From broad-spectrum biocides to quorum sensing disruptors and mussel repellents: antifouling profile of alkyl triphenylphosphonium salts

Antifouling activity and ecotoxicological profile of the cyanobacterial oxadiazine nocuolin A

Pursuing effective and biocompatible natural compounds to supplant current biocidal antifouling (AF) technologies remains crucial and challenging. Among natural products hosts, cyanobacteria are recognized as producers of bioactive secondary metabolites that are underexplored in terms of anti-biofilm and AF potential. Nocuolin A, a natural oxadiazine previously isolated and known to be produced by different cyanobacterial strains, has demonstrated bioactive potential, particularly against tumor cell lines. Considering this potential and its exquisite chemical structure, here nocuolin A was investigated as a potential natural AF agent through an integrative approach including AF bioactivity testing across distinct levels of biological organization, mode of action assessment, ecotoxicity evaluation, and ecological risk predictions. Nocuolin A was found to inhibit the settlement of mussel (Mytilus galloprovincialis) plantigrades (EC50 = 3.905 μM) while showing no toxicity to this biofouling species (LC50 > 100 μM). Additionally, while exhibiting no inhibitory activity against the growth of five marine biofilm-forming bacterial strains, it significantly suppressed the growth of the marine biofilm-forming diatom Navicula sp. (EC50 = 1.561 μM), and had a lethal effect on this diatom species (>3.1 μM). The AF targets of nocuolin A on mussel plantigrades revealed no correlation with acetylcholinesterase and tyrosinase metabolic processes; however, proteins involved in oxidative stress, muscle regulation, and energy production were highlighted. The results also provide insights into the ecological risk of nocuolin A, including its ecotoxicity against Artemia salina nauplii (LC50 = 2.480 μM), Amphibalanus amphitrite nauplii (LC50 = 0.0162 μM), and Danio rerio embryos (LC50 = 0.0584 μM). When matching these results with simulated environmental values, nocuolin A was deemed a considerable threat to the ecosystems. While this research highlights the AF activity of nocuolin A, it also emphasizes the potential adverse environmental impact when applied in preventive coatings.

Unleashing the antibacterial and antibiofilm potential of silica-based nanomaterials functionalized with an organotin(IV) compound

Bacterial diseases caused by superbugs are expected to be the main cause of death worldwide within a decade as a consequence of the resistance they are acquiring to the antibiotics currently in use, therefore, the field of new antibacterial treatments is currently being thoroughly studied. The present work focuses on the synthesis, functionalization, characterization and antibacterial behaviour of different systems based on three different silica-based nanostructured materials (MSN, mesoporous silica nanoparticles, SBA-15 Santa Barbara amorphous-15 and FSP fibrous slica nanoparticles) which serve as scaffolds for the support of different platforms to target and treat bacterial diseases and biofilm formation. Thus, (3-carboxypropyl)triphenylphosphonium bromide (PPh3+) and a cytotoxic organotin(IV) fragment (Sn) have been incorporated in the silica-based materials to study their potential activity in different antibacterial applications. After a complete characterization of the synthesized systems, which confirmed the incorporation of both the targeting and the therapeutic fragments within the nanostructured materials, the antibacterial study of the materials demonstrated bactericidal capacity against Escherichia coli and perturbation of the bacteria metabolism via oxidative stress through an enhanced ROS (reactive oxygen species) production. In addition, biofilm inhibition and eradication tests of bacterial strains were carried out, showing that the activity of the materials in both biofilm inhibition and eradication is dependent on the concentration of the material. Furthemore, the material MSN-AP(1:1)-PPh3+-Sn containing the targeting triphenylphosphonium and a "SnPh3" fragment is capable of inhibiting and eradicating up to 50% of the formation of biofilms, which is outstanding for metallodrug-functionalized silica-based systems compared with other materials based on metal nanoparticles supported on silica. Finally, a hemolysis study was carried out with the nanostructured systems proving to be non-toxic, making them adequate for their subsequent use in preclinical trials through in vivo models.

Chalcone derivatives as promising antifoulants: Molecular optimization, bioactivity evaluation and performance in coatings

Marine biofouling remains a huge concern for maritime industries and for environmental health. Although the current biocide-based antifouling coatings can prevent marine biofouling, their use has been associated with toxicity for the marine environment, being urgent to find sustainable alternatives. Previously, our research group has identified a prenylated chalcone (1) with promising antifouling activity against the settlement of larvae of the macrofouling species Mytilus galloprovincialis (EC50 = 16.48 µM and LC50 > 200 µM) and lower ecotoxicity when compared to Econea®, a commercial antifouling agent in use. Herein, a series of chalcone 1 analogues were designed and synthesized in order to obtain optimized antifouling compounds with improved potency while maintaining low ecotoxicity. Compounds 8, 15, 24, and 27 showed promising antifouling activity against the settlement of M. galloprovincialis larvae, being dihydrochalcone 27 the most potent. The effect of compound 24 was associated with the inhibition of acetylcholinesterase activity. Among the synthesized compounds, compound 24 also showed potent complementary activity against Navicula sp. (EC50 = 4.86 µM), similarly to the lead chalcone 1 (EC50 = 6.75 µM). Regarding the structure-activity relationship, the overall results demonstrate that the substitution of the chalcone of the lead compound 1 by a dihydrochalcone scaffold resulted in an optimized potency against the settlement of mussel larvae. Marine polyurethane (PU)-based coatings containing the best performed compound concerning anti-settlement activity (dihydrochalcone 27) were prepared, and mussel larvae adherence was reduced compared to control PU coatings.

Shifting from Ammonium to Phosphonium Salts: A Promising Strategy to Develop Next-Generation Weapons against Biofilms

Since they are difficult and sometimes impossible to treat, infections sustained by multidrug-resistant (MDR) pathogens, emerging especially in nosocomial environments, are an increasing global public health concern, translating into high mortality and healthcare costs. In addition to having acquired intrinsic abilities to resist available antibiotic treatments, MDR bacteria can transmit genetic material encoding for resistance to non-mutated bacteria, thus strongly decreasing the number of available effective antibiotics. Moreover, several pathogens develop resistance by forming biofilms (BFs), a safe and antibiotic-resistant home for microorganisms. BFs are made of well-organized bacterial communities, encased and protected in a self-produced extracellular polymeric matrix, which impedes antibiotics' ability to reach bacteria, thus causing them to lose efficacy. By adhering to living or abiotic surfaces in healthcare settings, especially in intensive care units where immunocompromised older patients with several comorbidities are hospitalized BFs cause the onset of difficult-to-eradicate infections. In this context, recent studies have demonstrated that quaternary ammonium compounds (QACs), acting as membrane disruptors and initially with a low tendency to develop resistance, have demonstrated anti-BF potentialities. However, a paucity of innovation in this space has driven the emergence of QAC resistance. More recently, quaternary phosphonium salts (QPSs), including tri-phenyl alkyl phosphonium derivatives, achievable by easy one-step reactions and well known as intermediates of the Wittig reaction, have shown promising anti-BF effects in vitro. Here, after an overview of pathogen resistance, BFs, and QACs, we have reviewed the QPSs developed and assayed to this end, so far. Finally, the synthetic strategies used to prepare QPSs have also been provided and discussed to spur the synthesis of novel compounds of this class. We think that the extension of the knowledge about these materials by this review could be a successful approach to finding effective weapons for treating chronic infections and device-associated diseases sustained by BF-producing MDR bacteria.

Amide/Amino-Based Functional Additives for Lubricants: Structure, Antimicrobial Activity and Wear Resistance

The lubricating properties of the lubricants were tested under boundary friction conditions; it was found that the surface-active additives had reduced the wear mark and thus the value of the Goz/40 parameter (limiting load of wear). The introduction of a surfactant containing amide compounds into the oils and greases was highly effective in slowing down the oxidation process. Lubricants containing mono-15 ([3-(N,N,N-dimethylbuthylamine)propyl]hexadecanamide chloride) and 15-4-15 (tetramethylene-bis [3-(N,N,N-dimethylamine)propyl]hexadecanamide) additives were characterised by higher oxidation stability compared to the unmodified lubricants. Both of the analysed substances showed bactericidal properties against Staphylococcus aureus and Salmonella enteritica. Tests of antibacterial activity in the lubricants with the addition of mono-15 and 15-4-15 confirmed that these lubricants can be considered bactericidal against Gram-positive and Gram-negative bacteria.

Role of inorganic foulants in the aging and deterioration of low-pressure membranes during the chemical cleaning process in surface water treatment: A review

Low-pressure membrane (LPM) filtration, including microfiltration (MF) and ultrafiltration (UF), is a promising technology for the treatment of surface water for drinking and other purposes. Various configurations and operational sequences have been developed to ensure the sustainable provision of clean water by overcoming fouling problems. In the literature, various periodic physical and/or chemical approaches to the cleaning of LPMs have been reported, but little data is available on the aging of MF/UF membranes that results from the interaction between the foulants and the cleaning agent. Periodic physical cleaning of the membrane is expected to return the membrane to its original performance capacity, but it only recovers to a certain level because the remaining foulants cause irreversible fouling. Chemical cleaning can then be employed to recover the membrane from this irreversible fouling but, in the process, it can cause irrecoverable damage to the membrane. In this review, the foulants responsible for irrecoverable damage to MF/UF membranes are summarized, and their interaction with cleaning agents and other foulants is described. The impact of these foulants on various membrane parameters, including filtration efficiency, flux decline, permeability, membrane characterization, and membrane integrity are also summarized and discussed in detail. In addition, mitigation options and future prospects are also discussed with regard to increasing the operational life span of a membrane in a cost-effective manner. Ultimately, this review suggests an advanced control system based on membrane-foulant interactions under the impact of various operational parameters to mitigate the integrity loss of membranes.

Bioactive Peptides from Barnacles and Their Potential for Antifouling Development

Barnacles, a prevalent fouler organism in intertidal zones, has long been a source of annoyance due to significant economic losses and ecological impacts. Numerous antifouling approaches have been explored, including extensive research on antifouling chemicals. However, the excessive utilization of small-molecule chemicals appears to give rise to novel environmental concerns. Therefore, it is imperative to develop new strategies. Barnacles exhibit appropriate responses to environmental challenges with complex physiological processes and unique sensory systems. Given the assumed crucial role of bioactive peptides, an increasing number of peptides with diverse activities are being discovered in barnacles. Fouling-related processes have been identified as potential targets for antifouling strategies. In this paper, we present a comprehensive review of peptides derived from barnacles, aiming to underscore their significant potential in the quest for innovative solutions in biofouling prevention and drug discovery.

Chemical diversity and antifouling activity of geniculate calcareous algae (Corallinales, Rhodophyta) from Brazil

Marine biofouling is a natural process by which many organisms colonize and grow in submerged structures, causing serious economic consequences for the maritime industry. Geniculate calcareous algae (GCA; Corallinales, Rhodophyta) produce bioactive secondary metabolites and are a promise for new antifouling compounds. Here, we investigated the antifouling activity of four GCA species-Amphiroa beauvoisii, Jania sagittata (formerly Cheilosporum sagittatum), Jania crassa, and Jania prolifera (formerly Amphiroa flabellata)-from the Brazilian coast against macro- and microorganisms. Simultaneously, metabolomic tools were applied to assess the chemical profiles of these seaweeds using gas chromatography coupled to mass spectrometry (GC-MS). Data analysis by principal component and molecular networking analyses used the global natural products social molecular networking platform (GNPS). Our results showed that all extracts were active against different strains of marine bacteria and that the J. sagittata (JsSI) extract showed the highest percentage of bacterial inhibition. The J. sagittata (JsSI) extract was the most active against the mussel Perna perna, showing 100% byssus inhibition. Regarding toxicity, only the J. crassa (JcP) extract showed a 20% mortality rate. The chemical profiles of the evaluated GCA extracts differed qualitatively and quantitatively. Yet, the steroid (3β)-cholest-5-en-3-ol was the major compound commonly identified in all extracts, with the exception of J. sagittata (JsSI). Moreover, we observed intra- and interspecific chemical variabilities among GCA extracts for the different populations, which could explain their antifouling activity variability. This study contributed new information about the chemical compounds produced by this group of seaweeds and showed its antifouling potential. These GCA species may be the subject of future studies to obtain new bioactive compounds with biotechnological potential in maritime areas.

Impact of flow regime on the performance of anti-biofouling coatings

Biofouling poses significant challenges for marine transportation due to increased skin drag, which results in increased fuel cost and associated [Formula: see text] emissions. Current antifouling methods involving polymer coating, biocides, and self-depleting layers harm marine ecosystems and contribute to marine pollution. Significant advancements have resulted in using bioinspired coatings to address this issue. However, prior investigations have predominantly focused on wettability and adhesion aspects, resulting in a limited understanding of the impact of flow regime on bioinspired structure patterns for antifouling. We conducted comprehensive experiments with two bioinspired coatings¹ under laminar and turbulent flow regimes and compared them with a smooth surface. The two coatings are composed of regular arrangements of micropillars measuring 85 μm in height and spaced at 180 μm (pattern A) and 50 μm high micropillars spaced at 220 μm (pattern B). Theoretical arguments indicate that wall-normal velocity fluctuations near the micropillars' top significantly contribute to reducing the onset of biofouling under turbulence compared to the smooth surface. Pattern A coating can effectively decrease biofouling by 90% for fouling sizes exceeding 80 microns when compared to a smooth surface subjected to a turbulent flow regime. The coatings exhibited comparable anti-biofouling properties under a laminar flow. Also, the smooth surface experienced substantially higher biofouling under laminar flow compared to turbulent conditions. This underscores how the effectiveness of anti-biofouling approaches is critically influenced by the flow regime.

Ship Hull-Fouling Diatoms on Korean Research Vessels Revealed by Morphological and Molecular Methods, and Their Environmental Implications

Ship biofouling is one of the main vectors for the introduction and global spread of non-indigenous organisms. Diatoms were the early colonizers of ship hulls; however, their community composition on ships is poorly understood. Herein, we investigated the diatom community on the hull samples collected from two Korean research vessels Isabu (IRV) and Onnuri (ORV) on September 2 and November 10, 2021, respectively. IRV showed low cell density (345 cells/cm2) compared to ORV (778 cells/cm2). We morphologically identified more than 15 species of diatoms from the two research vessels (RVs). The microalgae in both RVs were identified as Amphora, Cymbella, Caloneis, Halamphora, Navicula, Nitzschia, and Plagiogramma. Of them, the genus Halamphora was found to be predominant. However, both RVs had a varied dominant species with a significant difference in body size; Halamphora oceanica dominated at IRV, and Halamphora sp. at ORV, respectively. Molecular cloning showed similar results to morphological analysis, in which Halamphora species dominated in both RVs. The hull-attached species were distinct from species found in the water column. These results revealed diatoms communities that are associated with ship hull-fouling at an early stage of biofilm formation. Moreover, ships arriving from different regions could show some variation in species composition on their hull surfaces, with the potential for non-indigenous species introduction.

Spray-Drying Hydrophobic Cellulose Nanocrystal Coatings with Degradable Biocide Release for Marine Antifouling

With increasing awareness about the ecological environment, increased attention has been paid to the application of eco-friendly materials in the field of marine antifouling. In this work, a novel coating having good mechanical strength and static marine antifouling characteristics was fabricated using cellulose nanocrystals (CNCs) as the skeleton material, with in situ growth of SiO₂ as the basic superhydrophobic material and introducing hexadecyl trimethyl ammonium bromide (CTAB) and 4-bromo2-(4-chlorophenyl)-5-(trifluoromethyl)-1H-pyrrole-3-carbonitrile (Econea) into the coating. Due to the high strength and rod structure of CNCs, the coating maintained super-hydrophobicity after 50 cycles of abrasion tests. Moreover, the addition of CTAB during the synthesis of SiO₂ led to the hydrolysis and polycondensation of tetraethyl orthosilicate at the micellar interface. Econea was fully mixed with SiO₂ nanoparticles, thus slowing down the rate of release of Econea. Meanwhile, the adhesion between the coating and the substrate reached 1.9 MPa, which can meet the application requirements for marine environments. The bioassay using bacteria (Escherichia coli) and diatoms (Nitzschia closterium) showed that the rate of inhibition of the coating on bacteria and diatoms could reach 99 and 90%, respectively, after immersion in artificial seawater for 28 days. This research provides a facile and promising fabricating solution of an eco-friendly CNC-based coating having strong antifouling characteristics suitable for marine environments.

Effect of the structure of chitosan quaternary phosphonium salt and chitosan quaternary ammonium salt on the antibacterial and antibiofilm activity

N-(4-N', N', N'-trimethylphosphonium chloride) benzoyl chitosan (TMPCS), N-(4-N', N', N'-triphenylphosphonium chloride) benzoyl chitosan (TPPCS), and N-(4-N', N', N'-trimethylmethanaminium chloride) benzoyl chitosan (TMACS) were synthesized. The structures of the products were characterized by Fourier transform infrared spectroscopy, Nuclear magnetic resonance spectroscopy and ultraviolet-visible spectroscopy. Their antibacterial activities against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were investigated in vitro using the antibacterial rate, minimal inhibitory concentration (MIC) and minimum bactericidal concentration (MBC), the antibiofilm activity was investigated by crystal violet assay. The antibacterial assessment revealed that the chitosan quaternary phosphonium salts of similar structure had superior antibacterial activity than chitosan quaternary ammonium salt. The antibacterial rate of CS, TMPCS, TPPCS and TMACS against E. coli at 0.5 mg/mL was 10.4 %, 42.0 %, 58.5 % and 21.6 % respectively. At the same concentration, the antibacterial rate of TMPCS, TPPCS and TMACS against S.aureus was all up to 100 %. The biofilm inhibition rate of CS, TMPCS, TPPCS and TMACS at a half of MIC against E.coli was 28.4 %, 33.9 %, 56.6 % and 57.6 % respectively, and against S.aureus was 30.8 %, 53.8 %, 62.2 % and 58.5 % respectively. The biofilm removal rate of CS, TMPCS, TPPCS, TMACS against E.coli at 2.5 mg/mL was 20.6 %, 46.4 %, 48.9 % and 41.6 % respectively, and against S.aureus at 2.5 mg/mL was 41.5 %, 60.4 %, 69.9 % and 59.01 % respectively.

Exploring Efflux as a Mechanism of Reduced Susceptibility towards Biocides and Fluoroquinolones in Staphylococcus pseudintermedius

Staphylococcus pseudintermedius is the main bacterial cause of skin and soft tissue infections (SSTIs) in companion animals, particularly dogs. The emergence of methicillin-resistant S. pseudintermedius (MRSP) strains, frequently with multidrug resistance phenotypes is a public health concern. This study aimed to evaluate efflux, a resistance mechanism still poorly characterized in S. pseudintermedius, as a contributor to biocide and fluoroquinolone resistance. Susceptibility to the efflux pump substrates ethidium bromide (EtBr), tetraphenylphosphonium bromide (TPP) and ciprofloxacin (CIP) was evaluated by minimum inhibitory concentration (MIC) determination for 155 SSTIs-related S. pseudintermedius in companion animals. EtBr and TPP MIC distributions were analyzed to estimate cut-off (COWT) values. The effect of the efflux inhibitors (EIs) thioridazine and verapamil was assessed upon MICs and fluorometric EtBr accumulation assays, performed with/without glucose and/or EIs. This approach detected a non-wild type population towards TPP with increased efflux, showed to be strain-specific and glucose-dependent. Resistance to fluoroquinolones was mainly linked to target gene mutations, yet a contribution of efflux on CIP resistance levels could not be ruled out. In sum, this study highlights the relevance of efflux-mediated resistance in clinical S. pseudintermedius, particularly to biocides, and provides a methodological basis for further studies on the efflux activity on this important pathogen of companion animals.

Preparation and Properties of PED-TDI Polyurethane-Modified Silicone Coatings

To explore the influence mechanisms of polyurethane soft segments on modified silicone coatings, a series of modified coatings was prepared by introducing different contents of hydroxypropyl-terminated polydimethylsiloxane (PDMS2200) into the soft segment of polyurethane. ATR-FTIR, NMR, CLSM, AFM, contact angle measurement, the tensile test, bacterial adhesion, and the benthic diatom adhesion test were used to investigate the structure, morphology, roughness, degree of microphase separation, surface energy, tensile properties, and antifouling properties of the modified coatings. The results show that PDMS2200 could aggravate the microphase separation of the modified coatings, increase the surface-free energy, and reduce its elastic modulus; when the microphase separation exceeded a certain degree, increasing PDMS2200 would decrease the tensile properties. The PED-TDI polyurethane-modified silicone coating prepared with the formula of PU-Si17 had the best tensile properties and antifouling properties among all modified coatings.

Synthesis, biological evaluation and structure-activity relationship of 2-(2-hydroxyaryl)alkenylphosphonium salts with potency as anti-MRSA agents

Here we report the synthesis, in vitro antimicrobial activity, preliminary toxicity and mechanism study of a new series of 2-(2-hydroxyaryl)alkenylphosphonium salts with the variation of phosphonium moiety obtained by a two-step synthetic method from phosphine oxides. The salts showed pronounced activity against Gram-positive bacteria, including MRSA strains, and some fungi. Mechanism of action against S. aureus was studied by CV test, TEM and proteomic assay. No cell wall integrity loss was observed while proteomic assay results suggested interference in different metabolic processes of S. aureus. For this series, lipophilicity was determined as a key factor for the inhibition of Gram-positive bacteria growth and S. aureus killing. Biological properties of methylated derivatives were notably different with manifested action against Gram-negative bacteria.

A Fluorescent Cage for Supramolecular Sensing of 3-Nitrotyrosine in Human Blood Serum

3-Nitrotyrosine (NT) is generated by the action of peroxynitrite and other reactive nitrogen species (RNS), and as a consequence it is accumulated in inflammation-associated conditions. This is particularly relevant in kidney disease, where NT concentration in blood is considerably high. Therefore, NT is a crucial biomarker of renal damage, although it has been underestimated in clinical diagnosis due to the lack of an appropriate sensing method. Herein we report the first fluorescent supramolecular sensor for such a relevant compound: Fluorescence by rotational restriction of tetraphenylethenes (TPE) in a covalent cage is selectively quenched in human blood serum by 3-nitrotyrosine (NT) that binds to the cage with high affinity, allowing a limit of detection within the reported physiological concentrations of NT in chronic kidney disease.

Application of poly(vinylphosphonic acid) modified poly(amidoxime) in uptake of uranium from seawater

To enhance the anti-biofouling properties and adsorption capability of poly(amidoxime) (PAO), vinylphosphonic acid (VPA, CH2[double bond, length as m-dash]CH-PO3H2) was polymerized on poly(acrylonitrile) (PAN) surface by plasma technique, followed by amidoximation treatment to convert the cyano group (-C[triple bond, length as m-dash]N) into an amidoxime group (AO, -C(NH2)[double bond, length as m-dash]N-OH). The obtained poly(vinylphosphonic acid)/PAO (PVPA/PAO) was used as an adsorbent in the uptake of U(vi) from seawater. The effect of environmental conditions on the anti-biofouling property and adsorption capability of PVPA/PAO for U(vi) were studied. Results show that the modified PVPA enhances the anti-biofouling properties and adsorption capability of PAO for U(vi). The adsorption process is well described by the pseudo-second-order kinetic model and reached equilibrium in 24 h. Adsorption isotherms of U(vi) on PVPA/PAO can be well fitted by the Langmuir model, and the maximum adsorption capability was calculated to be 145 mg g-1 at pH 8.2 and 298 K. Experimental results highlight the application of PVPA/PAO in the extraction of U(vi) from seawater.

Sterically Hindered Quaternary Phosphonium Salts (QPSs): Antimicrobial Activity and Hemolytic and Cytotoxic Properties

Structure-activity relationships are important for the design of biocides and sanitizers. During the spread of resistant strains of pathogenic microbes, insights into the correlation between structure and activity become especially significant. The most commonly used biocides are nitrogen-containing compounds; the phosphorus-containing ones have been studied to a lesser extent. In the present study, a broad range of sterically hindered quaternary phosphonium salts (QPSs) based on tri-tert-butylphosphine was tested for their activity against Gram-positive (Staphylococcus aureus, Bacillus cereus, Enterococcus faecalis) and Gram-negative (Escherichia coli, Pseudomonas aeruginosa) bacteria and fungi (Candida albicans, Trichophyton mentagrophytes var. gypseum). The cation structure was confirmed to determine their biological activity. A number of QPSs not only exhibit high activity against both Gram-positive and -negative bacteria but also possess antifungal properties. Additionally, the hemolytic and cytotoxic properties of QPSs were determined using blood and a normal liver cell line, respectively. The results show that tri-tert-butyl(n-dodecyl)phosphonium and tri-tert-butyl(n-tridecyl)phosphonium bromides exhibit both low cytotoxicity against normal human cells and high antimicrobial activity against bacteria, including methicillin-resistant strains S. aureus (MRSA). The mechanism of QPS action on microbes is discussed. Due to their high selectivity for pathogens, sterically hindered QPSs could serve as effective tunable biocides.

Rational Design 2-Hydroxypropylphosphonium Salts as Cancer Cell Mitochondria-Targeted Vectors: Synthesis, Structure, and Biological Properties

It has been shown for a wide range of epoxy compounds that their interaction with triphenylphosphonium triflate occurs with a high chemoselectivity and leads to the formation of (2-hydroxypropyl)triphenylphosphonium triflates 3 substituted in the 3-position with an alkoxy, alkylcarboxyl group, or halogen, which were isolated in a high yield. Using the methodology for the disclosure of epichlorohydrin with alcohols in the presence of boron trifluoride etherate, followed by the substitution of iodine for chlorine and treatment with triphenylphosphine, 2-hydroxypropyltriphenylphosphonium iodides 4 were also obtained. The molecular and supramolecular structure of the obtained phosphonium salts was established, and their high antitumor activity was revealed in relation to duodenal adenocarcinoma. The formation of liposomal systems based on phosphonium salt 3 and L-α-phosphatidylcholine (PC) was employed for improving the bioavailability and reducing the toxicity. They were produced by the thin film rehydration method and exhibited cytotoxic properties. This rational design of phosphonium salts 3 and 4 has promising potential of new vectors for targeted delivery into mitochondria of tumor cells.

The challenges, achievements and applications of submersible superhydrophobic materials

Superhydrophobic materials have been widely reported throughout the scientific literature. Their properties originate from a highly rough morphology and inherently water repellent surface chemistry. Despite promising an array of functionalities, these materials have seen limited commercial development. This could be attributed to many factors, like material compatibility, low physical resilience, scaling-up complications, etc. In applications where persistent water contact is required, another limitation arises as a major concern, which is the stability of the air layer trapped at the surface when submerged or impacted by water. This review is aimed at examining the diverse array of research focused on monitoring/improving air layer stability, and highlighting the most successful approaches. The reported complexity of monitoring and enhancing air layer stability, in conjunction with the variety of approaches adopted, results in an assortment of suggested routes to achieving success. The review is addressing the challenge of finding a balance between maximising water repulsion and incorporating structures that protect air pockets from removal, along with challenges related to the variant approaches to testing air-layer stability across the research field, and the gap between the achieved progress and the required performance in real-life applications.

Structural preferences of an anti-fouling peptide: From single chain to small molecular assemblies

The structural features of a tripeptide constituted by two different non-coded amino acids, 3,4-dihydroxy-L-phenylalanine (L-DOPA) and 4-fluoro-Phenylalanine, (Phe(4F)), have been investigated by means of classical mechanics simulations. This tripeptide had been characterised as an antifouling agent with great adhesion capabilities. In this work, its conformational preferences have been described in two different environments (gas phase and water solution), at three different pHs and with different degrees of terminal capping. At the same time, the structural dynamics of small aggregates of the tripeptide have been investigated and their ability to stabilise β-sheet based assemblies has been studied. The reported results describe the complexity of the tripeptide conformational preferences due to both the amphiphilic nature of its side chains, and the effect of the ionisation state resulting from the solution conditions. The investigations performed with small tripeptide assemblies in water solution reproduced the previously reported structural features, such as the polymorphism of its aggregates as a function of the pH. At edge pH values, the electrostatic screening imposed by the ions present in the solution facilitates the aggregation of the tripeptide chains, while at neutral pH and low concentrations of ionised species, the polar groups and the hydrogen bond capable groups impose their strength and lead to the disaggregation of the peptide clusters by favouring the solvation of individual chains rather than stabilising the aggregated states.

The marine invasive seaweeds Asparagopsis armata and Sargassum muticum as targets for greener antifouling solutions

Biofouling is a complex phenomenon that affects all maritime dependent industries. The accumulation of both micro and macro-organisms in immerged structures increases significantly the maintenance expenses, and thus the use of antifouling substances is inevitable. Although with recognized antifouling properties, the available antifouling coatings are known to induce negative impacts in aquatic ecosystems. Therefore, greener alternatives are urgently required. Living underwater, marine organisms are prone to biofouling and some have developed strategies to defend themselves against undesirable organisms, which include the production of bioactive substances. As a result, marine organisms are promising sources of natural antifouling substances. Within this framework, the marine invasive seaweeds Sargassum muticum and Asparagopsis armata were addressed for antifouling compounds biodiscovery. Both seaweeds revealed antifouling properties against microfoulers, namely algicidal and anti-biofilm activities; however Asparagopsis armata stand out for its capacity to inhibit marine bacteria and microalgae growth, to decrease biofilm formation, and for acting as a neurotransmitter disruptor through the inhibition of acetylcholinesterase activity. By addressing invasive species, the problematic of the biological material supply for industrial purposes is surpassed while mitigating the negative impacts of invasive species through specimen's collection.

Flavonoid Glycosides with a Triazole Moiety for Marine Antifouling Applications: Synthesis and Biological Activity Evaluation

Over the last decades, antifouling coatings containing biocidal compounds as active ingredients were used to prevent biofouling, and eco-friendly alternatives are needed. Previous research from our group showed that polymethoxylated chalcones and glycosylated flavones obtained by synthesis displayed antifouling activity with low toxicity. In this work, ten new polymethoxylated flavones and chalcones were synthesized for the first time, including eight with a triazole moiety. Eight known flavones and chalcones were also synthesized and tested in order to construct a quantitative structure-activity relationship (QSAR) model for these compounds. Three different antifouling profiles were found: three compounds (1b, 11a and 11b) exhibited anti-settlement activity against a macrofouling species (Mytilus galloprovincialis), two compounds (6a and 6b) exhibited inhibitory activity against the biofilm-forming marine bacteria Roseobacter litoralis and one compound (7b) exhibited activity against both mussel larvae and microalgae Navicula sp. Hydrogen bonding acceptor ability of the molecule was the most significant descriptor contributing positively to the mussel larvae anti-settlement activity and, in fact, the triazolyl glycosylated chalcone 7b was the most potent compound against this species. The most promising compounds were not toxic to Artemia salina, highlighting the importance of pursuing the development of new synthetic antifouling agents as an ecofriendly and sustainable alternative for the marine industry.

Structure-Antifouling Activity Relationship and Molecular Targets of Bio-Inspired(thio)xanthones

The development of alternative ecological and effective antifouling technologies is still challenging. Synthesis of nature-inspired compounds has been exploited, given the potential to assure commercial supplies of potential ecofriendly antifouling agents. In this direction, the antifouling activity of a series of nineteen synthetic small molecules, with chemical similarities with natural products, were exploited in this work. Six (4, 5, 7, 10, 15 and 17) of the tested xanthones showed in vivo activity toward the settlement of Mytilus galloprovincialis larvae (EC50: 3.53-28.60 µM) and low toxicity to this macrofouling species (LC50 > 500 µM and LC50/EC50: 17.42-141.64), and two of them (7 and 10) showed no general marine ecotoxicity (<10% of Artemia salina mortality) after 48 h of exposure. Regarding the mechanism of action in mussel larvae, the best performance compounds 4 and 5 might be acting by the inhibition of acetylcholinesterase activity (in vitro and in silico studies), while 7 and 10 showed specific targets (proteomic studies) directly related with the mussel adhesive structure (byssal threads), given by the alterations in the expression of Mytilus collagen proteins (PreCols) and proximal thread proteins (TMPs). A quantitative structure-activity relationship (QSAR) model was built with predictive capacity to enable speeding the design of new potential active compounds.

Life and death of liquid-infused surfaces: a review on the choice, analysis and fate of the infused liquid layer

We review the rational choice, the analysis, the depletion and the properties imparted by the liquid layer in liquid-infused surfaces – a new class of low-adhesion surface.

Biomimetic Approaches for the Development of New Antifouling Solutions: Study of Incorporation of Macroalgae and Sponge Extracts for the Development of New Environmentally-Friendly Coatings

Biofouling causes major economic losses in the maritime industry. In our site study, the Bay of La Paz (Gulf of California), biofouling on immersed structures is a major problem and is treated mostly with copper-based antifouling paints. Due to the known environmental effect of such treatments, the search for environmentally friendly alternatives in this zone of high biodiversity is a priority to ensure the conservation and protection of species. The aim of this work was to link chemical ecology to marine biotechnology: indeed, the natural defense of macroalgae and sponge was evaluated against biofoulers (biofilm and macrofoulers) from the same geographical zone, and some coatings formulation was done for field assays. Our approach combines in vitro and field bioassays to ensure the selection of the best AF agent prospects. The 1st step consisted of the selection of macroalgae (5 species) and sponges (2 species) with surfaces harboring a low level of colonizers; then extracts were prepared and assayed for toxicity against Artemia, activity towards key marine bacteria involved in biofilm formation in the Bay of La Paz, and the potency to inhibit adhesion of macroorganisms (phenoloxidase assays). The most active and non-toxic extracts were further studied for biofouling activity in the adhesion of the bacteria involved in biofilm formation and through incorporation in marine coatings which were immersed in La Paz Bay during 40 days. In vitro assays demonstrated that extracts of Laurencia gardneri, Sargassum horridum (macroalgae), Haliclona caerulea and Ircinia sp. (sponges) were the most promising. The field test results were of high interest as the best formulation were composed of extracts of H. caerulea and S. horridum and led to a reduction of 32% of biofouling compared with the control.

Zwitterionic Protonophore Derived from 2-(2-Hydroxyaryl)alkenylphosphonium as an Uncoupler of Oxidative Phosphorylation

2-(2-Hydroxyaryl)alkenylphosphonium salts (here coined as PPR) representing derivatives of quaternary phosphonium with two phenyl (P) and one alkyl (R) substituents linked through alkenyl bridge to substituted phenol were applied here to planar bilayer lipid membranes (BLM), isolated mitochondria, and cell culture. PPR with six carbon atoms in R (PP6) induced proton-selective currents across BLM and caused mitochondrial uncoupling. In particular, PP6 at submicromolar concentrations accelerated respiration, decreased membrane potential, and reduced ATP synthesis in isolated rat liver mitochondria (RLM). Methylation of a hydroxyl group substantially suppressed the protonophoric activity of PP6 on BLM and its uncoupling potency in RLM. Of note, the methylated derivative PP6-OMe was synthesized here via a new synthetic route including cyclization of PP6 with subsequent ring opening. PPR were considered as protonophoric uncouplers of a zwitterionic type, capable of penetrating membranes both as a zwitterion composed of a deprotonated phenol and a cationic quaternary phosphonium, and as a protonated cation. The protonophoric and uncoupling properties of PPR found here were speculated to account for their strong antibacterial activity described previously.

Tailoring the self-assembly of a tripeptide for the formation of antimicrobial surfaces

The accumulation of bacteria on surfaces is currently one of the greatest concerns for the management of proper healthcare systems, water and energy. Here, we describe the mechanism by which a single peptide forms two pH-dependent supramolecular particles that resist bacterial contamination. By using NMR and molecular dynamics (MD), we determined the structures of the peptide monomers and showed the forces directing the self-assembly of each structure under different conditions. These peptide assemblies change the characteristics of bare glass and confer it with the ability to prevent biofilm formation. Furthermore, they can adsorb and release active compounds as demonstrated with an anticancer drug, antibiotic and enzyme. This synergism and the detailed understanding of the processes are necessary for developing new sterile surfaces for healthcare systems, water purification devices, food packaging or any environment that suffers from biocontamination.

Potential of synthetic chalcone derivatives to prevent marine biofouling

Biofouling represents a major economic, environmental and health concern for which new eco-friendly solutions are needed. International legislation has restricted the use of biocidal-based antifouling coatings, and increasing efforts have been applied in the search for environmentally friendly antifouling agents. This research work deals with the assessment of the interest of a series of synthetic chalcone derivatives for antifouling applications. Sixteen chalcone derivatives were synthesized with moderate yields (38-85%). Antifouling bioactivity of these compounds was assessed at different levels of biological organization using both anti-macrofouling and anti-microfouling bioassays, namely an anti-settlement assay using mussel (Mytilus galloprovincialis) larvae, as well as marine bacteria and microalgal biofilms growth inhibition bioassays. Results showed that three compounds (11, 12, and 16) were particularly active against the settlement of mussel larvae (EC₅₀ 7.24-34.63 μM), being compounds 12 and 16 also able to inhibit the growth of microfouling species (EC₅₀ 4.09-20.31 μM). Moreover, the most potent compounds 12 and 16 were found to be non-toxic to the non-target species Artemia salina (<10% mortality at 25 μM). A quantitative structure-activity relationship model predicted that descriptors describing the ability of molecules to form hydrogen bonds and encoding the shape, branching ratio and constitutional diversity of the molecule were implied in the antifouling activity against the settlement of mussel larvae. This work elucidates for the first time the relevance of synthesizing chalcone derivatives to generate new non-toxic products to prevent marine biofouling.

What determines sclerobiont colonization on marine mollusk shells?

Empty mollusk shells may act as colonization surfaces for sclerobionts depending on the physical, chemical, and biological attributes of the shells. However, the main factors that can affect the establishment of an organism on hard substrates and the colonization patterns on modern and time-averaged shells remain unclear. Using experimental and field approaches, we compared sclerobiont (i.e., bacteria and invertebrate) colonization patterns on the exposed shells (internal and external sides) of three bivalve species (Anadara brasiliana, Mactra isabelleana, and Amarilladesma mactroides) with different external shell textures. In addition, we evaluated the influence of the host characteristics (mode of life, body size, color alteration, external and internal ornamentation and mineralogy) of sclerobionts on dead mollusk shells (bivalve and gastropod) collected from the Southern Brazilian coast. Finally, we compared field observations with experiments to evaluate how the biological signs of the present-day invertebrate settlements are preserved in molluscan death assemblages (incipient fossil record) in a subtropical shallow coastal setting. The results enhance our understanding of sclerobiont colonization over modern and paleoecology perspectives. The data suggest that sclerobiont settlement is enhanced by (i) high(er) biofilm bacteria density, which is more attracted to surfaces with high ornamentation; (ii) heterogeneous internal and external shell surface; (iii) shallow infaunal or attached epifaunal life modes; (iv) colorful or post-mortem oxidized shell surfaces; (v) shell size (<50 mm² or >1,351 mm²); and (vi) calcitic mineralogy. Although the biofilm bacteria density, shell size, and texture are considered the most important factors, the effects of other covarying attributes should also be considered. We observed a similar pattern of sclerobiont colonization frequency over modern and paleoecology perspectives, with an increase of invertebrates occurring on textured bivalve shells. This study demonstrates how bacterial biofilms may influence sclerobiont colonization on biological hosts (mollusks), and shows how ecological relationships in marine organisms may be relevant for interpreting the fossil record of sclerobionts.

Review on Molecular Mechanisms of Antifouling Compounds: An Update since 2012

Better understanding of the mechanisms of antifouling compounds is recognized to be of high value in establishing sensitive biomarkers, allowing the targeted optimization of antifouling compounds and guaranteeing environmental safety. Despite vigorous efforts to find new antifouling compounds, information about the mechanisms of antifouling is still scarce. This review summarizes the progress into understanding the molecular mechanisms underlying antifouling activity since 2012. Non-toxic mechanisms aimed at specific targets, including inhibitors of transmembrane transport, quorum sensing inhibitors, neurotransmission blockers, adhesive production/release inhibitors and enzyme/protein inhibitors, are put forward for natural antifouling products or shelf-stable chemicals. Several molecular targets show good potential for use as biomarkers in future mechanistic screening, such as acetylcholine esterase for neurotransmission, phenoloxidase/tyrosinase for the formation of adhesive plaques, N-acyl homoserine lactone for quorum sensing and intracellular Ca²⁺ levels as second messenger. The studies on overall responses to challenges by antifoulants can be categorized as general targets, including protein expression/metabolic activity regulators, oxidative stress inducers, neurotransmission blockers, surface modifiers, biofilm inhibitors, adhesive production/release inhibitors and toxic killing. Given the current situation and the knowledge gaps regarding the development of alternative antifoulants, a basic workflow is proposed that covers the indispensable steps, including preliminary mechanism- or bioassay-guided screening, evaluation of environmental risks, field antifouling performance, clarification of antifouling mechanisms and the establishment of sensitive biomarkers, which are combined to construct a positive feedback loop.

Mitochondria-targeted antioxidants as highly effective antibiotics

Mitochondria-targeted antioxidants are known to alleviate mitochondrial oxidative damage that is associated with a variety of diseases. Here, we showed that SkQ1, a decyltriphenyl phosphonium cation conjugated to a quinone moiety, exhibited strong antibacterial activity towards Gram-positive Bacillus subtilis, Mycobacterium sp. and Staphylococcus aureus and Gram-negative Photobacterium phosphoreum and Rhodobacter sphaeroides in submicromolar and micromolar concentrations. SkQ1 exhibited less antibiotic activity towards Escherichia coli due to the presence of the highly effective multidrug resistance pump AcrAB-TolC. E. coli mutants lacking AcrAB-TolC showed similar SkQ1 sensitivity, as B. subtilis. Lowering of the bacterial membrane potential by SkQ1 might be involved in the mechanism of its bactericidal action. No significant cytotoxic effect on mammalian cells was observed at bacteriotoxic concentrations of SkQ1. Therefore, SkQ1 may be effective in protection of the infected mammals by killing invading bacteria.

Transcription Factor OsDOF18 Controls Ammonium Uptake by Inducing Ammonium Transporters in Rice Roots

Nitrogen is one of the most important mineral elements for plant growth. We studied the functional roles of Oryza sativa DNA BINDING WITH ONE FINGER 18 (OsDOF18) in controlling ammonium uptake. The growth of null mutants of OsDOF18 was retarded in a medium containing ammonium as the sole nitrogen source. In contrast, those mutants grew normally in a medium with nitrate as the sole nitrogen source. The gene expression was induced by ammonium but not by nitrate. Uptake of ammonium was lower in osdof18 mutants than in the wild type, while that of nitrate was not affected by the mutation. This indicated that OsDOF18 is involved in regulating ammonium transport. Among the 10 ammonium transporter genes examined here, expression of OsAMT1;1, OsAMT1;3, OsAMT2;1, and OsAMT4;1 was reduced in osdof18 mutants, demonstrating that the ammonium transporter genes function downstream of OsDOF18. Genes for nitrogen assimilation were also affected in the mutants. These results provide evidence that OsDOF18 mediates ammonium transport and nitrogen distribution, which then affects nitrogen use efficiency.

A Method for Evaluating the Efficacy of Antifouling Paints Using Mytilus galloprovincialis in the Laboratory in a Flow-Through System

A laboratory test with a flow-through system was designed and its applicability for testing antifouling paints of varying efficacies was investigated. Six different formulations of antifouling paints were prepared to have increasing contents (0 to 40 wt.%) of Cu₂O, which is the most commonly used antifouling substance, and each formulation of paint was coated on just one surface of every test plate. The test plates were aged for 45 days by rotating them at a speed of 10 knots inside a cylinder drum. A behavioral test was then conducted using five mussels (Mytilus galloprovincialis) that were pasted onto the coated surface of each aged test plate. The number of the byssus threads produced by each mussel generally decreased with increasing Cu₂O content of the paint. The newly designed method was considered valid owing to the high consistency of its results with observations from the field experiment.

Phosphonium pillar[5]arenes as a new class of efficient biofilm inhibitors: importance of charge cooperativity and the pillar platform

Inhibition of biofilm formation (MBIC₅₀ = 0.67–1.66 μM) by pillar[5]arene congugates.

Flavonoids from Piper delineatum modulate quorum-sensing-regulated phenotypes in Vibrio harveyi

Quorum sensing (QS), or bacterial cell-to-cell communication, is a key process for bacterial colonization of substrata through biofilm formation, infections, and production of virulence factors. In an ongoing investigation of bioactive secondary metabolites from Piper species, four new flavonoids (1-4), along with five known ones (5-9) were isolated from the leaves of Piper delineatum. Their stereostructures were established by spectroscopic and spectrometric methods, including 1D and 2D NMR experiments, and comparison with data reported in the literature. The compounds were screened for their ability to interfere with QS signaling in the bacterial model Vibrio harveyi. Four compounds from this series (2, 3, 6, and 7) exhibited remarkable activity in the micromolar range, being compounds 3 and 7 particularly attractive since they did not affect bacterial growth. The results suggest that these flavonoids disrupt QS-mediated bioluminescence by interaction with elements downstream LuxO in the QS circuit of V. harveyi, and also, they exhibited a strong dose-dependent inhibition of biofilm formation. The present findings shed light on the QS inhibition mechanisms of flavonoids, underlining their potential applications.