Practical: Investigating the Effects of Antiseptics & Antibiotics Flashcards

All 6 cards in this deck

What are the steps to investigate the effect of different antibiotics on a bacterial culture?

e.g. testing three antibiotics on one species of bacteria

1. Spread the bacteria evenly over sterile agar using a sterile spreader.
2. Place a paper disc soaked in each antibiotic on the agar, plus a control disc soaked in sterile water; tape the lid and incubate at 25 °C.
3. Measure the diameter of the clear zone of inhibition around each disc.

What is a zone of inhibition?

The clear area of agar around a disc where the bacteria have not grown.

True or false? A larger zone of inhibition means the antimicrobial substance is less effective.

False. A larger clear zone means more bacteria were killed or prevented from growing, so the substance is more effective.

Name three variables that must be controlled when comparing antiseptics or antibiotics on agar plates.

Same species and concentration of bacteria, same size of disc and volume of substance, same incubation temperature and time (and same agar).

What are the steps to calculate the cross-sectional area of a clear zone of inhibition?

e.g. a clear zone of diameter 10 mm

1. Halve the diameter to get the radius: $r = 5$ mm.
2. Use $A = \pi r^2$: $\pi \times 5^2$.
3. Give the answer with squared units: $78.5$ mm$^2$.

Why should the antimicrobial investigation be repeated and a mean zone size calculated?

Repeats make the results more reliable and let anomalous results be spotted and excluded from the mean.