New insights into the metal specificity of the Pseudomonas aeruginosa pyoverdine-iron uptake pathway

Pyoverdine (PvdI) is the major siderophore secreted by Pseudomonas aeruginosa PAOI in order to get access to iron. After being loaded with iron in the extracellular medium, PvdI is transported across the bacterial outer membrane by the transporter, FpvAI. We used the spectral properties of PvdI to show that in addition to Fe3+, this siderophore also chelates, but with lower efficiencies, all the 16 metals used in our screening. Afterwards, FpvAI at the cell surface binds Ag+, Al3+, Cd2+, Co2+, Cu2+, Fe3+, Ga3+, Hg2+, Mn2+, Ni2+ or Zn2+ in complex with PvdI. We used Inductively Coupled Plasma-Atomic Emission Spectrometry to monitor metal uptake in P. aeruginosa: TonB-dependent uptake, in the presence of PvdI, was only efficient for Fe3+. Cu2+, Ga3+, Mn2+ and Ni2+ were also transported into the cell but with lower uptake rates. The presence of Al3+, Cu2+, Ga3+, Mn2+, Ni2+ and Zn2+ in the extracellular medium induced PvdI production in P. aeruginosa. All these data allow a better understanding of the behaviour of the PvdI uptake pathway in the presence of metals other than iron: FpvAI at the cell surface has broad metal specificity at the binding stage and it is highly selective for Fe3+ only during the uptake process.