Which of the following is true about mycoplasma?
Answer: D
Correct answer: All the above
All three are true of mycoplasmas: they divide by binary fission, they are entirely without a cell wall, and being wall-less they are inherently resistant to penicillin and every other drug that acts on wall synthesis. They are treated with agents that act elsewhere, such as the macrolides and tetracyclines.
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Viroids are
Answer: B
Correct answer: Infective agents free of protein, with low molecular weight RNA
A viroid is a small circular RNA with no protein coat at all and no genes for one. Viroids cause a number of plant diseases, replicating by exploiting the host’s own polymerases; they are the smallest known infectious agents.
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Which of the following is false about the general characteristics of viruses
Answer: C
Correct answer: They are sensitive to antibiotics
That statement is false: viruses are not affected by antibiotics, which act on bacterial structures and processes - cell wall, ribosome, folate synthesis - that viruses simply do not have. Antiviral drugs must instead target viral enzymes such as polymerases and proteases. The other three statements are all true.
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During log phase
Answer: B
Correct answer: The cells start dividing and their number increase by geometric progression
In the log (exponential) phase the cells divide at a constant rate, so their number doubles at each generation - a geometric progression, which plots as a straight line on a logarithmic scale. This is the phase in which the population is most uniform and most susceptible to antibiotics, since so many cells are building walls.
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Subunit vaccines contain
Answer: B
Correct answer: immunogenic component of the bacteria
A subunit vaccine contains only the immunogenic part of the organism - a surface protein or a capsular polysaccharide - and none of the rest. Leaving out everything unnecessary means far fewer side effects; the hepatitis B and pneumococcal vaccines work this way.
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The capsid is composed of
Answer: C
Correct answer: Capsomers
The capsid is the protein shell around the genome, and it is built of repeating subunits called capsomers. Using many copies of one small protein is what lets a tiny genome specify a whole coat. Peplomers are the spikes on an envelope, which is a different structure again.
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Cross infection is
Answer: C
Correct answer: situation, when patient suffering from a disease and new infection is set up from external source
A cross infection is a fresh infection acquired from an outside source by a patient already ill with something else - classically in hospital, where it is called a nosocomial or cross infection. A first infection by an organism is a primary infection, and a later one by the same organism is reinfection.
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A gram-negative bacterium does not retain the crystal violet stain because
Answer: B
Correct answer: bacteria have thin peptidoglycan layer
A gram-negative cell has only a thin layer of peptidoglycan, and its outer membrane is dissolved by the alcohol used to decolourise. The crystal violet-iodine complex therefore washes straight out, and the cell takes up the safranin counterstain instead.
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Bacterial spores are demonstrated by
Answer: A
Correct answer: Ozeshki’s staining
Spores resist ordinary dyes, so they need a method that forces stain in - heat with a strong dye, then decolourisation, then a contrasting counterstain, so that the spore takes one colour and the vegetative cell another. That is what the spore stains of Ozeshki and of Schaeffer-Fulton do.
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Capsid in nature is a
Answer: A
Correct answer: Protein
The capsid is protein, assembled from many copies of one or a few polypeptides. Its job is to protect the nucleic acid and, in a naked virus, to attach to the host receptor.
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