Traditional treatment of infectious diseases is based on compounds that kill or inhibit growth of bacteria. A major concern with this approach is the frequent development of resistance to antibiotics. The discovery of communication systems (quorum sensing systems) regulating bacterial virulence has afforded a novel opportunity to control infectious bacteria without interfering with growth. Compounds that can override communication signals have been found in the marine environment. Using Pseudomonas aeruginosa PAO1 as an example of an opportunistic human pathogen, we show that a synthetic derivate of natural furanone compounds can act as a potent antagonist of bacterial quorum sensing. We employed GeneChip^® microarray technology to identify furanone target genes and to map the quorum sensing regulon. The transcriptome analysis showed that the furanone drug specifically targeted quorum sensing systems and inhibited virulence factor expression. Application of the drug to P.aeruginosa biofilms increased bacterial susceptibility to tobramycin and SDS. In a mouse pulmonary infection model, the drug inhibited quorum sensing of the infecting bacteria and promoted their clearance by the mouse immune response.

In cell‐free Pseudomonas aeruginosa culture supernatants, we identified two compounds capable of activating an N‐acylhomoserine lactone (AHL) biosensor. Mass spectrometry and NMR spectroscopy revealed that these compounds were not AHLs but the diketopiperazines (DKPs), cyclo(ΔAla‐l‐Val) and cyclo(l‐Pro‐l‐Tyr) respectively. These compounds were also found in cell‐free supernatants from Proteus mirabilis, Citrobacter freundii and Enterobacter agglomerans [cyclo(ΔAla‐l‐Val) only]. Although both DKPs were absent from Pseudomonas fluorescens and Pseudomonas alcaligenes, we isolated, from both pseudomonads, a third DKP, which was chemically characterized as cyclo(l‐Phe‐l‐Pro). Dose–response curves using a LuxR‐based AHL biosensor indicated that cyclo(ΔAla‐l‐Val), cyclo(l‐Pro‐l‐Tyr) and cyclo(l‐Phe‐l‐Pro) activate the biosensor in a concentration‐dependent manner, albeit at much higher concentrations than the natural activator N‐(3‐oxohexanoyl)‐l‐homoserine lactone (3‐oxo‐C6‐HSL). Competition studies showed that cyclo(ΔAla‐l‐Val), cyclo(l‐Pro‐l‐Tyr) and cyclo(l‐Phe‐l‐Pro) antagonize the 3‐oxo‐C6‐HSL‐mediated induction of bioluminescence, suggesting that these DKPs may compete for the same LuxR‐binding site. Similarly, DKPs were found to be capable of activating or antagonizing other LuxR‐based quorum‐sensing systems, such as the N‐butanoylhomoserine lactone‐dependent swarming motility of Serratia liquefaciens. Although the physiological role of these DKPs has yet to be established, their activity suggests the existence of cross talk among bacterial signalling systems.

Summary: Acylated homoserine lactone (AHL)-mediated gene expression controls phenotypes involved in colonization, often specifically of higher organisms, in both marine and terrestrial environments. The marine red alga Delisea pulchra produces halogenated furanones which resemble AHLs structurally and show inhibitory activity at ecologically realistic concentrations in AHL bioassays. Evidence is presented that halogenated furanones displace tritiated OHHL [N-3- (oxohexanoy1)-L-homoserine lactone] from Escherichia coli cells overproducing LuxR with potencies corresponding to their respective inhibitory activities in an AHL-regulated bioluminescence assay, indicating that this is the mechanism by which furanones inhibit AHL-dependent phenotypes. Alternative mechanisms for this phenomenon are also addressed. General metabolic disruption was assessed with two-dimensional PAGE, revealing limited non- AHL-related effects. A direct chemical interaction between the algal compounds and AHLs, as monitored by 1H NMR spectroscopy, was shown not to occur in vitro. These results support the contention that furanones, at the concentrations produced by the alga, can control bacterial colonization of surfaces by specifically interfering with AHL-mediated gene expression at the level of the LuxR protein.

Antimicrobial resistance is a multifaceted crisis, imposing a serious threat to global health. The traditional antibiotic pipeline has been exhausted, prompting research into alternate antimicrobial strategies. Inspired by nature, antimicrobial peptides are rapidly gaining attention for their clinical potential as they present distinct advantages over traditional antibiotics. Antimicrobial peptides are found in all forms of life and demonstrate a pivotal role in the innate immune system. Many antimicrobial peptides are evolutionarily conserved, with limited propensity for resistance. Additionally, chemical modifications to the peptide backbone can be used to improve biological activity and stability and reduce toxicity. This review details the therapeutic potential of peptide-based antimicrobials, as well as the challenges needed to overcome in order for clinical translation. We explore the proposed mechanisms of activity, design of synthetic biomimics, and how this novel class of antimicrobial compound may address the need for effective antibiotics. Finally, we discuss commercially available peptide-based antimicrobials and antimicrobial peptides in clinical trials.

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