Evidence for a functional quorum-sensing type AI-1 system in the extremophilic bacterium Acidithiobacillus ferrooxidans

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 (1) 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 acidophillic chemolithotrophic microorganism, and our results provide a very interesting opportunity to explore the control and regulation of biofilm formation during the bioleaching process.