Physical and Chemical Agents Questions and Answers

Answer: A

Correct answer: the pressure is raised above atmospheric

Saturated steam sits at the boiling point corresponding to its pressure, and at atmospheric pressure that is 100 degrees however much heat is supplied. Raising the pressure raises the boiling point, so at 15 pounds per square inch above atmospheric the steam is at 121 degrees. The pressure is therefore only the means; it is the temperature that sterilises.

Answer: B

Killing depends on steam condensing on the load, releasing its latent heat and wetting the surface. Steam heated beyond saturation stays gaseous, so it acts as dry heat and would need far higher temperatures and times.

Answer: C

Gamma radiation passes through the finished sealed package without heating it, so the item is sterilized in its final wrapping and stays sterile until opened. Ultraviolet light is stopped by the packaging and would never reach the contents.

Answer: D

Ethylene oxide adds alkyl groups to nucleic acid and protein, which is why it is sporicidal. Alcohol denatures protein and dissolves lipid, hypochlorite oxidises, and quaternary compounds disorganise the membrane as cationic surfactants.

Answer: A

Correct answer: removing the organisms rather than killing them

A membrane filter of 0.22 micrometre pore size retains bacteria and fungi physically, so the filtrate is free of them while nothing has been killed. That is why filtration is used for heat-labile solutions - sera, antibiotic solutions, vitamins - and why it is not reliable against viruses or mycoplasmas, which are small enough to pass.

Answer: B

Trapped air keeps steam from the surfaces it must condense on, so parts of the load never reach the sterilizing conditions even though the gauge reads correctly. Prevacuum sterilizers and correct packing both exist to prevent it.

Answer: C

Vancomycin acts by the inhibition of cell wall synthesis.

Vancomycin, a glycopeptide, binds the D-alanyl-D-alanine end of the peptidoglycan precursor. The bulky drug blocks the transglycosylation and transpeptidation steps that build and cross-link the cell wall, so growing Gram-positive bacteria lyse. It is too large to cross the outer membrane of most Gram-negative bacteria.

It does not inhibit nucleic acid synthesis (as rifampicin or the quinolones do), damage the cytoplasmic membrane (as polymyxins do) or affect DNA supercoiling (the target of quinolones acting on DNA gyrase).

Answer: A

Phenolics disinfect by rupturing the plasma membrane, followed by leakage of cellular contents.

Phenol and its derivatives, such as cresols, chloroxylenol and triclosan, dissolve in the lipid of the cytoplasmic membrane and disrupt it, so ions and metabolites leak out. At higher concentrations they also denature and coagulate proteins. This mode of action is why phenol, Lister's antiseptic, became the reference standard (the phenol coefficient) for other disinfectants.

Bond formation between adjacent pyrimidines describes ultraviolet light, which forms thymine dimers. Alkylation of proteins is the action of aldehydes such as formaldehyde and glutaraldehyde and of ethylene oxide. Adduct formation with amino acids and unsaturated fatty acids is not how phenolics are described as acting.

Answer: A

The incorrect statement is that Mycoplasma is naturally resistant to penicillins due to presence of R plasmid.

Penicillins act by blocking the synthesis of peptidoglycan in the cell wall. Mycoplasma has no cell wall at all, so it is intrinsically resistant: the drug has no target. That is natural resistance, not resistance carried on an R plasmid.

The other statements are correct. The outer membrane of Gram-negative bacteria keeps out penicillin G, β-lactamases hydrolyse the β-lactam ring, and efflux pumps that expel the drug are a recognised resistance mechanism.

Answer: C

Glutaraldehyde is used in cold sterilization.

A 2% alkaline glutaraldehyde solution kills bacteria, spores and viruses by cross-linking proteins, at room temperature. It is used to sterilise heat-sensitive instruments such as endoscopes, which cannot be autoclaved; this chemical sterilisation without heat is called cold sterilization.

Pasteurization and autoclaving use moist heat, and incineration uses burning; none relies on glutaraldehyde.