Which bacterial structure enables adherence to host tissue surfaces ?
Answer: B
Correct answer: Fimbriae
Fimbriae, also called common pili, are short surface filaments whose adhesins bind receptors on host cells, and that attachment is often the first step of infection. Flagella provide motility and the capsule resists phagocytosis.
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Salt agar is a
Answer: C
Correct answer: selective media
Salt (mannitol salt) agar contains about 7.5% sodium chloride, which inhibits most bacteria while staphylococci tolerate it. Suppressing the unwanted flora so that one group can grow is exactly what makes a medium selective.
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Which one of these has a Chinese letter arrangement?
Answer: D
Correct answer: Corynebacterium diphtheriae
Corynebacterium diphtheriae divides by snapping fission: the cells bend sharply apart but stay joined at one end, giving the V and L shapes described as a Chinese-letter or palisade arrangement. It is a useful diagnostic clue in a stained smear.
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LPS (endotoxin) consist of
Answer: D
Correct answer: all of the above
Lipopolysaccharide has three parts: lipid A, anchored in the outer membrane and responsible for the endotoxic activity; the core polysaccharide; and the O antigen, the variable chain of repeating sugars that projects outwards and determines the serotype.
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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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