The effects of palladium coordination complex speciation and concentration upon the ubiquitous bacterial species Pseudomonas aeruginosa

The toxicity of three different palladium (Pd) species to Pseudomonas aeruginosa, an environmentally ubiquitous bacterial species, is reported. Palladium was added to chemically-defined minimal media as three complex ion salts, namely sodium tetrachloropalladate (Na₂[PdCl₄]), tetraamminepalladium(II) chloride ([Pd(NH₃)₄]Cl₂), and potassium hexachloropalladate(IV) (K₂[PdCl₆]), inoculated with log-phase cultures and incubated for 24 h at 25 °C. Toxicity was tested for Pd concentrations ranging from 6.55 μg/L (0.06 μM Pd) to 250 μg/L (2.33 μM Pd). Minimum inhibitory concentrations (MICs) were determined and growth tracked via optical absorption at 600 nm. Viability and minimum bactericidal concentrations (MBCs) were measured in parallel with dilution, plating and colony forming unit (CFU) counting. MICs for all forms of Pd were 62.5 μg Pd/L, approximately 1000 times lower than previously reported values. The MBCs for PdCl₄^2- and Pd(NH₃)₄²⁺ were 62.5 μg Pd/L and 125 μg Pd/L for PdCl₆^2-. Pd(NH₃)₄²⁺ and PdCl₆^2- culture viability at 7.8–31.3 μg Pd/L was not different from controls. However, PdCl₄^2- culture viability was different from the other additives, with decreasing viability at sub-MBC concentrations down to 6.55 μg Pd/L. To understand the possible effect of speciation upon toxicity, the equilibrium speciation of Pd was modeled for all solutions using PHREEQC and found to be dominated by Pd(NH₃)₃Cl⁺ (65.6 % of total Pd) and Pd(NH₃)₄²⁺ (34.2 % total Pd). The juxtaposition of the equilibrium calculations and the toxicity results indicates that the kinetics of ligand exchange between the palladium complexes and the growth medium could influence bacterial response.