eye, but it is far more accurate to calculate the diameter of each clear zone around the paper disc and from this calculate the area of each inhibition zone

• Clear zones of inhibition are not always perfectly circular, so the diameter of each zone should be measured twice (at 90° angles to each other) and a mean diameter and area calculated for each clear zone

Record the diameter of each clear zone to the nearest whole mm, and remember to calculate the area using the radius