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Acidithiobacillus ferrooxidans is one of the main acidophilic chemolithotrophic bacteria involved in the bioleaching of metal sulfide ores. The bacterium-mineral interaction requires the development of biofilms, whose formation is regulated in many microorganisms by type AI-1 quorum sensing. Here, we report the existence and characterization of a functional type AI-1 quorum-sensing system in A. ferrooxidans. This microorganism produced mainly acyl-homoserine lactones (AHL) with medium and large acyl chains and different C-3 substitutions, including 3-hydroxy-C 8 -AHL, 3-hydroxy-C 10 -AHL, C 12 -AHL, 3-oxo-C 12 -AHL, 3-hydroxy-C 12 -AHL, C 14 -AHL, 3-oxo-C 14 -AHL, 3-hydroxy-C 14 -AHL, and 3-hydroxy-C 16 -AHL. A quorum-sensing genetic locus that includes two open reading frames, afeI and afeR, which have opposite orientations and code for proteins with high levels of similarity to members of the acyl synthase (I) and transcriptional regulator (R) protein families, respectively, was identified. Overexpression of AfeI in Escherichia coli and the associated synthesis of AHLs confirmed that AfeI is an AHL synthase. As determined by reverse transcription-PCR, the afeI and afeR genes were transcribed in A. ferrooxidans. The transcription levels of the afeI gene were higher in cells grown in sulfur and thiosulfate media than in iron-grown cells. Phosphate starvation induced an increase in the transcription levels of afeI which correlated with an increase in AHL levels. Two afe boxes which could correspond to the AfeR binding sites were identified upstream of the afeI gene. This is the first report of a functional type AI-1 quorum-sensing system in an acidophilic chemolithotrophic microorganism, and our results provide a very interesting opportunity to explore the control and regulation of biofilm formation during the bioleaching process.Quorum sensing (QS) is a widespread phenomenon that enables bacterial cells to establish cell-cell communication and to regulate the expression of specific genes in response to local changes in cell density (6,48,49). QS provides the means to coordinate the activities of cells so that they function as a multicellular unit and communicate with eukaryotic hosts (6,20,48,49). In gram-negative bacteria, depending on the autoinductor (AI) molecule, two QS processes have been described: type AI-1, which is involved mainly in intraspecies communication, and type AI-2, which is related to interspecies communication (20,49).The type AI-1 QS regulatory system is composed of four elements: (i) a transcriptional regulator (protein family R); (ii) a cis-acting DNA palindromic sequence; (iii) an acyl-homoserine lactone (AHL), which is the signaling molecule or autoinducer (AI-1); and (iv) the AHL synthase protein (protein family I), which synthesizes the AI (6,20,48,49). It is currently accepted that AI-1 diffuses freely between the cellular and external environments and complexes with the R protein only at a high cell density. The AHL-R complex binds through the R carboxyl domain to the specific site...

Acyl-homoserine lactones (acyl-HSLs) have emerged as important regulatory molecules for many gram-negative bacteria. We have found that Methylobacterium extorquens AM1, a member of the pink-pigmented facultative methylotrophs commonly present on plant surfaces, produces several acyl-HSLs depending upon the carbon source. A novel HSL was discovered with a double unsaturated carbon chain (N-(tetradecenoyl)) (C14:2) and characterized by MS and proton NMR. This long-chain acyl-HSL is synthesized by MlaI that also directs synthesis of C14:1-HSL. The Alphaproteobacterium also produces N-hexanoyl-HSL (C6-HSL) and N-octanoyl-HSL (C8-HSL) via MsaI.

In Pseudomonas aeruginosa, rhamnolipid production is controlled by the quorum-sensing system RhlRI, which itself depends on LasRI. These systems use cell-to-cell signal molecules: N-butyryl-l-homoserine lactone (C4-HSL) and N-(3-oxododecanoyl)-l-homoserine lactone (3OC(12)-HSL), respectively. Whereas both HSLs were produced in M63 medium, rhamnolipid synthesis was not achieved. Phosphate limitation reduced the HSL concentrations, while allowing rhamnolipid production. Hyperosmotic shock applied during the exponential growth phase stopped the accumulation of 3OC(12)-HSL, and prevented C4-HSL and rhamnolipid production. These defects result from lower expression of genes involved in C4-HSL and rhamnolipid syntheses. The osmoprotectant glycine betaine partially restored C4-HSL and rhamnolipid production.