Physical and Chemical Agents Questions and Answers

Q: The antimicrobial activity of vancomycin is due to the ____________.

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).

Q: Phenolics act as disinfectant by ____________.

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.

Q: Which one of the following is NOT correct about antibiotic resistance mechanism in microbes?

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.

Q: In which of following processes, glutaraldehyde is used as a sterilizing agent?

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.

Q: Cold sterilization refers to the preservation of food by :

Cold sterilization means preservation of food by radiation.Ionising radiation, such as gamma rays from cobalt-60, electron beams or X-rays, kills microorganisms and insects by damaging their DNA without significantly heating the food. Because sterilisation is achieved without heat, food irradiation is called cold sterilization (or cold pasteurisation).Dehydration and lyophilisation (freeze-drying) preserve food by removing water, which stops microbial growth but does not sterilise. Refrigeration only slows microbial growth. None of them kills the microbes in the way the term implies.

The antimicrobial activity of vancomycin is due to the ____________.

A.inhibition of nucleic acid synthesis
B.damage to the cytoplasmic membrane
C.inhibition of cell wall synthesis
D.regulation of DNA supercoiling

Exam Year(s)

GATE Life Sciences - 2022,

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).

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Phenolics act as disinfectant by ____________.

A.rupturing plasma membrane followed by leakage of cellular contents
B.bond formation between adjacent pyrimidine bases
C.forming adduct with amino acid and unsaturated fatty acids
D.alkylation of proteins

Exam Year(s)

GATE Life Sciences - 2022,

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.

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Which one of the following is NOT correct about antibiotic resistance mechanism in microbes?

A.Mycoplasma is naturally resistant to penicillins due to presence of R plasmid.
B.Gram-negative bacteria are impermeable to penicillin G.
C.β-lactamases of bacteria can cleave penicillins.
D.Selective microbes can efflux penicillins entering the cell and develop resistance.

Exam Year(s)

GATE Life Sciences - 2023,

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.

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In which of following processes, glutaraldehyde is used as a sterilizing agent?

A.Pasteurization
B.Incineration
C.Cold sterilization
D.Autoclaving

Exam Year(s)

GATE Life Sciences - 2024,

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.

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Determine the correctness or otherwise of the following Assertion [a] and the Reason [r].

Assertion [a]: Silver and copper display bacteriocidal or antiseptic activity.
Reason [r]: Silver and copper effectively demonstrate oligodynamic action.

A.Both [a] and [r] are true and [r] is the correct reason of [a].
B.Both [a] and [r] are true but [r] is not the correct reason of [a].
C.Both [a] and [r] are false.
D.[a] is true but [r] is false.

Exam Year(s)

GATE Life Sciences - 2026,

Answer: A

Both the assertion and the reason are true, and the reason correctly explains the assertion.

Silver and copper kill or inhibit microorganisms, which is why silver compounds are used on wound dressings and copper on touch surfaces and in water systems. The effect has a name, the oligodynamic action: very small amounts of these heavy-metal ions are toxic to microbes. The ions bind sulfhydryl and other groups on proteins and enzymes and so inactivate them.

The oligodynamic action is therefore the mechanism behind the bactericidal activity in the assertion, not merely a separate true fact. That rules out the option in which the reason does not explain the assertion, and both options that call either statement false.

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Cold sterilization refers to the preservation of food by :

A.Radiation
B.Dehydration
C.Lyophilization
D.Refrigeration

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: A

Cold sterilization means preservation of food by radiation.

Ionising radiation, such as gamma rays from cobalt-60, electron beams or X-rays, kills microorganisms and insects by damaging their DNA without significantly heating the food. Because sterilisation is achieved without heat, food irradiation is called cold sterilization (or cold pasteurisation).

Dehydration and lyophilisation (freeze-drying) preserve food by removing water, which stops microbial growth but does not sterilise. Refrigeration only slows microbial growth. None of them kills the microbes in the way the term implies.

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Sterilization is best defined as a process that

A.renders a product free of all forms of viable microorganisms
B.destroys pathogenic organisms but spares harmless ones
C.reduces contaminants to a level judged safe for public health
D.prevents further multiplication without killing anything

Answer: A

Sterilization is absolute: nothing viable is left, spores and all. Disinfection destroys pathogens and many other organisms but is not required to eliminate spores, and sanitization only lowers numbers to an agreed safe level, so the three terms are not interchangeable.

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Disinfection is defined as the

A.inhibition of growth for as long as the agent is present
B.thermal or chemical destruction of pathogenic and other microorganisms
C.validated removal of every viable microorganism including spores
D.mechanical washing away of soil and organic material

Answer: B

Disinfection kills pathogens and many other organisms by heat or chemicals, but unlike sterilization it carries no requirement to destroy bacterial spores. That is why a disinfected instrument is not a sterile one, however clean it looks.

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An agent that prevents or arrests the growth of microorganisms on living tissue is

A.a sanitizer
B.a detergent
C.an antiseptic
D.a sterilant for instruments

Answer: C

Antiseptics are formulated for skin and mucous membranes, so they must be active yet tolerable to tissue. Disinfectants are for inanimate surfaces and are often too irritant for skin, which is why the same chemical may appear at different strengths under both names.

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10.

A sanitizer is an agent that

A.eliminates every viable microorganism from a treated surface, spores included
B.arrests microbial growth only while it remains in contact
C.removes organic soil without any antimicrobial action
D.reduces bacterial numbers to levels judged safe by public health standards

Answer: D

Sanitization is a numbers game rather than an absolute: the count is brought down to an accepted threshold, which is the standard applied to eating utensils and dairy equipment. It neither sterilizes nor guarantees the removal of any particular pathogen.

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