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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The organ of locomotion of bacteria is
Answer: B
Correct answer: flagella
Flagella are the organelles of motility: a rotary motor in the membrane, driven by the proton-motive force, spins the helical filament like a propeller. Fimbriae are for attachment, and capsules and slime layers for protection.
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Which of this bacteria is resistant to penicillin as it lacks a cell wall?
Answer: C
Correct answer: Mycoplasmas
Penicillin blocks the cross-linking of peptidoglycan, so it can only act on a cell that is building a wall. Mycoplasmas have none - only a sterol-strengthened membrane - so the drug has nothing to attack. Spirochaetes, cyanobacteria and bdellovibrios all have walls.
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Hemolytic serum for complement fixation reaction is obtained in the following manner
Answer: A
Correct answer: rabbits are immunized by suspension of sheep erythrocytes
Haemolytic serum is anti-sheep-red-cell antibody, and it is raised by injecting rabbits with sheep erythrocytes. The rabbit serum is then heated to destroy its own complement, so that the only complement in the final test is the one being measured.
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Gram’s staining consist of
Answer: B
Correct answer: 4 stages
The Gram stain has four steps: the primary stain (crystal violet), the mordant (iodine), decolourisation with alcohol or acetone, and the counterstain (safranin). The third is the critical one - too long, and even gram-positive cells lose the dye.
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A gram-positive bacterium retains the crystal violet stain because
Answer: D
Correct answer: bacteria have thick peptidoglycan layer
Retention is a matter of wall thickness. The many layers of peptidoglycan in a gram-positive wall dehydrate and shrink when alcohol is applied, trapping the crystal violet-iodine complex inside. The teichoic acids and the absence of a periplasm are genuine features of such cells, but they are not what holds the dye.
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