Q.71 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

Correct Answer: (A) rupturing plasma membrane followed by leakage of cellular contents

Phenolics primarily function as disinfectants by disrupting microbial cell membranes, causing leakage and cell death. This mechanism aligns with their established antibacterial action in microbiology contexts like CSIR NET Life Sciences.

Option Analysis

(A) Rupturing Plasma Membrane

Phenolics penetrate the lipid-rich plasma membrane of bacteria due to their lipophilic nature, altering permeability and leading to rupture. This causes essential cellular contents like ions, proteins, and metabolites to leak out, resulting in cell lysis and death. Studies confirm this as the dominant initial step, with subsequent protoplasmic poisoning.

(B) Bond Formation Between Adjacent Pyrimidine Bases

This describes thymine dimer formation from UV radiation, which crosslinks DNA pyrimidines (thymine/cytosine) and blocks replication. Phenolics do not induce such photoproducts or UV-like DNA damage; no evidence links them to pyrimidine bonding.

(C) Forming Adduct with Amino Acid and Unsaturated Fatty Acids

Phenolics bind proteins via hydrogen bonding or hydrophobic interactions and may interact with lipids, but they do not form stable chemical adducts like covalent conjugates. Adduct formation is more typical of reactive electrophiles (e.g., epoxides), not phenolics’ denaturation-based action.

(D) Alkylation of Proteins

Alkylation involves adding alkyl groups to proteins/DNA, a mechanism of agents like formaldehyde or ethylene oxide that covalently modify sulfhydryl/amino groups. Phenolics denature proteins through disruption but lack alkylating reactivity.

Phenolics act as disinfectant by rupturing plasma membrane, a key mechanism in microbiology for CSIR NET aspirants. This action targets bacterial cell integrity, making phenolics effective against bacteria, fungi, and viruses.

Primary Mechanism

Phenolics, derived from phenol (carbolic acid), disrupt the lipid bilayer of microbial plasma membranes due to their amphipathic structure. Membrane rupture leads to leakage of vital intracellular contents, halting metabolic processes and causing lysis. This is supported by studies showing increased permeability and protoplasmic poisoning.

Why Not Other Options?

• Pyrimidine bonds (B): UV-specific, irrelevant to phenolics.

• Adducts (C): Phenolics denature, not covalently adduct.

• Alkylation (D): Exclusive to alkylating agents like formaldehyde.

Applications in Disinfection

Used at 1-5% concentrations, phenolics disinfect surfaces, equipment, and organic matter effectively, though limited against spores. Ideal for non-critical settings due to broad-spectrum activity.