Use of Whole-Cell Bioassays for Screening Quorum Signaling, Quorum Interference, and Biofilm Dispersion

RefAHL: a curated quorum sensing reference linking diverse LuxI-type signal synthases with their acyl-homoserine lactone products

Some bacteria use acyl-homoserine lactone (AHL) signals in quorum sensing, a type of cell-cell communication. Here, we present "RefAHL," an updated, curated collection of LuxI-type AHL synthases with their AHL products and associated metadata. RefAHL is publicly available as a community resource to help catalog LuxI-type diversity encoded in (meta) genomic data.

A method establishment and application for biofilm quorum quenching activity assay

The existence of quorum sensing (QS) and quorum quenching (QQ) plays important roles in biofilm formation. However, direct detection of QS ability is difficult due to the low concentrations of signal molecules inside the biofilm. Therefore, QQ activity is typically used to indicate the attribution of QS/QQ to the biofilm. Nevertheless, current detection methods of QQ activity based on biosensors present undesirable operability and accuracy. In this study, the 96-well plate assay based on a specific biosensor, Agrobacterium tumefaciens A136, and a colorimetric substance, X-gal was established. The reliable fitting results were obtained by standardizing the composition of the A136 X-gal assay solution and optimizing the operating conditions. This method improved the accuracy of QQ activity detection and reduced time and cost consumption. Finally, the 96-well plate assay was successfully applied to detect the QQ activities of biofilm samples and explore possible environmental influencing factors. In general, this study provided a new strategy for understanding the QQ effect in biofilm systems.

Methylotroph Quorum Sensing Signal Identification by Inverse Stable Isotopic Labeling

Natural products are an essential source of bioactive compounds. Isotopic labeling is an effective way to identify natural products that incorporate a specific precursor; however, this approach is limited by the availability of isotopically enriched precursors. We used an inverse stable isotopic labeling approach to identify natural products by growing bacteria on a ¹³C-carbon source and then identifying ¹²C-precursor incorporation by mass spectrometry. We applied this approach to methylotrophs, ecologically important bacteria predicted to have significant yet underexplored biosynthetic potential. We demonstrate that this method identifies N-acyl homoserine lactone quorum sensing signals produced by diverse methylotrophs grown on three different one-carbon compounds. We then apply this approach to simultaneously detect five previously unidentified signals produced by a methylotroph and link these compounds to their synthases. We envision that this method can be used to identify other natural product classes synthesized by methylotrophs and other organisms that grow on relatively inexpensive ¹³C-carbon sources.

Enterococcus faecalis inhibits Klebsiella pneumoniae growth in polymicrobial biofilms in a glucose-enriched medium

Catheter-related urinary tract infections are one of the most common biofilm-associated diseases. Within biofilms, bacteria cooperate, compete, or have neutral interactions. This study aimed to investigate the interactions in polymicrobial biofilms of Klebsiella pneumoniae and Enterococcus faecalis, two of the most common uropathogens. Although K. pneumoniae was the most adherent strain, it could not maintain dominance in the polymicrobial biofilm due to the lactic acid produced by E. faecalis in a glucose-enriched medium. This result was supported by the use of E. faecalis V583 ldh-1/ldh-2 double mutant (non-producer of lactic acid), which did not inhibit the growth of K. pneumoniae. Lyophilized cell-free supernatants obtained from E. faecalis biofilms also showed antimicrobial/anti-biofilm activity against K. pneumoniae. Conversely, there were no significant differences in planktonic polymicrobial cultures. In summary, E. faecalis modifies the pH by lactic acid production in polymicrobial biofilms, which impairs the growth of K. pneumoniae.

Terrein is an inhibitor of quorum sensing and c-di-GMP in Pseudomonas aeruginosa: a connection between quorum sensing and c-di-GMP

To address the drug-resistance of bacterial pathogens without imposing a selective survival pressure, virulence and biofilms are highly attractive targets. Here, we show that terrein, which was isolated from Aspergillus terreus, reduced virulence factors (elastase, pyocyanin, and rhamnolipid) and biofilm formation via antagonizing quorum sensing (QS) receptors without affecting Pseudomonas aeruginosa cell growth. Additionally, the effects of terrein on the production of QS signaling molecules and expression of QS-related genes were verified. Interestingly, terrein also reduced intracellular 3,5-cyclic diguanylic acid (c-di-GMP) levels by decreasing the activity of a diguanylate cyclase (DGC). Importantly, the inhibition of c-di-GMP levels by terrein was reversed by exogenous QS ligands, suggesting a regulation of c-di-GMP levels by QS; this regulation was confirmed using P. aeruginosa QS mutants. This is the first report to demonstrate a connection between QS signaling and c-di-GMP metabolism in P. aeruginosa, and terrein was identified as the first dual inhibitor of QS and c-di-GMP signaling.