| Date | May 2015 | Marks available | 2 | Reference code | 15M.3.SL.TZ2.16 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Describe | Question number | 16 | Adapted from | N/A |
Question
Vibrio cholerae live in aquatic environments and cause cholera. Some V. cholerae form aggregates that show characteristics not seen in individual bacteria. The bacteria in these aggregates monitor the population densities by quorum sensing. They produce quorum sensing proteins (QS + ). Some V. cholerae strains do not produce quorum sensing proteins (QS – ) and some only produce quorum sensing proteins in low amounts (QS c ).
V. cholerae strains isolated in China were examined. The pie charts show the percentage of different quorum-sensing systems in strains that contain cholera toxin genes and in strains that do not contain cholera toxin genes.
State the percentage of cholera producing strains that do not produce quorum sensing proteins (QS – ).
Determine the approximate percentage of non-cholera producing strains that produce quorum sensing proteins in low amounts (QSc ).
Compare the percentage of strains that do not produce quorum sensing proteins (QS – ) in strains with and without the cholera toxin genes.
Deduce, using the data, whether the genes for quorum sensing and for toxicity of cholera evolved together.
Vibrio cholerae is Gram-negative. Describe the structure of the cell wall of this bacterium.
Markscheme
50 (%) (allow answers in the range of 48 (%) to 52 (%))
16 (%) (allow answers in the range of 12 (%) to 20 (%))
a. there are less QS− strains that produce cholera than those that do not produce cholera;
b. approximately 50 % in cholera producing and approximately 70 % in noncholera producing;
c. greatest percentage in QS− in both so most are not quorum sensing;
(the hypothesis is supported as)
a. more sensing in bacteria that cause cholera than in those that do not;
b. forming aggregates to facilitate the propagation of the pathogen / bacteria working together can produce pathogenicity;
c. bacteria with QS+ and cholera producing strains are positively selected;
a. thin layer of peptidoglycan sandwiched between outer and inner membrane layer;
b. outer layer containing lipopolysaccharide and (protein);
c. high lipid and low peptidoglycan content;
Accept correct answers on a clearly labelled diagram.
Examiners report
Option F data was two pie charts comparing the amount of quorum sensing proteins in bacteria that produce cholera and those that do not. The questions were quite difficult in their language and this confused many students as there were negatives in both questions and responses.
Option F data was two pie charts comparing the amount of quorum sensing proteins in bacteria that produce cholera and those that do not. The questions were quite difficult in their language and this confused many students as there were negatives in both questions and responses.
Option F data was two pie charts comparing the amount of quorum sensing proteins in bacteria that produce cholera and those that do not. The questions were quite difficult in their language and this confused many students as there were negatives in both questions and responses.
Option F data was two pie charts comparing the amount of quorum sensing proteins in bacteria that produce cholera and those that do not. The questions were quite difficult in their language and this confused many students as there were negatives in both questions and responses.
Option F data was two pie charts comparing the amount of quorum sensing proteins in bacteria that produce cholera and those that do not. The questions were quite difficult in their language and this confused many students as there were negatives in both questions and responses.
