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All fluorescent pseudomonads (Pseudomonas aeruginosa, P. putida, P. fluorescens, P. syringae and others) are known to produce the high-affinity peptidic yellow-green fluorescent siderophore pyoverdine. These siderophores have peptide chains that are quite diverse and more than 50 pyoverdine structures have been elucidated. In the majority of the cases, a Pseudomonas species is also able to produce a second siderophore of lower affinity for iron. Pseudomonas fluorescens ATCC 17400 has been shown to produce a unique second siderophore, (thio)quinolobactin, which has an antimicrobial activity against the phytopathogenic Oomycete Pythium debaryanum. We show that this strain has the capacity to utilize 16 different pyoverdines, suggesting the presence of several ferripyoverdine receptors. Analysis of the draft genome of P. fluorescens ATCC 17400 confirmed the presence of 55 TonB-dependent receptors, the largest so far for Pseudomonas, among which 15 are predicted to be ferripyoverdine receptors (Fpv). Phylogenetic analysis revealed the presence of two different clades containing ferripyoverdine receptors, with sequences similar to the P. aeruginosa type II FpvA forming a separate cluster. Among the other receptors we confirmed the presence of the QbsI (thio)quinolobactin receptor, an ferri-achromobactin and an ornicorrugatin receptor, several catecholate and four putative heme receptors. Twenty five of the receptors genes were found to be associated with genes encoding extracytoplasmic sigma factors (ECF σ) and transmembrane anti-σ sensors.

During the last few years the knowledge about iron uptake and homeostasis in Pseudomonas has increased enormously. These very versatile bacteria can adapt to widely different ecological niches. It is therefore not surprising that Pseudomonas has a remarkable ability to take up iron and balance iron levels in the cell. The fluorescent pseudomonads, the best known species being P. aeruginosa, P. putida, P. syringae, and P. fluorescens, all produce a fluorescent pigment called pyoverdine, which serves as the major siderophore to capture iron (III). Pyoverdines are complex peptidic structures and each species produces its own pyoverdine siderophore and the corresponding receptor at the level of the outer membrane, meaning that both receptors and pyoverdines co-evolved. A peculiarity of the pyoverdine-mediated iron uptake is the release of iron since it takes place in the periplasm. Many pseudomonads produce a second siderophore of lesser affinity as well, such as pyochelin, enantio-pyochelin, pseudomonin, yersiniabactin, thioquinolobactin, achromobactin, and PDTC, which have other functions next to their role in iron uptake, such as antimicrobial and catalytic activity. A remarkable characteristic of fluorescent pseudomonads is their capacity to scavenge siderophores produced by other microorganisms (xenosiderophores) via a plethora of different outer membrane receptors. Heme is another source of iron that can be used by pseudomonads, animal pathogens such as P. aeruginosa and P. entomophila, having three different heme uptake systems. Finally, some pseudomonads have the capacity to take up iron (II). The regulation of iron homeostasis in fluorescent pseudomonads is quite elaborate and multi-layered, involving the master regulator Fur and secondary regulators, including sigma factors, two-component systems regulators, and small RNAs. Finally, we will present evidence that there is cross-talk between the quorum sensing regulon and iron homeostasis as well as between the response to oxidative stress and the control of iron uptake.

The Pseudomonas genus belongs to the gamma division of Proteobacteria and many species produce the characteristic yellow-green siderophore pyoverdine, and often a second siderophore, of lower affinity for iron. These bacteria are known for their ability to colonize different ecological niches and for their versatile metabolism. It is therefore not surprising that they are endowed with the capacity to take up exogenous xenosiderophores via different TonB-dependent receptors. Uptake of iron is controlled by the central regulator Fur, and via extracytoplasmic sigma factors or other types of regulators (two-component systems, AraC regulators). In this review the Fur regulon (experimentally proven and/or predicted) of P. aeruginosa will be presented. An interesting feature revealed by this analysis of Fur-regulated genes is the overlap between the iron and the sulfur regulons as well with the quorum sensing system.

Characteristic fragment ions of the various chromophores of the pyoverdin siderophore family obtained by collision activated dissociation of the [M+2H](2+) ions are reported allowing unambiguous identification. Tandem mass spectrometrical studies revealed the existence of the first example of a ferribactin with a succinamide side chain, and they add some information to the problem in which way a malic acid side chain is attached to the chromophore.

The production of pectic enzymes by phytopathogenic bacteria and fungi is one of the important factors of pathogenesis. Besides macerating and killing of parenchymatous tissues of the host, these enzymes may stimulate defence responses in the plant.