Flexibility and Adaptability of Quorum Sensing in Nature

Quorum Sensing Signaling Molecules Positively Regulate c-di-GMP Effector PelD Encoding Gene and PEL Exopolysaccharide Biosynthesis in Extremophile Bacterium Acidithiobacillus thiooxidans

Acidithiobacillus species are fundamental players in biofilm formation by acidophile bioleaching communities. It has been previously reported that Acidithiobacillus ferrooxidans possesses a functional quorum sensing mediated by acyl-homoserine lactones (AHL), involved in biofilm formation, and AHLs naturally produced by Acidithiobacillus species also induce biofilm formation in Acidithiobacillus thiooxidans. A c-di-GMP pathway has been characterized in Acidithiobacillus species but it has been pointed out that the c-di-GMP effector PelD and pel-like operon are only present in the sulfur oxidizers such as A. thiooxidans. PEL exopolysaccharide has been recently involved in biofilm formation in this Acidithiobacillus species. Here, by comparing wild type and ΔpelD strains through mechanical analysis of biofilm-cells detachment, fluorescence microscopy and qPCR experiments, the structural role of PEL exopolysaccharide and the molecular network involved for its biosynthesis by A. thiooxidans were tackled. Besides, the effect of AHLs on PEL exopolysaccharide production was assessed. Mechanical resistance experiments indicated that the loss of PEL exopolysaccharide produces fragile A. thiooxidans biofilms. qRT-PCR analysis established that AHLs induce the transcription of pelA and pelD genes while epifluorescence microscopy studies revealed that PEL exopolysaccharide was required for the development of AHL-induced biofilms. Altogether these results reveal for the first time that AHLs positively regulate pel genes and participate in the molecular network for PEL exopolysaccharide biosynthesis by A. thiooxidans.

Targeted and untargeted quantification of quorum sensing signalling molecules in bacterial cultures and biological samples via HPLC-TQ MS techniques

Quorum sensing (QS) is the ability of some bacteria to detect and to respond to population density through signalling molecules. QS molecules are involved in motility and cell aggregation mechanisms in diseases such as sepsis. Few biomarkers are currently available to diagnose sepsis, especially in high-risk conditions. The aim of this study was the development of new analytical methods based on liquid chromatography-mass spectrometry for the detection and quantification of QS signalling molecules, including N-acyl homoserine lactones (AHL) and hydroxyquinolones (HQ), in biofluids. Biological samples used in the study were Pseudomonas aeruginosa bacterial cultures and plasma from patients with sepsis. We developed two MS analytical methods, based on neutral loss (NL) and product ion (PI) experiments, to identify and characterize unknown AHL and HQ molecules. We then established a multiple-reaction-monitoring (MRM) method to quantify specific QS compounds. We validated the HPLC-MS-based approaches (MRM-NL-PI), and data were in accord with the validation guidelines. With the NL and PI MS-based methods, we identified and characterized 3 and 13 unknown AHL and HQ compounds, respectively, in biological samples. One of the newly found AHL molecules was C12-AHL, first quantified in Pseudomonas aeruginosa bacterial cultures. The MRM quantitation of analytes in plasma from patients with sepsis confirmed the analytical ability of MRM for the quantification of virulence factors during sepsis.

Integrated into Environmental Biofilm Chromobacterium vaccinii Survives Winter with Support of Bacterial Community

Chromobacterium species are common in tropical and subtropical zones in environmental samples according to numerous studies. Here, we describe an environmental case of resident Chromobacterium vaccinii in biofilms associated with Carex spp. roots in Moscow region, Russia (warm-summer humid continental climate zone). We performed broad characterization of individual properties as well as surrounding context for better understanding of the premise of C. vaccinii survival during the winter season. Genome properties of isolated strains propose some insights into adaptation to habit and biofilm mode of life, including social cheaters carrying ΔluxR mutation. Isolated C. vaccinii differs from previously described strains in some biochemical properties and some basic characteristics like fatty acid composition as well as unique genome features. Despite potential to modulate membrane fluidity and presence of several genes responsible for cold shock response, isolated C. vaccinii did not survive during exposure to 4 °C, while in the complex biofilm sample, it was safely preserved for at least half a year in vitro at 4 °C. The surrounding bacterial community within the same biofilm with C. vaccinii represented a series of psychrophilic bacterial species, which may share resistance to low temperatures with other species within biofilm and provide C. vaccinii an opportunity to survive during the cold winter season.

Impact of N-Acyl-Homoserine Lactones, Quorum Sensing Molecules, on Gut Immunity

Among numerous molecules found in the gut ecosystem, quorum sensing (QS) molecules represent an overlooked part that warrants highlighting. QS relies on the release of small molecules (auto-inducers) by bacteria that accumulate in the environment depending on bacterial cell density. These molecules not only are sensed by the microbial community but also interact with host cells and contribute to gut homeostasis. It therefore appears entirely appropriate to highlight the role of these molecules on the immune system in dysbiosis-associated inflammatory conditions where the bacterial populations are imbalanced. Here, we intent to focus on one of the most studied QS molecule family, namely, the type I auto-inducers represented by N-acyl-homoserine lactones (AHL). First described in pathogens such as Pseudomonas aeruginosa, these molecules have also been found in commensals and have been recently described within the complex microbial communities of the mammalian intestinal tract. In this mini-review, we will expound on this emergent field of research. We will first recall evidence on AHL structure, synthesis, receptors, and functions regarding interbacterial communication. Then, we will discuss their interactions with the host and particularly with agents of the innate and adaptive gut mucosa immunity. This will reveal how this new set of molecules, driven by microbial imbalance, can interact with inflammation pathways and could be a potential target in inflammatory bowel disease (IBD). The discovery of the general impact of these compounds on the detection of the bacterial quorum and on the dynamic and immune responses of eukaryotic cells opens up a new field of pathophysiology.