Question 20N.3.HL.TZ0.12
| Date | November 2020 | Marks available | [Maximum mark: 6] | Reference code | 20N.3.HL.TZ0.12 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Distinguish, Evaluate, State | Question number | 12 | Adapted from | N/A |
Bacteria sometimes form biofilms inside metal pipes in water systems. Distinguish between free bacteria and bacteria in biofilms.
[3]
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;
Good candidates managed to distinguish between free bacteria and bacteria in biofilms.
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.]
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]
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;
Most candidates stated that monochloramine was not a good choice as a large concentration is required, which could imply health risks.
State how viruses could be used to treat water systems, in order to avoid the use of a disinfectant.
[1]
viruses/(bacterio)phages that are specific to the bacterium are used to kill it;
Most knew that viruses were used but could not specify that they were specific to the biofilm bacteria; few stated that they were bacteriophages.
