Probing the viability of palladium-challenged bacterial cells using flow cytometry

BACKGROUND: The ability of bacterial cells to retain membrane integrity and membrane potential when challenged with Palladium (II) solution has not being examined previously, which would provide a platform towards the bio-fabrication of a multifunctional tandem bio-nanocatalyst. This study investigates the use of flow cytometry coupled with fluorescent probes to determine membrane integrity and membrane potential of cells of Desulfovibrio desulfuricans and Bacillus benzeovorans challenged with 1 mmol L^-1 of sodium tetrachloropalladate (II) (Na₂PdCl₄) solution at pH 2 followed by reduction of palladium (II) (Pd (II)) with formate to give 1wt% loading of Pd (0) on the cells.

RESULTS: Fluorescently labelled active bacterial cells retained over 80% of membrane potential when challenged with Pd (II) solutions except for Bacillus benzeovorans (Bb) with about 32% retention. Cell viability was also seen to be variable and strain-dependent while dead cells lack any membrane integrity. Since esterase activity is energy independent and unable to confirm the membrane potential of the bacterial cells, the dye 3, 3’-dihexyloxacarbocyanine iodide [DiO₆(3)] was used to determine and confirm the membrane potential of the bacterial cells.

CONCLUSION: The results revealed that since fluorescently labelled bacterial cells containing Pd (0) can retain metabolic activity when analysed with flow cytometry, it provides the potential for combining chemical catalysis with biochemical activity in reactions that require metabolic synergy.