Question 19M.3.SL.TZ1.c
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.3.SL.TZ1.c |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | Explain | Question number | c | Adapted from | N/A |
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]
The three species of bacteria are commonly found on skin. Explain why excessive handwashing with HCP might lead to a P. aeruginosa infection.
[2]
a. P. aeruginosa not killed by the HCP
OR
disinfectant kills Gram-positive bacteria/S. aureus ✔
b. leading to less competition for P. aeruginosa ✔
It was often recognized that HCP was ineffective against P. aeruginosa but decreased competition was not seen.
