Q.26
Mechanism of antibacterial action of polymyxins relies on the
(A) inhibition of 30S ribosomal subunit
(B) disruption of peptidoglycan synthesis
(C) inhibition of DNA replication of bacteria
(D) disruption of membrane architecture of bacteria
Polymyxins exert their antibacterial action primarily by disrupting the membrane architecture of Gram-negative bacteria. This makes option (D) the correct answer in the multiple-choice query.
Correct Answer
The correct answer is (D) disruption of membrane architecture of bacteria. Polymyxins, such as colistin and polymyxin B, are cationic lipopeptides that bind to the negatively charged lipid A component of lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria. This interaction displaces stabilizing divalent cations like Ca²⁺ and Mg²⁺, leading to outer membrane destabilization, insertion of polymyxin into the inner membrane, and subsequent leakage of cellular contents, causing cell death.
Option Explanations
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(A) Inhibition of 30S ribosomal subunit: Incorrect. This mechanism targets bacterial protein synthesis and is characteristic of aminoglycosides like gentamicin, which bind the 30S ribosomal subunit to cause mistranslation and membrane-disrupting faulty proteins.
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(B) Disruption of peptidoglycan synthesis: Incorrect. Beta-lactams (e.g., penicillins) and glycopeptides (e.g., vancomycin) inhibit peptidoglycan cross-linking or transport in cell wall synthesis, a process unrelated to polymyxins’ membrane-focused action.
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(C) Inhibition of DNA replication of bacteria: Incorrect. Quinolones like ciprofloxacin target DNA gyrase or topoisomerase IV to block DNA replication, distinct from polymyxins’ direct membrane disruption.
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(D) Disruption of membrane architecture of bacteria: Correct, as detailed above. Polymyxins’ amphipathic structure enables selective LPS binding and membrane lysis, primarily in Gram-negative bacteria.
