In Vitro Activity of a Novel Metal-Based Antimicrobial against Multidrug-Resistant Klebsiella pneumoniae

Multidrug-resistant (MDR) Klebsiella pneumoniae, classified by the World Health Organization (WHO) as a critical priority pathogen, represents a global health thereat requiring novel antimicrobials urgently. Here we evaluated the in vitro antimicrobial activity of a novel iridium-based compound (OMKP-3), against MDR K. pneumoniae. OMKP-3 exhibited robust antimicrobial activity in M9 minimal media (MIC=6.25{micro}g/mL) and rapid bactericidal effect (MBC=12.5{micro}g/mL) against the tested MDR K. pneumoniae strains. OMKP-3 showed antibiofilm ability and was active against multiple MDR Gram-negative pathogens, including Escherichia coli, Enterobacter cloacae, Pseudomonas aeruginosa and Serratia marcescens (MIC range:6.25-25{micro}g/mL). Importantly, OMKP-3 showed no cytotoxicity against mammalian cells after 24 hours of exposure. When combined with polymyxin B, OMKP-3 acted as an adjuvant, enhancing polymyxin B activity (FIC[≤]0.5). OMKP-3 was less prone to induce high-level resistance in MDR K. pneumoniae compared to ciprofloxacin, and supressed the growth of resistant bacteria at a low and non-cytotoxic concentration (4xMIC). K. pneumoniae strains harboring truncated Ompk35/36 porin genes exhibited higher OMKP-3 MICs, indicating that these porins may serve as an important entry pathway. Spectrometry analysis revealed that OMKP-3 was able to accumulate intracellularly (1.57{micro}g/mL), with minimal Resistance-Nodulation-Division (RND) efflux pump extrusion involvement. Furthermore, analysis of the resistant mutant, harboring a mutation in the outer membrane protein DegS, together with fluorescence microscopy, suggests that OMKP-3 induces membrane-associated damage. No cross-resistance between OMKP-3 and commonly used antibiotics was observed. Collectively, these findings identify OMKP-3 as a promising novel antimicrobial agent against MDR K. pneumoniae, likely acting through an unexplored bacterial target. ImportanceMultidrug-resistant (MDR) Klebsiella pneumoniae is a critical global health threat and is among the leading causes of hospital0hyphenorendash;associated mortality, largely due to the scarcity of effective therapeutic options. Alarmingly, the current antimicrobial pipeline fails to address this issue, relying largely on derivatives of existing scaffolds that offer only short-term clinical benefit due to rapid resistance emergence. Developing antibiotics against Gram-negative pathogens is particularly challenging because of their highly impermeable outer membrane and efficient efflux systems, limiting intracellular drug accumulation. Metal-based antimicrobials emerge as a promising alternative. Our findings showed that OMKP-3, an iridium complex, exhibits potent bactericidal activity against MDR K. pneumoniae without selecting for high-level resistance, suggesting the potential for sustained therapeutic efficacy. Additionally, it demonstrated to accumulate intracellularly with minimal efflux involvement. Together, these features position OMKP-3 as a valuable and underexplored novel antimicrobial strategy for addressing the escalating threat of MDR K. pneumoniae infections.