Quorum sensing going wild

Virulence regulation in plant-pathogenic bacteria by host-secreted signals

Bacterial pathogens manipulate host signaling pathways and evade host defenses using effector molecules, coordinating their deployment to ensure successful infection. However, host-derived metabolites as signals, and their critical role in regulating bacterial virulence requires further insights. Effective regulation of virulence, which is essential for pathogenic bacteria, involves controlling factors that enable colonization, defense evasion, and tissue damage. This regulation is dynamic, influenced by environmental cues including signals from host plants like exudates. Plant exudates, comprising of diverse compounds released by roots and tissues, serve as rich chemical signals affecting the behavior and virulence of associated bacteria. Plant nutrients act as signaling molecules that are sensed through membrane-localized receptors and intracellular response mechanisms in bacteria. This review explains how different bacteria detect and answer to secreted chemical signals, regulating virulence gene expression. Our main emphasis is exploring the recognition process of host-originated signaling molecules through molecular sensors on cellular membranes and intracellular signaling pathways. This review encompasses insights into how bacterial strains individually coordinate their virulence in response to various distinct host-derived signals that can positively or negatively regulate their virulence. Furthermore, we explained the interruption of plant defense with the perception of host metabolites to dampen pathogen virulence. The intricate interplay between pathogens and plant signals, particularly in how pathogens recognize host metabolic signals to regulate virulence genes, portrays a crucial initial interaction leading to profound influences on infection outcomes. This work will greatly aid researchers in developing new strategies for preventing and treating infections.

Unravelling the Roles of Bacterial Nanomachines Bistability in Pathogens' Life Cycle

Bacterial nanomachines represent remarkable feats of evolutionary engineering, showcasing intricate molecular mechanisms that enable bacteria to perform a diverse array of functions essential to persist, thrive, and evolve within ecological and pathological niches. Injectosomes and bacterial flagella represent two categories of bacterial nanomachines that have been particularly well studied both at the molecular and functional levels. Among the diverse functionalities of these nanomachines, bistability emerges as a fascinating phenomenon, underscoring their dynamic and complex regulation as well as their contribution to shaping the bacterial community behavior during the infection process. In this review, we examine two closely related bacterial nanomachines, the type 3 secretion system, and the flagellum, to explore how the bistability of molecular-scale devices shapes the bacterial eco-pathological life cycle.

Pseudomonas fuscovaginae quorum sensing studies: 5% dominates cell-to-cell conversations

A common feature of N-acyl-l-homoserine lactone (AHL) quorum-sensing (QS) systems is that the AHL signal is autoinducing. Once induced, a cell will further amplify the signal via a positive feedback loop. Pseudomonas fuscovaginae UPB0736 has two fully functional AHL QS systems, called PfsI/R and PfvI/R, which are inactive in a standard laboratory setting. In this work, we induce the QS systems with exogenous AHL signals and characterize the AHL signal amplification effect and QS activation dynamics at community and single-cell level. While the cognate signal is in both cases significantly further amplified to physiologically relevant levels, we observe only a limited response in terms of AHL synthase gene promoter activity. Additionally, the PfsI/R QS system exhibits a unique dramatic phenotypic heterogeneity, where only up to 5% of all cells amplify the signal further and are, thus, considered to be QS active.

IMPORTANCE

Bacteria use N-acyl-l-homoserine lactone (AHL) quorum-sensing (QS) systems for population-wide phenotypic coordination. The QS configuration in Pseudomonas fuscovaginae is dramatically different from other model examples of AHL QS signaling and, thus, represents an important exception to the norm, which usually states that QS triggers population-wide phenotypic transitions in relation to cell density. We argue that the differences in QS dynamics of P. fuscovaginae highlight its different evolutionary purpose, which is ultimately dictated by the selective pressures of its natural habitat. We hope that this example will further expand our understanding of the complex and yet unknown QS-enabled sociomicrobiology. Furthermore, we argue that exemptions to the QS norm will be found in other plant-pathogenic bacterial strains that grow in similar environments and that molecularly similar QS systems do not necessarily share a similar evolutionary purpose; therefore, generalizations about bacterial cell-to-cell signaling systems function should be avoided.

Control of bacterial quorum threshold for metabolic homeostasis and cooperativity

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.

RsaM: a unique dominant regulator of AHL quorum sensing in bacteria

Quorum sensing (QS) in proteobacteria is a mechanism to control gene expression orchestrated by the LuxI/LuxR protein family pair, which produces and responds to N-acyl homoserine lactone (AHL) diffusible signal molecules. QS is often regarded as a cell density response via the sensing of/response to the concentrations of AHLs, which are constantly basally produced by bacterial cells. The luxI/R systems, however, undergo supra-regulation in response to external stimuli and many regulators have been implicated in controlling QS in bacteria, although it remains unclear how most of these regulators and cues contribute to the QS response. One regulator, called RsaM, has been reported in a few proteobacterial species to have a stringent role in the control of AHL QS. RsaMs are small, in the range of 140-170 aa long, and are found in several genera, principally in Burkholderia and Acinetobacter. The gene encoding RsaM is always located as an independent transcriptional unit, situated adjacent to QS luxI and/or luxR loci. One of the most remarkable aspects of RsaM is its uniqueness; it does not fall into any of the known bacterial regulatory families and it possesses a distinct and novel fold that does not exhibit binding affinity for nucleic acids or AHLs. RsaM stands out as a distinctive regulator in bacteria, as it is likely to have an important ecological role, as well as unravelling a novel way of gene regulation in bacteria.