Community quorum sensing signalling and quenching: microbial granular biofilm assembly

BACKGROUND: Recent reports exploring the role of gradients of quorum sensing (QS) signals in functional activated sludge have raised the question of whether shared systems of signalling synthesis and degradation, or quorum quenching (QQ), across the community inform of the means by which QS biology regulate floccular and granular biofilm assembly. AIMS: In this study, we aimed to explore the species origin and interactive role of QS and QQ activities in such highly diverse microbial biofilm communities. METHODS: Here, such aims were addressed systematically by a comprehensive multi-pronged RNA-sequencing, microbiological and analytical chemistry experimental approach, using two related but independently evolved floccular and granular sludge communities. RESULTS: Our data revealed a distinct difference between the QS and QQ potentials of the two communities, with different species largely displaying either QS or QQ functions. The floccular sludge community showed a high rate of QQ activity, and this rate was dependent on the acyl chain length demonstrating specificity of degradation. When the floccular biomass was transformed into the granular sludge, the QQ activity of the community was reduced by 30%. N-acyl homoserine lactones with four to eight carbons on the acyl chain accumulated at the granular stage, and their concentrations were at least threefold higher than those of the floccular stage. These findings corroborated meta-community analysis where a major shift in the dominant species from potential signal quenchers to producers was observed during the transition from flocs to granules, indicating the role of species composition and associated signalling activities in coordinating community behaviours. CONCLUSIONS: This study suggests that QQ has an important function in regulating community level QS signalling, and provides a mechanistic insight into the role of QS biology in complex community assembly.