Deciphering rapid proliferation of filamentous bacteria in activated sludge community via cross-feeding of public goods mediated by diffusible signal factor

The induced mechanisms underlying filamentous bulking by quorum sensing (QS) remain unclear. This study investigated how diffusible signal factors (DSF)-mediated QS facilitates filamentous bacterial proliferation via public goods cross-feeding. The results indicated that DSF concentrations increased by 4.32– and 5.03-fold in the aqueous and solid phases, respectively, correlating positively with the proliferation of the filamentous bacterium Thiothrix and upregulation of DSF-related genes (rpfF, rpfC, rpfG). Activation of the DSF-QS system stimulated threonine and serine biosynthesis and extracellular protease secretion, supporting the auxotrophic growth of Thiothrix. Correlation and phylogenetic analyses suggested that DSF-QS regulates the cross-feeding of these metabolites between Thiothrix, Acinetobacter bohemicus B and Ca. Competibacter B. Excessive proliferation of Thiothrix disrupted the physicochemical properties of sludge flocs and impaired aggregation, leading to sludge bulking. This study reveals that DSF-QS promotes filamentous bulking in activated sludge by regulating the cross-feeding of public goods.