Presence of the siderophores pyoverdine and pyochelin in the extracellular medium reduces toxic metal accumulation in Pseudomonas aeruginosa and increases bacterial metal tolerance

In order to get access to iron, Pseudomonas aeruginosa strain PAO1 produces two major siderophores pyoverdine (PVD) and pyochelin (PCH). Both siderophores are able to chelate many other metals in addition to iron. However, despite this property, only iron is transported efficiently into the bacteria by the PVD and PCH uptake pathways. Growth studies with P. aeruginosa strains showed a lower sensitivity to toxic metals for the siderophore-producing strain than for the mutants unable to produce siderophores. Moreover, addition of PVD or PCH to the growth medium of a siderophore-deficient strain considerably reduced the toxicity of toxic metals present at concentrations of 100 µM in iron-limited and iron-supplemented growth conditions. Measurement by Inductively Coupled Plasma-Atomic Emission Spectrometry of the concentration of metals present in bacteria incubated with metals in the presence or absence of PVD or PCH indicated that both siderophores were able to sequester metals from the extracellular medium of the bacteria, decreasing metal diffusion into the bacteria. Pyoverdine was able to sequester Al(3+) , Co(2+) , Cu(2+) , Eu(3+) , Ni(2+) , Pb(2+) , Tb(3+) and Zn(2+) from the extracellular medium, and PCH, Al(3+) , Co(2+) , Cu(2+) , Ni(2+) , Pb(2+) and Zn(2+) . Moreover, the presence of 100 µM Cu(2+) and Ni(2+) increased PVD production by 290% and 380%, respectively, in a medium supplemented with iron. All these data suggest that PVD and PCH may contribute to P. aeruginosa resistance to heavy metals.