Ir-CONJUGATED NISIN: ANTIMICROBIAL ACTIVITY AGAINST METHICILLIN-RESISTANT Staphylococcus aureus (MRSA)

Bhattacharya, Rajdeep (2025) Ir-CONJUGATED NISIN: ANTIMICROBIAL ACTIVITY AGAINST METHICILLIN-RESISTANT Staphylococcus aureus (MRSA). Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS Dissertation of Rajdeep Bhattacharya (20MS059)) 20MS059_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (6MB)

Abstract

The global escalation of antimicrobial resistance, particularly in methicillin-resistant Staphylococcus aureus (MRSA), necessitates the development of innovative therapeutic strategies. Nisin, a lantibiotic produced by Lactococcus lactis, demonstrates potent activity against Gram-positive bacteria, including MRSA, but its clinical utility is increasingly compromised by emerging resistance. This study explores the antimicrobial, photodynamic, and imaging potential of a novel Iridium-conjugated Nisin complex (Ir-Nisin), hypothesized to enhance Nisin’s efficacy through Iridium-mediated reactive oxygen species (ROS) generation under light activation. The complex was synthesized and evaluated through a series of in vitro assays, including MIC-50 determination, colony-forming unit (CFU) enumeration, ROS quantification, scanning electron microscopy (SEM), confocal imaging, and cytotoxicity testing in Huh7 human liver cells. MIC-50 and CFU/mL assays revealed that Ir-Nisin exhibited improved antimicrobial activity over native Nisin, yet remained less potent than the Iridium complex alone under both irradiated and unirradiated conditions. This observation suggests that while conjugation elevates Nisin’s performance, the bulky nature of the Ir-Nisin complex may sterically hinder Iridium’s full photodynamic potential. SEM analysis confirmed substantial morphological damage in MRSA cells following Ir-Nisin treatment. ROS assays demonstrated minimal oxidative stress in the dark but a concentration-dependent increase in ROS under 427 nm light, validating Iridium’s role in photodynamic activation. Confocal microscopy confirmed Ir-Nisin’s imaging capability at low concentrations, though Ir alone exhibited stronger fluorescence. MTT assays indicated that all three compounds—Nisin, Ir, and Ir-Nisin—were non-cytotoxic to Huh7 cells, preserving over 80% viability under all conditions. Collectively, these results position Ir-Nisin as a multifunctional therapeutic with improved antimicrobial activity over Nisin, added photodynamic and imaging capabilities, and a favorable safety profile. Although Iridium alone displayed superior antimicrobial potency, Ir-Nisin represents a balanced candidate for localized photodynamic antimicrobial therapy, particularly in surface infections where light exposure is feasible. This work advances the rationale for metal-peptide conjugates in combating multidrug-resistant pathogens.

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