The standard quantitative agglutination method is
Answer: C
Correct answer: tube agglutination
The tube method is the quantitative one: the serum is diluted serially and a fixed amount of antigen added to each tube, so the result is a titre - the highest dilution still giving agglutination. Slide agglutination is quick but only qualitative.
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During decline phase
Answer: D
Correct answer: Population of the cells decreases
In the decline (death) phase deaths outnumber divisions, so the viable population falls - often exponentially. Nutrients are exhausted and toxic products have accumulated; some cells lyse altogether.
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For biosynthesis, viruses use
Answer: D
Correct answer: The synthetic machinery of host cell
A virus carries no ribosomes and, in most cases, no metabolism of its own. For biosynthesis it takes over the host cell’s machinery - ribosomes, tRNAs, amino acids and ATP - to make its own proteins and nucleic acid. That total dependence is what makes viruses obligate intracellular parasites, and what makes them so hard to attack with drugs.
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Spirochetes can be visualized by
Answer: D
Correct answer: All the above
Spirochaetes are too slender to be seen by ordinary bright-field microscopy of a stained film, so all three are used: dark-field microscopy on a fresh preparation, which shows them alive and moving; silver impregnation by Morozov, which thickens them; and the Giemsa and India ink methods.
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Positive result of the complement fixation reaction is expressed in
Answer: A
Correct answer: complete inhibition of haemolysis
A positive result means the antigen and antibody did correspond, so the complement was fixed by their complex. None is left for the indicator system, the red cells stay intact, and they settle as a button - no haemolysis. It is an inversion that catches people out: no lysis means the test is positive.
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The spore protects the genomic DNA from
Answer: D
Correct answer: all of the above
The endospore protects its DNA against heat, chemicals and radiation alike. Its defences are the dehydrated core, the calcium dipicolinate that stabilises the DNA, the small acid-soluble proteins that coat it, and the thick cortex and coats outside. This is why boiling will not sterilise and autoclaving is required.
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Strict anaerobes lack which of the following enzymes?
Answer: D
Correct answer: All the above
Oxygen metabolism produces superoxide and hydrogen peroxide, and three enzymes normally deal with them: superoxide dismutase, catalase and peroxidase. Strict anaerobes lack them, so these products accumulate and destroy the cell - which is why oxygen is lethal to such organisms.
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This about cell wall of gram-positive bacteria is true
Answer: D
Correct answer: All of the above
All three describe the gram-positive wall: it is built of many layers of peptidoglycan, is much thicker than the gram-negative wall, and contains teichoic acids threaded through it. Those layers are what hold the crystal violet-iodine complex against decolourisation.
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What is a cluster of polar flagella called?
Answer: D
Correct answer: Lophotrichous
The prefixes describe the arrangement: lophotrichous is a tuft of several flagella at one pole; monotrichous a single one; amphitrichous flagella at both poles; peritrichous flagella all over the surface.
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Ziehl-Neelsen staining consist of
Answer: C
Correct answer: 3 stages
Ziehl-Neelsen has three steps: carbol fuchsin applied with heat, decolourisation with acid-alcohol, and a methylene blue counterstain. Acid-fast organisms such as mycobacteria keep the fuchsin, because their waxy mycolic acid wall resists the acid-alcohol.
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