| Date | May 2019 | Marks available | 2 | Reference code | 19M.3.SL.TZ1.8 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Explain | Question number | 8 | Adapted from | N/A |
Question
Hexochlorophene (HCP) and ortho-phenylphenol (OPP) are two ingredients of handwashing formulations. The antiseptic effectiveness of the two agents was tested by soaking discs in the agents and applying them to plates inoculated with Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa. The resulting zones of inhibition are depicted.
[Source: © International Baccalaureate Organization 2019]
Estimate the diameter of the zone of inhibition around the disc containing OPP in the S. aureus culture.
E. coli and P. aeruginosa are both Gram-negative bacteria and S. aureus is Gram-positive. Explain how it could be verified that S. aureus is a Gram-positive bacterium.
The three species of bacteria are commonly found on skin. Explain why excessive handwashing with HCP might lead to a P. aeruginosa infection.
P. aeruginosa is a concern in hospitals because it tends to form biofilms. Outline one reason for biofilms being particularly difficult to eradicate.
Markscheme
14–16 mm or 1.5–3 mm «around the disc» ✔
Units required
a. use the Gram staining procedure ✔
b. Gram-positive bacteria take up/retain «crystal violet» stain ✔
c. «Gram-positive bacteria» appear purple-coloured seen through a microscope ✔
a. P. aeruginosa not killed by the HCP
OR
disinfectant kills Gram-positive bacteria/S. aureus ✔
b. leading to less competition for P. aeruginosa ✔
microorganisms growing in a biofilm are resistant to antimicrobial agents ✔
Examiners report
The zone of inhibition was surprisingly poorly measured, even given the range and alternative measurements in the mark scheme. It is possible that when asked to estimate, candidates did not use a ruler.
Some answers referred back to the experiment, but where there was knowledge of Gram staining there were good answers.
It was often recognized that HCP was ineffective against P. aeruginosa but decreased competition was not seen.
Some correct answers were seen.
