Four synthetic Schiff bases (PSB1 [(E)-2-(((4-aminopyridin-3-yl)imino)methyl)-4,6-dibromophenol], PSB2 [(E)-2-(((4-aminopyridin-3-yl)imino)methyl)-4,6-diiodophenol], PSB3 [(E)-2-(((4-aminopyridin-3-yl)imino)methyl)-4-iodophenol], and PSB4 [(E)-2-(((4-aminopyridin-3-yl)imino)methyl)-4-chloro-6-iodophenol]) were fully characterized. These compounds exhibit an intramolecular hydrogen bond between the hydroxyl group of the phenolic ring and the nitrogen of the azomethine group, contributing to their stability. Their antimicrobial activity was evaluated against various Gram-negative and Gram-positive bacteria, and it was found that the synthetic pyridine Schiff bases, as well as their precursors, showed no discernible antimicrobial effect on Gram-negative bacteria, including Salmonella Typhi (and mutant derivatives), Salmonella Typhimurium, Escherichia coli, and Morganella morganii. In contrast, a more pronounced biocidal effect against Gram-positive bacteria was found, including Bacillus subtilis, Streptococcus agalactiae, Streptococcus pyogenes, Enterococcus faecalis, Staphylococcus aureus, and Staphylococcus haemolyticus. Among the tested compounds, PSB1 and PSB2 were identified as the most effective against Gram-positive bacteria, with PSB2 showing the most potent biocidal effects. Although the presence of reactive oxygen species (ROS) was noted after treatment with PSB2, the primary mode of action for PSB2 does not appear to involve ROS generation. This conclusion is supported by the observation that antioxidant treatment with vitamin C only partially mitigated bacterial inhibition, indicating an alternative biocidal mechanism.

Salmonella enterica serovar Typhi (S. Typhi) produces outer membrane vesicles (OMVs) that remain comparatively underexplored as potential biotechnological tools. Here, we investigated how hypervesiculating S. Typhi mutants (ΔtolR and ΔdegS) can be engineered to load and deliver the fluorescent reporter protein mCherry, targeting human epithelial cells and the murine immune system. Deletions in tolR and degS led to distinct OMV phenotypes characterized by higher vesicle production and altered cargo composition, underscoring the impact of disrupted membrane integrity and envelope stress on OMV biogenesis. By fusing mCherry with the S. Typhi OmpA signal peptide (SP_ompA), we achieved robust and functionally intact intravesicular packaging in all strains. Flow cytometry and confocal microscopy revealed that the ΔtolR mutant exhibited particularly high cargo loading in the OMV fraction and pronounced mCherry delivery to epithelial cells, highlighting the potential of hypervesiculation to enhance OMV-based protein transport. However, immunization studies in mice showed that wild-type OMVs, despite carrying less mCherry than their hypervesiculating counterparts, induced the strongest anti-mCherry IgG responses. These findings indicate that, at least under these conditions, antigen loading alone is not sufficient to fully determine immunogenicity. Instead, the intrinsic composition or adjuvant-like properties of OMVs play a pivotal role in driving robust immune activation. Our results establish S. Typhi OMVs, especially when genetically modified with a Sec-dependent targeting signal (SP_ompA), as versatile platforms for heterologous protein delivery. Although hypervesiculation facilitates increased protein encapsulation and delivery to epithelial cells, native OMVs appear to better preserve and/or present antigens for effective immunogenic responses in vivo. These insights set the stage for further optimization of S. Typhi OMVs in vaccine development and protein therapeutics, where balancing cargo loading with immunostimulatory features may be key to achieving maximal efficacy.

A new Schiff base, (E)-2-(((1,10-phenanthrolin-5-yl)imino)methyl)-4,6-di-tert-butylphenol (Fen-IHB), was designed to incorporate an intramolecular hydrogen bond (IHB) between the phenolic OH and the azomethine nitrogen with the goal of modulating its physicochemical and biological properties. Fen-IHB was synthesized by condensation of 5-amino-1,10-phenanthroline with 3,5-di-tert-butyl-2-hydroxybenzaldehyde and exhaustively characterized by HR-ESI-MS, FTIR, 1D/2D NMR (¹H, ¹³C, DEPT-45, HH-COSY, CH-COSY, D₂O exchange), and UV–Vis spectroscopy. Cyclic voltammetry in anhydrous CH₃CN revealed a single irreversible cathodic peak at −1.43 V (vs. Ag/Ag⁺), which is consistent with the intramolecular reductive coupling of the azomethine moiety. Density functional theory (DFT) calculations, including MEP mapping, Fukui functions, dual descriptor analysis, and Fukui potentials with dual descriptor potential, identified the exocyclic azomethine carbon as the principal nucleophilic site and the phenolic ring (hydroxyl oxygen and adjacent carbons) as the main electrophilic region. Noncovalent interaction (NCI) analysis further confirmed the strength and geometry of the intramolecular hydrogen bond (IHB). In vitro antimicrobial assays indicated that Fen-IHB was inactive against Gram-negative facultative anaerobes (Salmonella enterica serovar Typhimurium and Typhi, Escherichia coli) and strictly anaerobic Gram-positive species (Clostridioides difficile, Roseburia inulinivorans, Blautia coccoides), as any growth inhibition was indistinguishable from the DMSO control. Conversely, Fen-IHB displayed measurable activity against Gram-positive aerobes and aerotolerant anaerobes, including Bacillus subtilis, Streptococcus pyogenes, Enterococcus faecalis, Staphylococcus aureus, and Staphylococcus haemolyticus. Overall, these comprehensive characterization results confirm the distinctive chemical and electronic properties of Fen-IHB, underlining the crucial role of the intramolecular hydrogen bond and electronic descriptors in defining its reactivity profile and selective biological activity.

Metal-free aminobenzoic acid-derived Schiff bases are attractive antimicrobial leads because their azomethine (–C=N–) functionality enables tunable electronic properties and target engagement. We investigated whether halogenation on the phenolic ring would modulate the redox behavior and enhance antibacterial potency, and hypothesized that heavier halogens would favorably tune physicochemical and electronic descriptors. We synthesized three derivatives (SB-3/Cl, SB-4/Br, and SB-5/I) and confirmed their structures using FTIR, ¹H- and ¹³C-NMR, UV-Vis, and HRMS. For SB-5, single-crystal X-ray diffraction and Hirshfeld analysis verified the intramolecular O–H⋯N hydrogen bond and key packing contacts. Cyclic voltammetry revealed an irreversible oxidation (aminobenzoic ring) and, for the halogenated series, a reversible reduction associated with the imine; peak positions and reversibility trends are consistent with halogen electronic effects and DFT-based MEP/LHS descriptors. Antimicrobial testing showed that SB-5 was selectively potent against Gram-positive aerobes, with low-to-mid micromolar MICs across the panel. Among anaerobes, activity was more substantial: Clostridioides difficile was inhibited at 0.1 µM, and SB-3/SB-5 reduced its sporulation at sub-MICs, while Blautia coccoides was highly susceptible (MIC 0.01 µM). No activity was detected against Gram-negative bacteria at the tested concentrations. In the fungal assay, Botrytis cinerea displayed only a transient fungistatic response without complete growth inhibition. In mammalian cells (HeLa), the compounds displayed clear concentration-dependent behavior. Overall, halogenation, particularly iodination, emerges as a powerful tool to couple redox tuning with selective Gram-positive activity and a favorable cellular tolerance window, nominating SB-5 as a promising scaffold for further antimicrobial optimization.

Correction for ‘Canonical Ru( ii ) tris-polypyridyl complexes as confocal-compatible stains for lignified secondary-wall domains in Arabidopsis thaliana stems’ by Ignacio Fuentes et al. , RSC Adv. , 2026, https://doi.org/10.1039/d6ra04630d.