The pathogen Pseudomonas aeruginosa optimizes the production of the siderophore pyochelin upon environmental challenges.

Siderophores are iron chelators produced by bacteria to access iron, an essential nutrient. The pathogen Pseudomonas aeruginosa produces two siderophores, pyoverdine and pyochelin, the former with a high affinity for iron and the latter with a lower affinity. Furthermore, the production of both siderophores involves a positive auto-regulatory loop: the presence of the ferri-siderophore complex is essential for their large production. Since pyochelin has a lower affinity for iron it was hard to consider the role of pyochelin in drastic competitive environments where the host or the environmental microbiota produce strong iron chelators and may inhibit iron chelation by pyochelin. We showed here that the pyochelin pathway overcomes this difficulty through a more complex regulating mechanism for pyochelin production than previously described. Indeed, in the absence of pyoverdine, and thus higher difficulty to access iron, the bacteria are able to produce pyochelin independently of the presence of ferri-pyochelin. The regulation of the pyochelin pathway appeared to be more complex than expected with a more intricate tuning between repression and activation. Consequently, when the bacteria cannot produce pyoverdine they are able to produce pyochelin even in the presence of strong iron chelators. Such results support a more complex and varied role for this siderophore than previously described, and complexify the battle for iron during P. aeruginosa infection.

Non-specific interference of cobalt with siderophore-dependent iron uptake pathways.

Much data shows that biological metals other than Fe3+ can interfere with Fe3+ acquisition by siderophores in bacteria. Siderophores are small Fe3+ chelators produced by the microorganisms to obtain access to Fe3+. Here, we show that Co2+ is imported into Pseudomonas aeruginosa cells in a complex with the siderophore pyochelin (PCH) by the ferri-PCH outer membrane transporter FptA. Moreover, the presence of Co2+ in the bacterial environment strongly affects the production of PCH. Proteomic and transcriptomic approaches showed that a decrease of PCH production is associated with repression of the expression of the genes involved in PCH biosynthesis. We used various molecular biology approaches to show that this repression is not Fur-(ferric uptake transcriptional regulator) dependent but due to competition of PCH-Co with PCH-Fe for PchR (transcriptional activator), thus inhibiting the formation of PchR-PCH-Fe and consequently the expression of the PCH genes. We observed a similar mechanism of repression of PCH production, but to a lesser extent, by Ni2+, but not for Zn2+, Cu2+, or Mn2+. Here, we show, for the first time at a molecular level, how the presence of a contaminant metal can interfere with Fe3+ acquisition by the siderophores PCH and PVD.