High-efficiency particulate air filters are used chiefly to
Answer: C
These filters trap the particles that carry organisms, delivering effectively organism-free air to operating theatres, safety cabinets and isolation rooms. They control the air supply and do nothing about surfaces or instruments.
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Lyophilization preserves cultures by
Answer: D
Freeze-drying removes water by sublimation from the frozen state, leaving organisms viable but metabolically inert for years. It preserves rather than kills, which is why it is a storage method and not a sterilization method.
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High concentrations of salt or sugar inhibit microbial growth by
Answer: A
A strongly hypertonic surrounding pulls water from the cytoplasm and leaves the cell unable to grow, which is the principle behind salting and preserving in syrup. Organisms adapted to such conditions, the halophiles and osmophiles, are the exceptions that spoil these foods.
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Which form of microbial life is the most resistant to heat and chemicals?
Answer: B
Endospores are dehydrated, densely coated structures with very little free water, and they survive boiling, drying and many disinfectants. Their resistance is the reason sterilization is defined against them and that spores are used as biological indicators.
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The biological indicator used to monitor steam sterilization contains spores of
Answer: C
These spores are markedly more resistant to moist heat than anything expected in the load, so their death is good evidence the cycle worked. A different organism, Bacillus atrophaeus, serves the same purpose for dry heat and ethylene oxide.
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Prions are of particular concern in sterilization because they
Answer: D
Having no nucleic acid and being an unusually stable misfolded protein, a prion survives standard cycles and routine chemicals, so special extended cycles and strong sodium hydroxide or hypochlorite are recommended. They sit beyond spores at the top of the resistance scale.
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Ethyl alcohol is most effective as a disinfectant at a concentration of about
Answer: A
Water is needed for alcohol to denature proteins, so absolute alcohol is markedly less effective than a 70 per cent solution. Undiluted alcohol also dehydrates the cell surface and can leave the interior protected, which is the paradox this question rests on.
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Alcohols act on microorganisms by
Answer: B
Alcohol unfolds proteins and disrupts the lipid of the membrane, killing vegetative bacteria, fungi and enveloped viruses quickly. It is not sporicidal, which is why alcohol hand rub is inadequate against Clostridioides difficile.
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Alcohol-based hand rub is unsuitable against Clostridioides difficile because alcohol
Answer: C
The organism is shed as spores, and alcohol has no useful sporicidal activity at any practical concentration. Washing with soap and water is recommended instead, because it removes the spores mechanically rather than trying to kill them.
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The phenol coefficient of a disinfectant expresses its activity
Answer: D
The test compares the dilution of the agent that kills a test organism in a set time with the dilution of phenol that does the same. A coefficient above one means the agent is more potent than phenol, and below one less so.
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