Cholera toxin produces watery diarrhoea by ________.
Answer: A
Correct answer: irreversibly activating adenylate cyclase and raising cyclic AMP
The A subunit ADP-ribosylates the Gs regulatory protein, locking adenylate cyclase in the active state. The resulting flood of cyclic AMP drives chloride and water into the lumen, which is why the mucosa remains intact and the stool contains no blood or pus.
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The test(s) used for the assay of cholera toxin is/are
Answer: D
Correct answer: All of the above
All three detect the toxin, and they answer slightly different questions. Radioimmunoassay and ELISA measure it immunologically and are quantitative; the Chinese hamster ovary cell assay detects biological activity, the cells elongating characteristically in response. A biological assay confirms that what has been detected is functional toxin rather than an inactive fragment.
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Which of the following biotypes of Vibrio cholerae are prevalent in India?
Answer: A
Correct answer: EI Tor
El Tor displaced the classical biotype worldwide during the seventh pandemic and is the one prevalent in India, along with the O139 serogroup that appeared in 1992. El Tor persists better in the environment and produces a higher proportion of mild and asymptomatic infections, which is part of why it spread so successfully.
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The transport medium for Vibrio cholerae can be
Answer: A
Correct answer: Venkatraman-Ramakrishnan medium
VR medium is a buffered sea-salt solution designed to keep vibrios viable in transit without allowing other organisms to overgrow them, and Cary-Blair serves the same purpose. Selenite F and tetrathionate broths are enrichment media for salmonellae and shigellae and will not preserve a vibrio.
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Which of the followings can be used to differentiate between classical and El Tor biotypes of Vibrio cholerae?
Answer: D
Correct answer: All of these
The three tests separate the biotypes in three different ways. The classical biotype is sensitive to Mukerjee’s group IV phage and to polymyxin B and does not agglutinate fowl red cells; El Tor is resistant to both and does agglutinate them. The Voges-Proskauer reaction and haemolysis are used alongside them, and no single test is relied on alone.
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The motility of Vibrio cholerae seen under a hanging drop preparation is described as ________.
Answer: D
Correct answer: darting motility
The single polar flagellum drives the organism in rapid, shooting movements, described as darting or shooting-star motility. Tumbling motility is characteristic of Listeria, and the finding is inhibited by specific antiserum, which was long used as a rapid presumptive test.
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The DNA coding for the production of cholera toxin in Vibrio cholerae is on the
Answer: C
Correct answer: chromosome
The ctxAB genes are carried by the filamentous bacteriophage CTX, which lysogenises the organism and integrates into the bacterial chromosome, so the genes sit on the chromosome as part of an integrated prophage. That is why a non-toxigenic strain can be converted to a toxigenic one, and why the toxin genes are inherited stably once acquired.
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Vibrio vulnificus and V. parahaemolyticus both gram-negative, motile bacteria are mostly found in warm coastal areas. What is/are the common source through which these bacteria can be transferred to human and cause infections?
Answer: D
Correct answer: All of the above
Both organisms live in warm coastal water, so exposure comes through the water itself - typically through a wound - and through the filter-feeding shellfish that concentrate them, oysters above all. Eating raw shellfish and swimming or fishing with an open wound are therefore the two routes, and both are seasonal.
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Which one of these is not the common laboratory tests for the identification of Helicobacter pylori?
Answer: D
Correct answer: Urine test
The routine tests are the urea breath test, the stool antigen test, serology, and endoscopic biopsy for histology, culture or the rapid urease test. There is no established urine test in ordinary use. The first three are the non-invasive options and are also the ones used to confirm eradication after treatment.
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Which of the following toxin resembles cholera toxin?
Answer: C
Correct answer: Labile toxin of Escherichia coli
Cholera toxin and the heat-labile toxin of enterotoxigenic E. coli share the same A-B structure and the same mechanism: the B subunits bind the GM1 ganglioside, the A subunit enters and ADP-ribosylates the G protein, adenylate cyclase is locked on, and the resulting cyclic AMP drives the secretory diarrhoea. The two are so alike that antibody to one neutralises the other.
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