Metagenomics revealed a quorum quenching lactonase QlcA from yet unculturable soil bacteria

Quorum sensing (QS) is a signal mediated cell-cell communication system that couples bacterial cell density to a synchronized gene expression (Fuqua et al., 1994). Mostly, in Gram negative bacteria QS signals are N-acylhomoserine lactones (NAHLs) that coordinate important functions such as virulence and pathogenicity. QS signals or the elements involved in their production or perception could be targeted to disrupt QS, a phenomenon called Quorum quenching (QQ). QQ properties (chemicals and enzymes) are naturally found in various Living organisms, like bacteria (Rhodococcus and Commamonas), plants (carrot, soybean, pea seedling, chilli, garlic etc), and animals (human sera, pork kidney tissues). Consequently, various bacterial genes encoding for NAHL degrading enzymes, like NAHL lactonases (AiiA in Bacillus, AiiB and AttM in Agrobacterium tumefaciens) and acylase/-amidohydrolase (AiiD in Ralstonia) were identified (Givskov et al., 2006). In Pectobacterium carotovorum (causal agent of soft rot diseases) production of various virulence factors and cell wall maceration enzymes is QS dependant, and relies upon successful production, stability, emission and perception of NAHLs (C-8, oxo-C8 and C-10). Disruption of QS signalling by NAHL degrading bacteria, modified bacteria or plants expressing NAHL lactonases resulted in the reduced virulence of the pathogen (Faure et al., 2007). Until recently, investigations on QQ enzymes were carried out mostly on cultivable bacteria, that represent a tiny fraction of soil and root-associated bacteria. In this study, a metagenomics approach (Handelsman, 2004) was employed to access the hidden diversity of uncultivable soil bacteria that revealed a QQ enzyme, an NAHL lactonase, in these bacteria (Riaz et al., 2008).