This would in turn result in a decreased TWS119 substrate availability for the PAO1 autoinducer synthase and consequently result in reduced levels of formed autoinducers and in an absence of autoinduction of lasI transcription. Unfortunately, we have no chemical data available to prove this hypothesis. However, the almost complete lack of lasI and rhlI expression support this hypothesis. Furthermore BpiB09 acts as SDR on 3-oxo-C12-HSL and due to the structural similarity of the substrate it is very well possible that the protein also acts on the 3-oxo-acyl-ACP available in the cell. Within this framework, it is worthwhile noting that BpiB09 is a novel SDR and no Dasatinib enzyme with a similar activity has been described yet. A phylogenetic analysis suggested that the protein is different from known SDRs. The most similar proteins found in GenBank were SDRs from Acidobacterium and from Koribacter. However, the overall similarity was not higher than 58% suggesting that BpiB09 originates from a not yet cultivated microbe. Since the protein was derived from a metagenome it will not be possible to speculate on the original host. It appears that BpiB09 represents probably the first NADPdependent SDR derived from a non-cultivated microbe whose structure is deposited at PDB. Concerning a potential application of the protein for the prevention of microbial biofilms, we can currently only speculate about the success of such an attempt. However, taking into account the strong phenotypes observed in our biofilm tests it might indeed be possible to use the protein for quenching the QS signal and thereby suppressing bacterial biofilm formation at a very early stage. In fact, several examples have been published demonstrating that the expression of quorum quenching enzymes can result in the reduction of pathogenicity and virulence. Since BpiB09 requires NADPH as a cofactor it might however, require the additional supply of the cofactor at sufficiently high concentrations for the reduction of the respective autoinducer molecules. Current work needs to assess the feasibility of this approach by immobilization of BpiB09 on a catheter or other surfaces. Those immobilized proteins can then be used to analyze the role of the BpiB09 protein on developing P. aeruginosa or mixed species microbial biofilms. The Screening was performed as previously published. The positive clones were verified at least three times. In addition, we used C. violaceum to also verify the observed results. For these tests a cell free extract was prepared of the Bio5 clone and of E. coli XL1 blue containing an empty vector. The protein concentration was determined and adjusted to 10 mg/ml. An overnight culture of C. violaceum was prepared and 50 ml were mixed with 50 ml of the cell extracts and incubated overnight at 30uC. Absence or impairment of purple coloration indicated quorum quenching.