Control of bacterial quorum threshold for metabolic homeostasis and cooperativity

Abstract

ABSTRACT Many Proteobacteria employ an acyl-homoserine lactone (AHL)-mediated quorum sensing (QS) system to control diverse social behaviors in a cell density-dependent manner. Various QS modulation mechanisms securing QS initiation at high cell density have been described. However, how QS bacteria determine the quorum threshold is less well known than expected, and little is known about how their physiological and social traits are affected by problems making such early decisions. Here, we show that the RNA-binding protein TofM binds to the mRNA of the QS signal synthase gene tofI and prevents QS signal biosynthesis at low cell density (LCD), thereby defining a stringent QS threshold for the rice pathogen Burkholderia glumae . The tofM mutant produced significant amounts of QS signals at LCD, resulting in a reduced growth rate due to advanced metabolic slowing and metabolic imbalance. When tofM mutants were grown in closed batch culture, mutants of qsmR , encoding a QS-dependent master regulator, spontaneously emerged. The same type of qsmR mutation was observed when low-density wild-type cells were cultured at AHL concentrations above the QS threshold. These data showed that translational control of the QS signal synthase gene at LCD is a stringent mechanism to maintain metabolic homeostasis and cooperativity in B. glumae . Our findings reveal that the bacterial genetic system has diversified to ensure the social activity of QS bacteria, as well as the possible consequences of QS bacteria at LCD encountering environments in which signals generated by their natural neighbors exceed their QS threshold. IMPORTANCE The mechanisms used by various bacteria to determine whether their density is sufficient to meet the QS threshold, how stringently bacterial cells block QS initiation until the QS threshold is reached, and the impacts of low-density bacterial cells encountering conditions that exceed the QS threshold are longstanding gaps in QS research. We demonstrated that translational control of the QS signaling biosynthetic gene creates a stringent QS threshold to maintain metabolic balance at low cell densities. The emergence of non-cooperative cells underlines the critical role of stringent QS modulation in maintaining the integrity of the bacterial QS system, demonstrating that a lack of such control can serve as a selection pressure. The fate of quorum-calling cells exposed to exceeding the QS threshold clarifies QS bacteria evolution in complex ecosystems.