DP Biology (last assessment 2024)

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Question 23M.3.SL.TZ1.10

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Date May 2023 Marks available [Maximum mark: 6] Reference code 23M.3.SL.TZ1.10
Level SL Paper 3 Time zone TZ1
Command term Deduce, Describe, Outline Question number 10 Adapted from N/A
10.
[Maximum mark: 6]
23M.3.SL.TZ1.10

The effectiveness of three different treatments for killing the pathogenic bacterium Staphylococcus aureus found on solid surfaces was assessed. Each treatment contained a probiotic cleaning hygiene system (PCHS), specific bacteriophages alone (SBP), or a combination of both (SBP + PCHS). A control was set up using water. 

The bar chart shows the percentage of S. aureus remaining on solid surfaces at two different times after applying the treatments and the control.

[Source: D’Accolti, M., Soffritti, I., Piffanelli, M., Bisi, M., Mazzacane, S. and Caselli, E., 2018. Efficient removal of hospital
pathogens from hard surfaces by a combined use of bacteriophages and probiotics: potential as sanitizing agents.
Infection and Drug Resistance, [e-journal] 11, pp. 1015–1026. https://doi.org/10.2147/IDR.S170071. Open access.]

(a)

Deduce with a reason the most efficient treatment to eliminate S. aureus from solid surfaces.

[1]

Markscheme

SBP+PCHS as more bacteria removed/killed
OR
SBP+PCHS as the effect lasts longer/more than 3 days;

OWTTE

Examiners report

The treatment was correctly identified but the reason was not always given so a mark could not be awarded.

(b)

S. aureus often forms biofilms in water systems together with Pseudomonas aeruginosa. Outline the benefits of using bacteriophages rather than chemicals to eliminate biofilms from water systems.

[2]

Markscheme
  1. bacteriophages (are viruses that) infect bacteria;
  2. they can spread through entire biofilm to kill bacteria;
    OR
    chemicals only kill bacteria near the surface of biofilm
  3. polysaccharide matrix/EPS/biopolymers act as a (physical) barrier to chemicals;
  4. bacteriophages are specific/can be added to destroy a specific pathogen;
  5. chemicals can contaminate water
    OR
    can lead to bacteria resistant to the chemical;
Examiners report

Sometimes one mark was awarded for contamination of the water supply or specificity of bacteriophages.

(c)

Describe how Pseudomonas can be used to reduce pollution caused by methyl mercury.

[3]

Markscheme
  1. bioremediation;
  2. Pseudomonas putida used;
  3. methyl mercury converted/decomposed to (methane and) mercury ions;
  4. mercury ions turned into mercury (by other bacteria);
  5. insoluble mercury can be separated from waste water;
Examiners report

No correct answers were seen.