Antimicrobial and toxicological evaluations of binuclear mercury(ii)bis(alkynyl) complexes containing oligothiophenes and bithiazoles

Abstract

Metals and metal complexes are promising antimicrobials due to their powerful inhibitory activity against microbes. However, the potential for metal toxicity to life remains unsolved. Mercury is a toxic element with no known positive biological functions, and its toxicity to humans is well known. With the known toxicity of Hg and its complexes, their usefulness for antimicrobial therapy should not be neglected. In this work, we investigated the antimicrobial activity of bis-(alkynyl)mercury(II) complexes with oligothiophene and bithiazole linking units against MRSA and C. albicans (pathogens commonly co-inhabit infected sites), and their cytotoxicity was tested on NIH 3T3 cells. Among the Hg(II) complexes, complex 2 showed potent microcidal activity against MRSA (MIC and MBC values = 0.2 μg mL⁻¹) and C. albicans (MIC and MBC values = 0.4 μg mL⁻¹). Complex 2 displayed a highly distinguished zone of clearance with comparably sharp zone edges on microbial agar plates. Time–kill kinetic studies showed that MRSA colonies were reduced by approximately 5 logs during the initial 3 h exposure to complex 2 at an 8× MIC level. The possible antimicrobial mechanism might be due to the intracellular ROS generation by complex 2 that caused the loss in MRSA viability. Complex 2 exhibited a higher IC₅₀ value (0.8 μg mL⁻¹) towards NIH 3T3 cells that was greater than its corresponding MIC values against tested microbes. Hg(II) complexes would be potent antimicrobials, but a challenge is to fully utilize their potential inhibitory effects on microbes while lowering their toxic levels on human health.