What is responsible for the endotoxin activity of LPS
Answer: A
Correct answer: lipid A
Lipid A is the endotoxin. Released when gram-negative cells lyse, it triggers macrophages to pour out cytokines, producing fever, hypotension and, in quantity, septic shock. The O antigen and core polysaccharide are antigenic but not themselves toxic.
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Reduction of virulence is known as
Answer: A
Correct answer: attenuation
Attenuation is the reduction of virulence while the organism is kept alive - achieved by repeated subculture, growth under unfavourable conditions, or passage through an unnatural host. It is the basis of live vaccines such as BCG. The opposite, enhancing virulence by passage through a susceptible host, is called exaltation.
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Reinfection is
Answer: B
Correct answer: subsequent infection by the same organism
Reinfection is a later infection by the same organism, after the first has resolved. It is distinct from a relapse, in which the original infection was never cleared and flares up again.
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Nutrient agar is prepared by addition of
Answer: B
Correct answer: 3% agar in nutrient broth
Agar is added at about 2-3% to set nutrient broth into a solid plate. Less than that gives a soft or semi-solid medium, used for motility testing; more would make the gel needlessly hard and hard to pour. Agar itself is not a nutrient - almost no bacterium can digest it, which is precisely why it is used as the gelling agent.
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Living microorganisms are studied using the
Answer: A
Correct answer: hanging drop technique
The hanging drop keeps the organisms alive and suspended in liquid under a cover slip over a cavity slide, so their true motility can be watched. Staining methods all begin by fixing - that is, killing - the cells, so they can never show movement.
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The material being tested contains
Answer: A
Correct answer: unknown antigen
The material being tested - a specimen from the patient or a culture - supplies the unknown antigen, and it is identified by reacting it against a diagnostic serum of known antibody.
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For adsorption, viruses use
Answer: A
Correct answer: Specific receptors
Adsorption is the first step of infection: a protein on the virus surface binds a specific receptor molecule on the host cell. Which receptor a virus recognises is what determines its host range and its tissue tropism - HIV binds CD4, so it infects helper T cells.
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This is analogous to mesosomes of bacteria
Answer: C
Correct answer: Mitochondria of eukaryotes
The mesosome is an infolding of the bacterial plasma membrane, long regarded as the seat of the respiratory enzymes - hence the comparison with the mitochondrion, which performs that role in eukaryotes.
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Negative result of the complement fixation reaction is expressed in
Answer: B
Correct answer: complete haemolysis
A negative result means antigen and antibody did not correspond, so no complex formed and the complement was not fixed. It is still free when the indicator system is added, and it lyses the sensitised red cells - complete haemolysis, giving a clear red solution.
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Volutin granules are demonstrated by
Answer: C
Correct answer: Loeffler’s technique
Volutin (metachromatic) granules are polyphosphate stores, and they stain a different colour from the rest of the cell with methylene blue - the metachromasia that gives them their name. Loeffler’s alkaline methylene blue is the classic method, and Neisser’s stain is used for the same purpose in Corynebacterium diphtheriae.
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