Iron is essential in many biological processes. However, its bioavailability is reduced in aerobic environments, such as soil. To overcome this limitation, microorganisms have developed different strategies, such as iron chelation by siderophores. Some bacteria have even gained the ability to detect and utilize xenosiderophores, i.e., siderophores produced by other organisms. We illustrate an example of such an interaction between two soil bacteria, Pseudomonas fluorescens strain BBc6R8 and Streptomyces ambofaciens ATCC 23877, which produce the siderophores pyoverdine and enantiopyochelin and the siderophores desferrioxamines B and E and coelichelin, respectively. During pairwise cultures on iron-limiting agar medium, no induction of siderophore synthesis by P. fluorescens BBc6R8 was observed in the presence of S. ambofaciens ATCC 23877. Cocultures with a Streptomyces mutant strain that produced either coelichelin or desferrioxamines, as well as culture in a medium supplemented with desferrioxamine B, resulted in the absence of pyoverdine production; however, culture with a double mutant deficient in desferrioxamines and coelichelin production did not. This strongly suggests that P. fluorescens BBbc6R8 utilizes the ferrioxamines and ferricoelichelin produced by S. ambofaciens as xenosiderophores and therefore no longer activates the production of its own siderophores. A screening of a library of P. fluorescens BBc6R8 mutants highlighted the involvement of the TonB-dependent receptor FoxA in this process: the expression of foxA and genes involved in the regulation of its biosynthesis was induced in the presence of S. ambofaciens. In a competitive environment, such as soil, siderophore piracy could well be one of the driving forces that determine the outcome of microbial competition. Bacteria detect, assimilate, and integrate different environmental signals in order to better adapt to their habitat and cope with changes in environmental conditions. Multiple signaling pathways allow them to communicate with each other within the same species or between different species (1). This can be achieved through the production and detection of diffusible molecules in the environment. In response to these interactions, the microorganisms have developed complex metabolic and physiological responses. One of the essential environmental factors vital for organisms is iron. It plays an essential role in many biological processes, such as DNA synthesis, respiration, and photosynthesis. Iron can adopt two different ionic forms, Fe 2ϩ and Fe 3ϩ . This property makes it an important player in the oxidation-reduction reactions in the cell. However, while iron is an abundant element on earth, its bioavailability is reduced in aerobic environments, such as soil. Ferric iron (Fe 3ϩ ) forms insoluble ferric hydroxides (with a solubility product of ϳ10 Ϫ39 ) in the presence of oxygen (2-4). Therefore, iron is a limiting factor for the growth of microorganisms.To overcome the limitation of iron bioavailability, aerobic bacteria have develop...

Streptomyces are ubiquitous soil bacteria well known for their ability to produce a wide range of secondary metabolites including antibiotics. In their natural environments, they co-exist and interact with complex microbial communities and their natural products are assumed to play a major role in mediating these interactions. Reciprocally, their secondary metabolism can be influenced by the surrounding microbial communities. Little is known about these complex interactions and the underlying molecular mechanisms. During pairwise co-culture experiments, a fluorescent Pseudomonas, Pseudomonas fluorescens BBc6R8, was shown to prevent the production of the diffusible blue pigment antibiotic γ-actinorhodin by Streptomyces coelicolor A3(2) M145 without altering the biosynthesis of the intracellular actinorhodin. A mutant of the BBc6R8 strain defective in the production of gluconic acid from glucose and consequently unable to acidify the culture medium did not show any effect on the γ-actinorhodin biosynthesis in contrast to the wild-type strain and the mutant complemented with the wild-type allele. In addition, when glucose was substituted by mannitol in the culture medium, P. fluorescens BBc6R8 was unable to acidify the medium and to prevent the biosynthesis of the antibiotic. All together, the results show that P. fluorescens BBc6R8 impairs the biosynthesis of the lactone form of actinorhodin in S. coelicolor by acidifying the medium through the production of gluconic acid. Other fluorescent Pseudomonas and the opportunistic pathogen Pseudomonas aeruginosa PAO1 also prevented the γ-actinorhodin production in a similar way. We propose some hypotheses on the ecological significance of such interaction.

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