Private link between signal and response in Bacillus subtilis quorum sensing

Abstract

Bacteria coordinate their behavior using quorum sensing (QS), whereby cells secrete diffusible signals that generate phenotypic responses associated with group living. The canonical model of QS is one of extracellular signaling, where signal molecules bind to cognate receptors and cause a coordinated response across many cells. Here we study the link between QS input (signaling) and QS output (response) in the ComQXPA QS system of Bacillus subtilis by characterizing the phenotype and fitness of comQ null mutants. These lack the enzyme to produce the ComX signal and do not activate the ComQXPA QS system in other cells. In addition to the activation effect of the signal, however, we find evidence of a second, repressive effect of signal production on the QS system. Unlike activation, which can affect other cells, repression acts privately: the de-repression of QS in comQ cells is intracellular and only affects mutant cells lacking ComQ. As a result, the QS signal mutants have an overly responsive QS system and overproduce the secondary metabolite surfactin in the presence of the signal. This surfactin overproduction is associated with a strong fitness cost, as resources are diverted away from primary metabolism. Therefore, by acting as a private QS repressor, ComQ may be protected against evolutionary competition from loss-of-function mutations. Additionally, we find that surfactin participates in a social selection mechanism that targets signal null mutants in coculture with signal producers. Our study shows that by pleiotropically combining intracellular and extracellular signaling, bacteria may generate evolutionarily stable QS systems.social interactions | social evolution B acteria secrete and share quorum-sensing (QS) signaling molecules that bind to specific receptors, and upon reaching critical concentration induce cell density-dependent adaptive responses within the population (1). In Gram-positive bacteria small peptide QS signals are typically produced from oligopeptide precursors that are modified by a signal-processing enzyme before they are secreted from the cell. It is assumed that these bacteria secrete QS signals during growth, and then in the stationary phase when the signal threshold concentration is reached, the signals activate specific histidine kinase receptors. These activate specific response regulators through phosphotransfer, which then initiate a QS response (2). The QS response often involves expression of adaptive extracellular factors (such as food-degrading enzymes, virulence factors, antibiotics, or biosurfactants) that are considered public goods (3, 4), as they can be shared within the population. Recently, however, it was shown that the QS of Gramnegative bacteria may also regulate adaptive metabolic pathways that produce molecules that remain private as they are not secreted by the responsive cells (5).The ComQXPA QS system of Bacillus subtilis is a typical QS system of Gram-positive bacteria that controls expression of nearly 200 genes, including both extracellular and private...

Facultative cheating supports the coexistence of diverse quorum-sensing alleles

“…S2, t test, P value = 0.0026, n = 9). This probably reflects the reduction in the total signal level with the increase of the ΔcomQXP mutant frequency (30).…”

Potential Emergence of Multi-quorum Sensing Inhibitor Resistant (MQSIR) Bacteria

“…In Bacillus cereus group members, QSS: (1) LuxS, (2) PapR/PlcR and (3) ComQXPA produce signals such as (a) AIP, (b) ComX peptide, (c) Competence and sporulation factor, and (d) AI-2 (Table 1) [9,19,20]. These QSS are responsible for the production of toxins, antibiotics phospholipases, proteases, sporulation and competence.…”

Inhibition of Cell Differentiation in Bacillus subtilis by Pseudomonas protegens

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