Introduction:
The urgent demand for new antimicrobial strategies has highlighted transition metal complexes as promising therapeutic candidates. Among them, Co(III) complexes with diamine chelate ligands exhibit notable stability and biomolecular affinity. To elucidate their antimicrobial activity and mode of action, phosphorescent Ru(II)- and Pt(II)-based probes are particularly advantageous due to their oxygen-sensitive phosphorescence. Phosphorescence optical respirometry (POR) enables real-time monitoring of microbial respiration, providing a sensitive measure of metabolic inhibition induced by metal complexes.

Materials and Methods:
Co(III)-diamine complexes were synthesized and characterized by elemental analysis, an ATR technique, and a scan method. Their antimicrobial properties were examined against representative gram-positive (Staphylococcus aureus, Staphylococcus epidermidis) and gram-negative (Escherichia coli, Pseudomonas aeruginosa) reference and clinical bacterial strains. Ru(II) and Pt(II) phosphorescent probes were employed in POR assays to monitor oxygen consumption as a marker of cellular respiration.

Results:
The Co(III) complexes displayed strain-dependent antimicrobial effects, with more pronounced activity against gram-positive bacteria. POR measurements revealed a dose-dependent suppression of microbial respiration, consistent with metabolic inhibition. Changes in phosphorescence lifetimes of Ru(II) and Pt(II) probes confirmed oxygen depletion dynamics and allowed kinetic profiling of bacterial response to the complexes.

Conclusions:
Ru(II) and Pt(II) phosphorescent biosensors, applied through phosphorescence optical respirometry, provide a robust platform for investigating the antimicrobial activity of Co(III)-diamine complexes. as well as other antimicrobial compounds. This method not only enables sensitive detection of respiratory inhibition but also permits the determination of viable aerobic microbial cell counts through analysis of the phosphorescence value and offers mechanistic insights into the biological action of metal-based therapeutics and. These findings establish a foundation for further development the PRO method to investigate the interactions of antibiotics and antimycotics with newly developed metal complex–derived antimicrobial agents targeting microorganisms.