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Quorum Sensing (QS) is a well-studied intercellular communication mechanism in bacteria, regulating collective behaviors such as biofilm formation, virulence, and antibiotic resistance. However, cell–cell signaling in haloarchaea remains largely unexplored. The coexistence of bacteria and archaea in various environments, coupled with the known cell–cell signaling mechanisms in both prokaryotic and eukaryotic microorganisms and the presence of cell–cell signaling mechanisms in both prokaryotic and eukaryotic microorganisms, suggests a possibility for haloarchaea to possess analogous cell–cell signaling or QS systems. Recently, N-acylhomoserine lactone (AHL)-like compounds were identified in haloarchaea; yet, their precise role—for example, persister cell formation—remains ambiguous. This study investigated the capacity of crude supernatant extract from the haloarchaeon Halorubrum saccharovorum CSM52 to stimulate bacterial AHL-dependent QS phenotypes using bioreporter strains. Our findings reveal that these crude extracts induced several AHL-dependent bioreporters and modulated pyocyanin and pyoverdine production in Pseudomonas aeruginosa. Importantly, our study suggests cross-domain communication between archaea and bacterial pathogens, providing evidence for archaea potentially influencing bacterial virulence. Using Thin Layer Chromatography overlay assays, lactonolysis, and colorimetric quantification, the bioactive compound was inferred to be a chemically modified AHL-like compound or a diketopiperazine-like molecule, potentially involved in biofilm formation in H. saccharovorum CSM52. This study offers new insights into putative QS mechanisms in haloarchaea and their potential role in interspecies communication and coordination, thereby enriching our understanding of microbial interactions in diverse environments.

Large regions of Earth's surface are underlain by salt deposits that evaporated from ancient oceans and are populated by extreme halophilic microbes. Some of these halophiles may have been preserved over geological timescales within hypersaline fluid inclusions, but ingresses of water and/or anthropogenic activities can lead to the formation of alternative habitats, including NaCl stalactites or speleothems. While the microbiology of ancient evaporites has been well-studied, the ecology of these recently formed structures is less-well understood. Here, the microbiology of a NaCl stalactite ('salticle') in a Triassic halite mine is characterised. The specific aims were to: determine the presence of fluid inclusions; determine the microbial structure of the salticle compared with a nearby brine-pool and surficial soil; and characterise the ecophysiological capabilities of this unique ecosystem. The salticle contained fluid inclusions, and their microbiome was composed of Euryarchaetota, Proteobacteria, and Actinobacteria, with Haloarchaea in greater abundance than brine-pool or soil microbiomes. The salticle metagenome exhibited a greater abundance of genes involved in osmoregulation, anaerobic respiration, UV resistance, oxidative stress, and stress-protein synthesis relative to the soil microbiome. We discuss the potential astrobiological implications of salticles as enclosed salt-saturated habitats that are protected from ionising radiation and have a stable water-activity. This article is protected by copyright. All rights reserved.

Kilroot salt mine, a Triassic halite deposit located in County Antrim, Northern Ireland, is the only permanent hypersaline environment on the island of Ireland. In this study, the microbiome of this unstudied environment was profiled for the first time using conventional and enhanced culturing techniques, and culture independent metagenomic approaches. Using both conventional isolation plates and iChip devices, 89 halophilic archaeal isolates from six known genera, and 55 halophilic or halotolerant bacterial isolates from 18 genera were obtained, based on 16S rRNA gene sequencing. The archaeal isolates were similar to those previously isolated from other ancient halite deposits, and as expected, numerous genera were identified in the metagenome which were not represented among the culturable isolates. Preliminary screening of a selection of isolates from this environment identified antimicrobial activities against a panel of clinically important bacterial pathogens from 15 of the bacterial isolates and one of the archaea. This, alongside previous studies reporting the discovery of novel biocatalysts from the Kilroot mine microbiome, suggests that this environment may be a new, untapped source of of chemical diversity with high biodiscovery potential.

Biofilms present a major problem to industry and healthcare worldwide. Composed of a population of surface-attached microbial cells surrounded by a protective extracellular polysaccharide matrix, they are responsible for increased tolerance to antibiotics, treatment failure and a resulting rise in antimicrobial resistance. Here we demonstrate that self-assembled peptide nanostructures composed of a diphenylalanine motif provide sufficient antibacterial activity to eradicate mature biofilm forms of bacteria widely implicated in hospital infections. Modification of terminal functional groups to amino (–NH 2 ), carboxylic acid (–COOH) or both modalities, and switch to d-isomers, resulted in changes in antibacterial selectivity and mammalian cell toxicity profiles. Of the three peptide nanotubes structures studied (NH 2 -FF-COOH, NH 2 -ff-COOH and NH 2 -FF-NH 2 ), NH 2 -FF-COOH demonstrated the most potent activity against both planktonic (liquid, free-floating) and biofilm forms of bacteria, possessing minimal mammalian cell toxicity. NH 2 -FF-COOH resulted in greater than 3 Log 10 CFU/mL viable biofilm reduction (>99.9%) at 5 mg/mL and total biofilm kill at 10 mg/mL against Staphylococcus aureus after 24 h exposure. Scanning electron microscopy proved that antibiofilm activity was primarily due to the formation of ion channels and/or surfactant-like action, with NH 2 -FF-COOH and NH 2 -ff-COOH capable of degrading the biofilm matrix and disrupting cell membranes, leading to cell death in Gram-positive bacterial isolates. Peptide-based nanotubes are an exciting platform for drug delivery and engineering applications. This is the first report of using peptide nanotubes to eradicate bacterial biofilms and provides evidence of a new platform that may alleviate their negative impact throughout society. Statement of Significance We outline, for the first time, the antibiofilm activity of diphenylalanine (FF) peptide nanotubes. Biofilm bacteria exhibit high tolerance to antimicrobials 10–10,000 times that of free-flowing planktonic forms. Biofilm infections are difficult to treat using conventional antimicrobial agents, leading to a rise in antimicrobial resistance. We discovered nanotubes composed of NH 2 -FF-COOH demonstrated potent activity against staphylococcal biofilms implicated in hospital infections, resulting in complete kill at concentrations of 10 mg/mL. Carboxylic acid terminated FF nanotubes were able to destroy the exopolysaccharide architecture of staphylococcal biofilms expressing minimal toxicity, highlighting their potential for use in patients. Amidated (NH 2 -FF-NH 2 ) forms demonstrated reduced antibiofilm efficacy and significant toxicity. These results contribute significantly to the development of innovative antibacterial technologies and peptide nanomaterials.

The marine brown alga Halidrys siliquosa is known to produce compounds with antifouling activity against several marine bacteria. The aim of this study was to evaluate the antimicrobial and antibiofilm activity of organic extracts obtained from the marine brown alga H. siliquosa against a focused panel of clinically relevant human pathogens commonly associated with biofilm-related infections. The partially fractionated methanolic extract obtained from H. siliquosa collected along the shores of Co. Donegal, Ireland, displayed antimicrobial activity against bacteria of the genus Staphylococcus, Streptococcus, Enterococcus, Pseudomonas, Stenotrophomonas, and Chromobacterium with MIC and MBC values ranging from 0.0391 to 5 mg/mL. Biofilms of S. aureus MRSA were found to be susceptible to the algal methanolic extract with MBEC values ranging from 1.25 mg/mL to 5 mg/mL respectively. Confocal laser scanning microscopy using LIVE/DEAD staining confirmed the antimicrobial nature of the antibiofilm activity observed using the MBEC assay. A bioassay-guided fractionation method was developed yielding 10 active fractions from which to perform purification and structural elucidation of clinically-relevant antibiofilm compounds.