Quorum sensing was first discovered in the bioluminescent marine bacterium Vibrio fischeri, which glows only when its population reaches a critical density. The mechanism relies on small signaling molecules called autoinducers, which bacteria release into their environment. When the concentration of these molecules crosses a threshold, a signal cascade triggers coordinated gene expression across the entire population. In Gram-negative bacteria, the dominant autoinducers are N-acyl-homoserine lactones, or AHLs, produced by LuxI-type synthases and detected by LuxR-type receptor proteins. Gram-positive bacteria instead use modified oligopeptides that interact with membrane-bound sensor kinases, while autoinducer-2, built from the precursor 4,5-dihydroxy-2,3-pentanedione, serves as a potential universal signal shared across both bacterial groups.

The stakes of this communication system are enormous. In opportunistic pathogens such as Pseudomonas aeruginosa, more than six percent of genes linked to pathogenesis are controlled by quorum sensing. During the early stages of infection, virulence genes remain largely silent, but once population density reaches a threshold, the system upregulates the production of toxins, lytic enzymes, adhesion molecules, and siderophores, driving disease progression. Biofilm formation, one of the most clinically troublesome bacterial behaviors, is also quorum-regulated, which explains why biofilm-based infections are among the most persistent and difficult to treat in modern medicine.

Marine eukaryotes have evolved sophisticated chemical defenses against bacterial neighbors, and these defenses are proving to be a treasure trove of quorum sensing inhibitors. The red macroalga Delisea pulchra produced the first naturally discovered quorum sensing inhibitor: halogenated furanones, compounds that structurally mimic AHLs except for a furan ring in place of the homoserine lactone ring. These furanones bind to LuxR-type proteins, destabilizing them and accelerating their turnover, thereby preventing the activation of quorum-regulated genes. The same alga has become one of the most extensively studied sources of anti-quorum sensing compounds, and its surface-associated furanones also explain its powerful antifouling properties.

Other algae contribute their own chemical arsenals. The red macroalga Ahnfeltiopsis flabelliformis yielded three AHL antagonists identified through bioactivity-guided fractionation: floridoside, betonicine, and isethionic acid. Notably, while commercial isethionic acid alone showed no activity, the combination of floridoside and isethionic acid effectively inhibited quorum sensing, suggesting synergistic interactions between algal metabolites. The marine macroalga Asparagopsis taxiformis produced 2-dodecanoyloxyethanesulfonate, which blocked quorum sensing in both Chromobacterium violaceum CV026 and Serratia liquefaciens MG44. Even microalgae participate in this chemical interference: Chlorella saccharophila extracts suppressed violacein production and bioluminescence without affecting cell density, while Chlamydomonas reinhardtii secretes compounds that mimic bacterial signals and scramble their communication networks.

Marine fungi represent an equally promising but less explored reservoir. Kojic acid, isolated from the fungus Alternaria sp. associated with the green alga Ulva pertusa, suppressed quorum-dependent luminescence in reporter strains at concentrations above 36 micromolar. Equisetin, derived from the marine fungus Fusarium sp. Z10, reduced biofilm formation, swarming motility, pyocyanin biosynthesis, and elastase production in Pseudomonas aeruginosa, implicating inhibition of the las, rhl, and Pseudomonas quinolone signal systems. Asteltoxin from Penicillium sp. QF046 outperformed the synthetic furanone positive control in suppressing violacein production. Meleagrin, obtained from Penicillium chrysogenum isolated from coastal slime in Korea, inhibited quorum sensing in C. violaceum CV017 with a minimum inhibitory concentration of 138.42 millimolar, while also acting as a novel bacterial enoyl-acyl carrier protein reductase inhibitor.

The diversity of fungal compounds continues to impress. From Penicillium sp. SCS-KFD08, recovered from the marine mammal Sipunculus nudus in Haikou Bay, researchers isolated six quorum sensing inhibitors including aculene C-E, penicitor B, and aspergillumarin A and B, which reduced violacein production by 46 and 49 percent at sub-inhibitory concentrations. Endophytic fungi from coral reefs, including species of Sarocladium, Fusarium, Epicoccum, and Khuskia, showed strong activity at doses between 50 and 500 micrograms per milliliter. Fusaric acid and linoleic acid, isolated from these strains, disrupt quorum sensing systems, and microwave-assisted synthesis of 39 fusaric acid analogs revealed that the C-2 ester group is essential for inhibition because it mimics the intermolecular interactions required by the lactone moiety of natural signal molecules.

Perhaps most clinically relevant is the activity of these compounds against the ESKAPE pathogens, the group of highly virulent bacteria that includes Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species. Citrinin from Penicillium sp. JH1, found in offshore waters near Qingdao, China, suppressed violacein synthesis and reduced the production of elastase, rhamnolipid, and pyocyanin in P. aeruginosa PAO1, while also downregulating nine quorum sensing genes. Tyrosol from Penicillium chrysogenum DXY-1 reduced pyocyanin synthesis by 63.3 percent and elastase activity by 57.8 percent. Extracts from Pestalotiopsis sydowiana PPR decreased biofilm formation by over 90 percent, and in silico docking suggested its metabolites cyclo-leucine-proline and 4-hydroxyphenylacetamide interact with the LasR and RhlR receptor proteins in ways resembling native signaling molecules.

Against Gram-positive ESKAPE members, the results are equally striking. An Aspergillus fumigatus extract from Moringa oleifera inhibited S. aureus and K. pneumoniae biofilm development by 69.2 and 57.66 percent respectively. Emodin from the mangrove-derived fungus Eurotium chevalieri reduced staphylococcal biofilm formation by 80 percent, while vulculic acid from Chaetosphaeronema achilleae achieved 96.82 percent biofilm inhibition at 256 micrograms per milliliter. The cyclic dipeptide cyclo-tyrosine-proline from Penicillium chrysogenum DXY-1 reduced quorum-mediated virulence factors in P. aeruginosa at sub-inhibitory concentrations, demonstrating that these compounds can attenuate pathogenicity without killing cells, precisely the property that minimizes resistance development.

Beyond medicine, quorum quenching holds promise for biotechnology and environmental engineering. Biofouling, the unwanted accumulation of organisms on submerged surfaces, costs naval transport, aquaculture, and water treatment industries billions annually, and traditional antifouling agents like tributyltin have been banned for their devastating ecological effects. Vanillin combined with cellulose acetate membranes prevents biofilm in reverse osmosis systems, and AHL-degrading bacteria improve wastewater treatment performance. Quorum sensing inhibitors can also enhance conventional antibiotics: cinnamic acid derivatives increased biofilm sensitivity to tobramycin. The review’s authors caution, however, that the field lacks standardized methods for evaluating inhibitor efficacy, and that future studies must test compounds under conditions that closely mimic in vivo infections rather than relying solely on laboratory-adapted strains. As research moves from the phycosphere to the clinic, the ocean’s chemical conversations may well provide the next generation of anti-virulence therapies.

Subject of Research: Marine algae- and fungi-derived bioactive compounds that inhibit bacterial quorum sensing

Article Title: Bioactive compounds from marine algae and fungi in down-regulating quorum sensing

Article References: Bioactive compounds from marine algae and fungi in down-regulating quorum sensing. (n.d.). https://doi.org/10.1186/s44315-024-00018-2

Image Credits: AI Generated

DOI: 10.1186/s44315-024-00018-2

Keywords: quorum sensing, quorum quenching, marine algae, marine fungi, bioactive compounds, antibiotic resistance, biofilm, ESKAPE pathogens, halogenated furanones, AHL, anti-virulence, natural products