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Date November 2020 Marks available 3 Reference code 20N.3.HL.TZ0.12
Level Higher level Paper Paper 3 Time zone TZ0 / no time zone
Command term Distinguish Question number 12 Adapted from N/A

Question

Disinfectants can be used to break up the biofilms. One of the most commonly used disinfectants is chlorine. Hypochlorous acid forms when chlorine mixes with water. Two different disinfectants were tested experimentally: monochloramine and hypochlorous acid. The concentration of disinfectant needed to kill 99 % of the bacteria was determined, both for free bacteria and for bacteria in biofilms. The table shows the results.

[Source: LeChevallier, M W et al. “Inactivation of biofilm bacteria.” Applied and environmental
microbiology vol. 54,10 (1988): 2492–9. doi:10.1128/AEM.54.10.2492-2499.1988 reproduced/amended with
permission from American Society for Microbiology.]

Bacteria sometimes form biofilms inside metal pipes in water systems. Distinguish between free bacteria and bacteria in biofilms.

[3]
a.

The use of monochloramine is replacing the use of chlorine, as it is more stable, but it can produce by-products that pose possible health risks. Evaluate the data to see whether monochloramine is a good choice as a disinfectant for water systems.

[2]
b.

State how viruses could be used to treat water systems, in order to avoid the use of a disinfectant.

[1]
c.

Markscheme

a. biofilm bacteria are all together while free bacteria do not interact with others;

b. biofilm bacteria present emergent properties not present in free bacteria;

c. quorum sensing only found in biofilm;

d. EPS matrix only in biofilm;

e. biofilm bacteria are more resistant to antibiotics/disinfectants;

a.

Monochloramine not a good choice [1 max]
a. high concentrations/amounts are needed;

b. health risks need to be assessed;

Monochloramine a good choice [1 max]
c. monochloramine is more stable;

d. monochloramine range for biofilm is very extensive
OR
some biofilm bacteria must be highly resistant;

b.

viruses/(bacterio)phages that are specific to the bacterium are used to kill it;

c.

Examiners report

Good candidates managed to distinguish between free bacteria and bacteria in biofilms.

a.

Most candidates stated that monochloramine was not a good choice as a large concentration is required, which could imply health risks.

b.

Most knew that viruses were used but could not specify that they were specific to the biofilm bacteria; few stated that they were bacteriophages.

c.

Syllabus sections

Option B: Biotechnology and bioinformatics » Option B: Biotechnology and bioinformatics (Core topics) » B.3 Environmental protection
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Option B: Biotechnology and bioinformatics » Option B: Biotechnology and bioinformatics (Core topics)
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