There are many different forms of Vibrio cholerae, a bacterium that is found naturally in aquatic environments. The bacterium is motile (can move) and uses a cell structure known as a flagellum to allow it to move through water. Fig. 4.1 is a drawing of four cells of one form of V. cholerae. [Figure 4.1] Two main forms of V. cholerae, O1 and O139, are able to colonise the small intestine and cause cholera. These two forms are able to produce a toxin, choleragen, which causes the symptoms of diarrhoeal disease. Mutant V. cholerae that lack flagella are less able to cause disease.

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

1. [2] image length; A magnification triangle magnification 250 (nm); (8 mm) A 220 (for 7 mm) A 230 (for 7.5 mm) A 270 (for 8.5 mm) A 280 (for 9 mm)
2. [1] pathogen ;
3. [1] ingests / drinks / AW, contaminated water / water containing the pathogen / food washed in contaminated water / contaminated food ; A examples of accidental ingestion e.g. swimming in contaminated rivers A faecal / oral, transmission A bacteria / Vibrio R if described as virus
4. [3] A mRNA or messenger RNA any three from: (one strand only needed) to form mRNA / mRNA is single-stranded ; (m)RNA is, used / needed, to produce, subunit (s) / polypeptide(s) ; (only) one strand (of DNA) is the, template / transcribed, strand ; idea that (complementary) copying / transcribing, other DNA strand would not result in, desired / AW, mRNA / polypeptide ;
5. [2] any two from: I receptor I active site (mAbs) specific / different, antigen binding sites / binding sites for antigen ; A (each) mAb binds to a, specific / particular, antigen R if antibody described as an enzyme (each mAb has) specific / different, tertiary structure / variable region(s) / primary structure / sequence of amino acids ; binding site and antigen have complementary shapes or ZAC-3 complementary shape to core polysaccharide and lipid A or 2D6 complementary shape to O-polysaccharide;
6. [4] any four from: yes general points (agglutination/motility prevented, so) bacteria less able to, penetrate mucus / attach to intestinal epithelial cells / colonise intestine / AW ; A idea that fewer bacteria able to, attach / colonise intestine less/no, choleragen / toxin, released ; bacteria passed out in faeces not able to cause disease in others / AW ; ref. to phagocytosis more effective; e.g. macrophages stimulated to carry out phagocytosis prevention/treatment (to prevent disease) needs to be given, at early stages / before colonising (intestine); passive immunity / passive vaccine ; must be in context of treatment or immediate protection idea that in addition to immune response (so increased effect); e.g. acts beside, immune system / immune system cells acts before the immune response can become effective ref. to quicker recovery (if a person has cholera); useful when, there is antibiotic resistance / antibiotics cannot be given ; specific mAb mAb ZAC-3 may be more effective for cholera caused by, wider range of V. cholerae / AW; mAb ZAC-3 may be useful if exact form of V. cholerae not known ; mAb 2D6, needs to be targeted against specific V. cholerae forms / may not be effective against other V. cholerae forms; A not all forms of V. cholerae were tested in lab AVP; e.g. (only) short-term protection / not active immunity / not long term ref. difficulty in delivering mAbs to intestine e.g. may be, digested / destroyed, as pass through gut need to find a way to get mAbs to the intestine if given intravenously need to pass through to gut lumen may be able to prevent multiplication of bacteria mAbs have been used successfully for other diseases