Required practical activity 2

• Describe a method to investigate effect of antibiotics on bacterial growth

  1. Prepare an uncontaminated culture of bacteria

  using aseptic technique on an agar gel plate

  ● Spread bacteria evenly over plate

  2. Place paper discs soaked in different antibiotics

  on agar plate

  3. Include a control disc - soaked in sterile water

  4. Incubate plate at 25 C for 48 hours

  5. Use a ruler to measure diameter of zones of

  inhibition (area where bacteria are killed or

  growth is prevented) around each disc and

  calculate their areas using πr
• Rp 2:What are the independent, dependent and control variables?

  ● Independent - type of antibiotic / antiseptic on disc

  ● Dependent - zone of inhibition

  ● Control - size of disc, concentration of antiseptic / antibiotic used, incubation temperature and time
• RP2 Why is it important that cultures were

  not contaminated? (2)

  ● Unwanted microorganisms can affect growth of cultures so

  affect validity of results

  ● Contamination could result in growth of pathogens
• RP 2 How could a student overcome

  measuring zones of inhibition that are

  irregular in shape? (2)

  ● Repeat readings in different positions around the clear zone

  ● Then calculate a mean
• RP 2 Why use a control such as discs soaked

  in water? (1)

  Ensure differences in bacterial growth can be attributed to

  presence of antibiotic / antiseptic not another factor eg. paper
• RP 2 How could the investigation be

  improved? (1)

  ● Repeat and eliminate anomalies / calculate a mean

  ● Use different types of bacteria
• Describe two aseptic techniques used

  in this experiment. (2)

  ● Sterilise equipment / surfaces (before use)

  ● Use sterilised agar

  ● Secure lid of the Petri dish with (adhesive) tape

  ● Only lift lid of Petri dish a little (when setting up plate)