| Date | May 2019 | Marks available | 2 | Reference code | 19M.3.HL.TZ1.9 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Explain | Question number | 9 | Adapted from | N/A |
Question
Discs were soaked in different antibiotics and were placed on a culture of Bacillus subtilis spread on sterile agar in a Petri dish. The Petri dish was left in an incubator, after which zones of inhibition were observed surrounding some of the discs. The photograph is to scale.
[Source: Tasha L. Sturm. 2009. Kirby-Bauer disk diffusion susceptibility test. Visual Resources. American Society for
Microbiology, Washington, DC. www.microbelibrary.org Accessed 29 September 2014]
Estimate the diameter of the zone of inhibition of chloramphenicol.
Distinguish between the action of tetracycline and penicillin on B. subtilis.
Suggest a reason for the result with disc X.
Explain how it could be determined that B. subtilis is a Gram-positive bacterium.
B. subtilis colonies form biofilms through quorum sensing. Define quorum sensing.
B. subtilis colonies form biofilms through quorum sensing. State three possible advantages to B. subtilis of forming a biofilm.
1.
2.
3.
Markscheme
16 mm or 1.6 cm ✔
units required
Allow 15 mm to 17 mm, or 1.5 cm to 1.7 cm
tetracycline was more effective in inhibiting/killing the bacteria ✔
a. it contained no antibiotic ✔
b. it was a control «with only water» ✔
c. the antibiotic it contained was not effective against the bacterium/bacterium is resistant to antibiotic ✔
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 system of» behaviours triggered as a function of population density ✔
a. allows a broader range of habitat «for colonization» ✔
b. biofilms are resistant to physical forces/heat shock/desiccation/physical wiping ✔
c. biofilms can withstand nutrient deprivation ✔
d. resistance to changes in pH ✔
e. resistance to antibiotics ✔
f. avoid phagocytosis ✔
Examiners report
Although this question was correctly answered by the majority of students, there were students who found it difficult, possibly because they did not bring a ruler to the examination. Students should also be reminded to use SI units only. Measurements in inches are not acceptable.
Students had no trouble identifying tetracycline as being more effective.
This question was also well answered, with most students recognising the disc as a control or the fact that the bacteria were resistant. Weaker students struggled with subject-specific terminology and mistakenly used terms such as “antibodies” (instead of “antibiotics”) and “immune” (instead of “resistant”).
Most candidates who attempted this option were familiar enough with the technique to score at least 1 mark.
This question was a good discriminator, as weaker students were unable to use correct terminology such as “population density,” instead resorting to vague phrases like “when the bacteria sense something.”
For two years in a row, option B questions about biofilms have been well answered. The most commonly identified advantages of forming a biofilm were resistance to antibiotics and resistance to physical forces, such as wiping.
