The opportunistic pathogen Pseudomonas aeruginosa uses three interwoven quorum-sensing (QS) circuits—Las, Rhl, and Pqs—to regulate the global expression of myriad virulence-associated genes. Interception of these signaling networks with small molecules represents an emerging strategy for the development of anti-infective agents against this bacterium. In the current study, we applied a chemical approach to investigate how the Las-Rhl-Pqs QS hierarchy coordinates key virulence phenotypes in wild-type P. aeruginosa. We screened a focused library of synthetic, non-native N-acyl l-homoserine lactones and identified compounds that can drastically alter production of two important virulence factors: pyocyanin and rhamnolipid. We demonstrate that these molecules act by targeting RhlR in P. aeruginosa, a QS receptor that has seen far less scrutiny to date relative to other circuitry. Unexpectedly, modulation of RhlR activity by a single compound induces inverse regulation of pyocyanin and rhamnolipid, a result that was not predicted using genetic approaches to interrogate QS in P. aeruginosa. Further, we show that certain RhlR agonists strongly repress Pqs signaling, revealing disruption of Rhl-Pqs cross-regulation as a novel mechanism for QS inhibition. These compounds significantly expand the known repertoire of chemical probes available to study RhlR in P. aeruginosa. Moreover, our results suggest that designing chemical agents to disrupt Rhl-Pqs crosstalk could be an effective antivirulence strategy to fight this common pathogen.

Many bacteria regulate gene expression through a cell-cell signaling process called quorum sensing (QS). In proteobacteria, QS is largely mediated by signaling molecules known as N-acylated L-homoserine lactones (AHLs) and their associated intracellular LuxR-type receptors. The design of non-native small molecules capable of inhibiting LuxR-type receptors, and thereby QS, in proteobacteria is an active area of research, and numerous lead compounds are AHL derivatives that mimic native AHL signals. Much of this past work has focused on the pathogen Pseudomonas aeruginosa, which controls an arsenal of virulence factors and biofilm formation through QS. The MexAB-OprM drug efflux pump has been shown to play a role in the secretion of the major AHL signal in P. aeruginosa, N-(3-oxododecanoyl) L-homoserine lactone. In the current study, we show that a variety of non-native AHLs and related derivatives capable of inhibiting LuxR-type receptors in P. aeruginosa display significantly higher potency in a P. aeruginosa Δ(mexAB-oprM) mutant, suggesting that MexAB-OprM also recognizes these compounds as substrates. We also demonstrate that the potency of 5,6-dimethyl-2-aminobenzimidazole, recently shown to be a QS and biofilm inhibitor in P. aeruginosa, is not affected by the presence or absence of the MexAB-OprM pump. These results have implications for the use of non-native AHLs and related derivatives as QS modulators in P. aeruginosa and other bacteria, and provide a potential design strategy for the development of new QS modulators that are resistant to active efflux.

Pseudomonas aeruginosa uses N-acylated L-homoserine lactone signals and a triumvirate of LuxR-type receptor proteins – LasR, RhlR, and QscR – for quorum sensing (QS). Each of these receptors can contribute to QS activation or repression, and thereby, the control of myriad virulence phenotypes in this pathogen. LasR has traditionally been considered at the top of the QS receptor hierarchy in P. aeruginosa; however, recent reports suggest that RhlR plays a more prominent role in infection than originally predicted, in some circumstances superseding LasR. Herein, we report the characterization of a set of synthetic, small molecule agonists and antagonists of RhlR. Using E. coli reporter strains, we demonstrate that many of these compounds can selectively activate or inhibit RhlR instead of LasR and QscR. Moreover, several molecules maintain their activities in P. aeruginosa at concentrations analogous to native RhlR-signal levels. These compounds represent useful chemical probes to study the role of RhlR in the complex QS circuitry of P. aeruginosa, its direct (and indirect) effects on virulence, and its overall merit as a target for anti-infective therapy.

In an effort to identify novel antibacterial chemotypes, we performed a whole‐cell screen for inhibitors of Staphylococcus aureus growth and pursued those compounds with previously uncharacterized antibacterial activity. This process resulted in the identification of a benzothiazolium salt, ABTZ‐1, that displayed potent antibacterial activity against Gram‐positive pathogens. Several clinically desirable qualities were demonstrated for ABTZ‐1 including potent activity against multidrug‐resistant clinical isolates of methicillin‐resistant S. aureus (MRSA) and vancomycin‐resistant enterococci (VRE), retention of this activity in human serum, and low hemolytic activity. The antibacterial activity of ABTZ‐1 was attributed to its inhibition of bacterial translation, as this compound prevented the incorporation of [35S]methionine into S. aureus proteins, and ABTZ‐1‐resistant strains were cross‐resistant to known inhibitors of bacterial translation. ABTZ‐1 represents a promising new class of antibacterial agents.

The inside cover picture shows the inhibition of ribosomal protein synthesis in Staphylococcus aureus by the benzothiazolium salt, ABTZ-1. This antibacterial, which inhibits the growth of multidrug-resistant Gram-positive bacteria, prevents the incorporation of radiolabeled methionine into S. aureus protein.Strains resistant to ABTZ-1 also display cross-resistance to fusidic acid and streptogramins. For more information on the discovery and characterization of ABTZ-1 by P. J. Hergenrother et al., see p. 574 ff.

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The pipelines and vast infrastructure required for the production and transport of oil and gas are largely constructed of carbon steel. This material is highly susceptible to damage and failure as a result of direct or indirect Microbially Influenced Corrosion (MIC). One approach to reduce and/or remediate MIC is the application of microbicides to affected assets such as storage tanks, pipelines, heat exchangers and pumps. The objective of this paper is to generate efficacy data for biocidal formulations against corrosion-associated biofilms grown under anoxic and flowing conditions, which is difficult to obtain due to the distinct nature of biofilms and growth conditions. This data can be more representative when mimicking petroleum and water transporting pipelines. Thus, to facilitate the testing and comparison of products that successfully attenuate MIC under field-relevant conditions, a method that allows consistent corrosion measurements to be made, for corroding biofilms grown on steel surfaces under flowing and anoxic conditions, has been developed. This method utilizes microorganisms that were enriched on carbon steel from a North Sea sediment sample that, when grown in recirculating flow circuits, can generate corrosion rates of up to 65 mpy after four weeks of growth. The samples taken for the enrichment of corroding cultures came from a low tide anoxic sediment sample (black sand). In this presentation and paper, this method and its application to the benchmarking of field-relevant biocidal chemistries in MIC control experiments are described. Attention will be given to the reproducibility of the method as well. Distinct differences can be observed in biocidal performance against biofilms developed on metal surfaces.

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