Background/Objectives: Brown algae, particularly Eisenia bicyclis, produce various bioactive chemicals with significant application potential in the food, cosmetics, and pharmaceutical industries. This study aimed to evaluate the antibacterial, antibiofilm, and antivirulence properties of the ethyl acetate fraction (EA) of E. bicyclis and its synthesized gold nanoparticles (EA-AuNPs), with a focus on their potential applications against both Gram-positive and Gram-negative bacteria. Methods: The bioactive component in the ethyl acetate fraction was identified using a gas chromatography-mass spectroscopy (GC-MS) device and a liquid chromatography-mass spectrometer/mass spectrometry (LC-MS) system. The crystal violet method was utilized to evaluate the biofilm inhibition experiments. Several instruments, including dynamic light scattering, Fourier transform infrared, X-ray diffraction, field emission transmission electron microscopy, and energy-dispersive spectroscopy, were employed to completely characterize the produced EA-AuNPs. The cytotoxicity of the EA-AuNPs was determined using the MTT assay, and the expression of genes linked with biofilm and virulence in Pseudomonas aeruginosa and Staphylococcus aureus was investigated using real-time polymerase chain reaction (RT-PCR). Results: Various bioactive compounds were identified from the EA using GC-MS and LC-MS, including fatty acids and phlorotannins such as eckol, dieckol, 6,6’-bieckol, and phlorofucofuroeckol in high amounts, highlighting EA as a phlorotannin-rich fraction. The EA also demonstrated significant antibiofilm activity, with 79.86% inhibition at 512 μg/mL against P. aeruginosa and 87.00% at 64 μg/mL against S. aureus. EA was then used in the synthesis of gold nanoparticles (AuNPs) to improve their stability and safety. The synthesized EA-AuNPs were determined to have an average size of 165.04 nm, with a zeta potential of −29.86 mV, indicating good stability. In antibiofilm activity assays, EA-AuNPs demonstrated 45.76% inhibition against P. aeruginosa at 1024 μg/mL and 44.64% inhibition against S. aureus at 128 μg/mL. At sub-MIC levels, EA-AuNPs significantly inhibited biofilm formation and virulence factors, including the motility of P. aeruginosa and staphyloxanthin synthesis in S. aureus. The RT-PCR analysis revealed the downregulation of key genes involved in biofilm formation and virulence in P. aeruginosa and S. aureus. Conclusions: These findings highlight the potential of E. bicyclis solvent-soluble extracts and EA-AuNPs as effective antibacterial, antibiofilm, and antivirulence agents, with significant application potential in the pharmaceutical and food industries. To the best of our knowledge, this is the first report of antibiofilm activity against both Gram-positive and Gram-negative bacteria using EA-AuNPs.

Background/Objectives: Titanium dental implants, while highly successful, face challenges due to polymicrobial infections leading to peri-implantitis and implant failure. Biofilm formation on implant surfaces is the primary cause of these infections, with factors such as matrix production and cross-kingdom interactions contributing to the microbial accumulation of bacterial and fungal pathogens species. To combat this issue, naturally derived molecules have been reported to overcome the hurdle of antimicrobial resistance against the application of conventional antibiotics and antifungals. Methods: The present study aimed to employ the lichen-derived molecules, usnic acid (UA), to retard the development of biofilms of bacterial and fungal pathogens on the surface of titanium kept in the human artificial saliva (HAS) working as a growth-supporting, host-mimicking media. Results: The minimum inhibitory concentration of UA in HAS towards Candida albicans was >512 µg/mL, whereas against Staphylococcus aureus and Streptococcus mutans, it was determined to be 512 µg/mL. Whereas, in the standard growth media, the MIC value of UA towards S. mutans and S. aureus were 8 and 16 µg/mL; however, against C. albicans, it was 512 µg/mL. UA synergistically enhanced the efficacy of the antibiotics toward bacterial pathogens and the efficacy of antifungals against C. albicans. The antibiofilm results depict the fact that in the HAS, UA significantly reduced both mono-species of S. mutans, S. aureus, and C. albicans and mixed-species biofilm of C. albicans with S. mutans and S. aureus on the surface of the titanium. Conclusions: The present study showed that UA is a promising natural drug that can control oral polymicrobial disease as a result of the application of dental implants.

Background/Objectives: Lactic acid bacteria produce biofilms in meat products that contribute to the products’ deterioration, reduction in quality, and shortened shelf life. Although LAB are generally considered benign, certain strains create slime and cause significant drops in pH. The study’s goal was to identify and characterize LAB strains from sausage products that are capable of biofilm formation, and to evaluate the inhibitory effects of E. bicyclis methanol extract, its ethyl acetate fraction, and phloroglucinol, as well as to synthesize AuNPs, and assess their efficacy in controlling biofilm formation. Methods: Slime or biofilm-producing LAB bacteria were isolated from commercial sausages and identified using 16S rRNA gene sequencing. Lactobacillus sakei S10, which can tolerate high salt concentrations and cold temperatures, was chosen as a representative strain. The isolates were subsequently tested for hemolytic activity, salt and temperature tolerance, and carbohydrate consumption patterns. To evaluate antibiofilm potential, marine-derived compounds from Eisenia bicyclis, such as phloroglucinol (PG), crude methanolic extracts, ethyl acetate fractions, and gold nanoparticle (AuNP) formulations, were tested in situ on sausage surfaces against L. sakei S10 and common pathogens (Pseudomonas aeruginosa and Staphylococcus aureus). The biofilm-inhibitory effects of the extracts, PG, and PG-AuNPs were estimated using the colony-counting method. Results: The PG-AuNPs had an average particle size of 98.74 nm and a zeta potential of −29.82 mV, indicating nanoscale dimensions and considerable colloidal stability. Structural analysis confirmed their spherical form and crystalline structure, as well as the presence of phenolic groups in both reduction and stabilization processes. Among the studied treatments, the PG and PG-AuNPs had the strongest antibiofilm activities, dramatically lowering biofilm biomass, particularly for P. aeruginosa and L. sakei S10. However, the inhibitory effects were less prominent in in situ conditions than in in vitro testing, highlighting the complexity of real food matrices. Conclusions: The results of this study indicate that polyphenolic compounds obtained from marine sources, particularly in nano-formulated forms, have a great deal of potential as natural antibiofilm products. Enhancing the microbiological safety of processed meat products and extending their shelf life could be accomplished through the application of these polyphenolic compounds in food packaging or surface treatments.