Search for novel antimicrobial agents, including plant-derived flavonoids, and evaluation of the mechanisms of their antibacterial activities are pivotal objectives. The goal of this study was to compare the antihemolytic activity of flavonoids, quercetin, naringenin and catechin against sheep erythrocyte lysis induced by α-hemolysin (αHL) produced by the Staphylococcus aureus strain NCTC 5655. We also sought to investigate the membrane-modifying action of the flavonoids. Lipophilic quercetin, but not naringenin or catechin, effectively inhibited the hemolytic activity of αHL at concentrations (IC₅₀ = 65 ± 5 µM) below minimal inhibitory concentration values for S. aureus growth. Quercetin increased the registered bacterial cell diameter, enhanced the fluidity of the inner and surface regions of bacterial cell membranes and raised the rigidity of the hydrophobic region and the fluidity of the surface region of erythrocyte membranes. Our findings provide evidence that the antibacterial activities of the flavonoids resulted from a disorder in the structural organization of bacterial cell membranes, and the antihemolytic effect of quercetin was related to the effect of the flavonoid on the organization of the erythrocyte membrane, which, in turn, increases the resistance of the target cells (erythrocytes) to αHL and inhibits αHL-induced osmotic hemolysis due to prevention of toxin incorporation into the target membrane. We confirmed that cell membrane disorder could be one of the direct modes of antibacterial action of the flavonoids.

Dental caries (DC) is the most common oral pathology. The main bacteria responsible for DC is Streptococcus mutans. One of the strategies that can decrease or eliminate the risk of DC development is using compounds that will inhibit both the growth and virulence factors of S. mutans. Tannins are plant polyphenols that have strong antibacterial activity. The purpose of this study was to assess the antibacterial activity of three tannins against S. mutans. In this investigation, microbiological tests (MIC and MBC) and physicochemical techniques like the fluorescence measurements of tannins’ interaction with S. mutans cell membrane and membrane proteins, zeta potential, and thermodynamic analyses were used to obtain knowledge about the antibacterial potential of the investigated compounds against S. mutans as well as about the mechanisms associated with antibacterial activity. The obtained results demonstrate that the used compounds exhibit high antibacterial activity against S. mutans. The mechanisms of their antibacterial activity are linked to the strong change in the S. mutans membrane fluidity and potential, and to their interaction with membrane proteins that can result in great disturbance of bacterial physiology and ultimately the inhibition of bacterial growth, triggering their death. Therefore, it can be concluded that the investigated compounds can be potentially used as natural factors in the prevention of dental caries.

Polyphenols, including tannins, are phytochemicals with pronounced antimicrobial properties. We studied the activity of two hydrolysable tannins, (i) gallotannin—1,2,3,4,5-penta-O-galloyl-β-D-glucose (PGG) and (ii) ellagitannin—1,2-di-O-galloyl-4,6-valoneoyl-β-D-glucose (dGVG), applied alone and in combination with antibiotics against Staphylococcus aureus strain 8324-4. We also evaluated the effect of these tannins on bacterial membrane integrity and fluidity and studied their interaction with membrane proteins and lipids. A correlation between the antimicrobial activity of the tannins and their membranotropic action depending on the tannin molecular structure has been demonstrated. We found that the antibacterial activity of PGG was stronger than dGVG, which can be associated with its larger flexibility, dipole moment, and hydrophobicity. In addition, we also noted the membrane effects of the tannins observed as an increase in the size of released bacterial membrane vesicles.

Vitamin E is the most active natural lipophilic antioxidant with a broad spectrum of biological activity. α-Tocopherol (α-T), the main representative of the vitamin E family, is a strong inhibitor of lipid peroxidation as a chain-breaking antioxidant. Antioxidant and antiradical properties of vitamin E result from the presence of a phenolic hydroxyl group at the C-6 position. Due to stereoelectronic effects in the dihydropyranyl ring, the dissociation enthalpy for phenolic O–H bond (BDE_OH) is reduced. The high chain-breaking reactivity of α-T is mainly attributed to orbital overlapping of the 2p-type lone pair on the oxygen atom (O1) in para position to the phenolic group, and the aromatic π-electron system. The influence of the O1 atom on the antioxidant activity of vitamin E was estimated quantitatively. The all-rac-1-carba-α-tocopherol was synthesized for the first time. Along with model compounds, 1-carba-analog of Trolox and its methyl ester were screened for their in vitro antioxidant activity by inhibition of styrene oxidation, and for the radical-reducing properties by means of 2,2-diphenyl-1-picrylhydrazyl free radical (DPPH) scavenging assay. To study the antioxidant activity, density functional theory (DFT) was also applied. Reaction enthalpies related to HAT (hydrogen atom transfer), SET–PT (sequential electron transfer—proton transfer), and SPLET (sequential proton loss—electron transfer) mechanisms were calculated.

The chemical composition of propolis of four species of stingless bees (SLBs) from Argentina was determined, and its antibacterial and anticancer activity was evaluated on selected types of microbes and cancer cell lines. Volatile secretions of all propolis samples are formed by 174 C₂–C₁₅ organic compounds, mainly mono- and sesquiterpenes and their derivatives. The chromatograms of ether extracts showed 287 peaks, of which 210 were identified. The most representative groups in the extracts of various propolis samples were diterpenoids (mainly resin acids), triterpenoids and phenolic compounds: long-chain alkenyl phenols, resorcinols and salicylates. The composition of both volatile and extractive compounds turned out to be species-specific; however, in both cases, the pairwise similarity of the propolis of Scaptotrigona postica and Tetragonisca fiebrigi versus that of Tetragona clavipes and Melipona quadrifasciata quadrifasciata was observed, which indicated the similarity of the preferences of the respective species when choosing plant sources of resin. The composition of the studied extracts completely lacked flavonoids and phenolcarboxylic acids, which are usually associated with the biological activity and medicinal properties of propolis. However, tests on selected microbial species and cancer cell lines showed such activity. All propolis samples tested against Paenibacillus larvae, two species of Bacillus and E. coli showed biofilm inhibition unrelated to the inhibition of bacterial growth, leading to a decrease in their pathogenicity. Testing the anticancer activity of ether extracts using five types of cell cultures showed that all four types of propolis studied inhibit the growth of cancer cells in a dose- and time-dependent manner. Propolis harvested by T. clavipes demonstrated the highest cytotoxicity on all tested cell lines.

The action of UVA radiation (both that derived from solar radiation and that used in the treatment of skin diseases) modifies the function and composition of keratinocyte membranes. Therefore, this study aimed to assess the effects of phytocannabinoids (CBD and CBG), used singly and in combination, on the contents of phospholipids, ceramides, lipid rafts and sialic acid in keratinocyte membranes exposed to UVA radiation, together with their structure and functionality. The phytocannabinoids, especially in combination (CBD+CBG), partially prevented increased levels of phosphatidylinositols and sialic acid from occurring and sphingomyelinase activity after the UVA exposure of keratinocytes. This was accompanied by a reduction in the formation of lipid rafts and malondialdehyde, which correlated with the parameters responsible for the integrity and functionality of the keratinocyte membrane (membrane fluidity and permeability and the activity of transmembrane transporters), compared to UVA-irradiated cells. This suggests that the simultaneous use of two phytocannabinoids may have a protective effect on healthy cells, without significantly reducing the therapeutic effect of UV radiation used to treat skin diseases such as psoriasis.