Enhanced Antibacterial Activity of Ent-Labdane Derivatives of Salvic Acid (7α-Hydroxy-8(17)-ent-Labden-15-Oic Acid): Effect of Lipophilicity and the Hydrogen Bonding Role in Bacterial Membrane Interaction

Production of new ent-hardwickiic acid derivatives by microbial transformation and their antifungal activity

Ent-hardwickiic acid is the major compound of Copaifera pubiflora Benth oleoresin traditionally used in Brazilian folk medicine as an antimicrobial agent. Microbial transformation of ent-hardwickiic by Cunninghamella elegans ATCC 10028b resulted in two and five antifungal derivatives (four new ones) produced in the Czapek modified and Koch's K1 media, respectively. The derivatives were isolated and their structures were determined by spectral analysis, namely 1D/2D NMR and HR-ESIMS. All compounds were tested for cytotoxic and antifungal activities and they were not cytotoxic to the tested cell lines, but all derivatives showed fungicidal activity against Candida glabrata and Candida krusei, which have emerged as resistant to fluconazole. One of the yet unreported biotransformation products displayed the strongest activity with minimum fungicidal concentration values smaller than the other compounds, including fluconazole.

Discovery of Novel Small Molecule Growth Inhibitors to Manage Pseudomonas Leaf Spot Disease on Peppers (Capsicum sp.)

Pseudomonas leaf spot (PLS) disease in peppers caused by Pseudomonas syringae pv. syringae (Pss) is an emerging seedborne phytopathogen. Pss infection can severely reduce the marketable yield of peppers in favorable environmental conditions and cause significant economic losses. The intensive use of copper-sulfate and streptomycin-sulfate to control PLS and other bacterial diseases is associated with antimicrobial-resistant Pss strains, making these control methods less effective. So, there is an urgent need to develop novel antimicrobials effective against Pss in peppers. Several studies, including those done in our laboratory, have shown that small molecule (SM) antimicrobials are ideal candidates as they can be effective against multidrug resistant bacteria. Therefore, our study aims to identify novel SM growth inhibitors of Pss, assess their safety, and evaluate their efficacy on Pss-infected pepper seeds and seedlings. Using high-throughput screening, we identified 10 SMs (PC1 to PC10) that inhibited the growth of Pss strains at 200 µM or lower concentrations. These SMs were effective against both copper- and streptomycin-resistant as well as biofilm-embedded Pss. These SMs were effective against other plant pathogens (n = 22) at low concentrations (<200 μM) and had no impact on beneficial phytobacteria (n = 12). Furthermore, these SMs showed better or equivalent antimicrobial activity against Pss in infested pepper seeds and inoculated seedlings compared with copper-sulfate (200 μM) and streptomycin (200 μg/ml). Additionally, none of the SMs were toxic to pepper tissues (seeds, seedlings, or fruits), human Caco-2 cells, and pollinator honeybees at 200 μM. Overall, the SMs identified in this study are promising alternative antimicrobials for managing PLS in pepper production.

Whey Proteins as a Potential Co-Surfactant with Aesculus hippocastanum L. as a Stabilizer in Nanoemulsions Derived from Hempseed Oil

The use of natural surfactants including plant extracts, plant hydrocolloids and proteins in nanoemulsion systems has received commercial interest due to demonstrated safety of use and potential health benefits of plant products. In this study, a whey protein isolate (WPI) from a byproduct of cheese production was used to stabilize a nanoemulsion formulation that contained hempseed oil and the Aesculus hippocastanum L. extract (AHE). A Box-Behnken experimental design was used to set the formulation criteria and the optimal nanoemulsion conditions, used subsequently in follow-up experiments that measured specifically emulsion droplet size distribution, stability tests and visual quality. Regression analysis showed that the concentration of HSO and the interaction between HSO and the WPI were the most significant factors affecting the emulsion polydispersity index and droplet size (nm) (p < 0.05). Rheological tests, Fourier transform infrared spectroscopy (FTIR) analysis and L*a*b* color parameters were also taken to characterize the physicochemical properties of the emulsions. Emulsion systems with a higher concentration of the AHE had a potential metabolic activity up to 84% in a microbiological assay. It can be concluded from our results that the nanoemulsion system described herein is a safe and stable formulation with potential biological activity and health benefits that complement its use in the food industry.

Compound Prioritization through Meta-Analysis Enhances the Discovery of Antimicrobial Hits against Bacterial Pathogens

The development of informatic tools to improve the identification of novel antimicrobials would significantly reduce the cost and time of drug discovery. We previously screened several plant (Xanthomonas sp., Clavibacter sp., Acidovorax sp., and Erwinia sp.), animal (Avian pathogenic Escherichia coli and Mycoplasma sp.), and human (Salmonella sp. and Campylobacter sp.) pathogens against a pre-selected small molecule library (n = 4182 SM) to identify novel SM (hits) that completely inhibited the bacterial growth or attenuated at least 75% of the virulence (quorum sensing or biofilm). Our meta-analysis of the primary screens (n = 11) using the pre-selected library (approx. 10.2 ± 9.3% hit rate per screen) demonstrated that the antimicrobial activity and spectrum of activity, and type of inhibition (growth versus virulence inhibitors) correlated with several physico-chemical properties (PCP; e.g., molecular weight, molar refraction, Zagreb group indexes, Kiers shape, lipophilicity, and hydrogen bond donors and acceptors). Based on these correlations, we build an in silico model that accurately classified 80.8% of the hits (n = 1676/2073). Therefore, the pre-selected SM library of 4182 SM was narrowed down to 1676 active SM with predictable PCP. Further, 926 hits affected only one species and 1254 hits were active against specific type of pathogens; however, no correlation was detected between PCP and the type of pathogen (29%, 34%, and 46% were specific for animal, human foodborne and plant pathogens, respectively). In conclusion, our in silico model allowed rational identification of SM with potential antimicrobial activity against bacterial pathogens. Therefore, the model developed in this study may facilitate future drug discovery efforts by accelerating the identification of uncharacterized antimicrobial molecules and predict their spectrum of activity.

Antimicrobial properties of novel ionic liquids derived from imidazolium cation with phenolic functional groups

Bacterial infections are nowadays among the major threats to public health worldwide. Thus, there is an urgent and increased need for new antimicrobial agents. As a result, the exploration of the antimicrobial properties of different substances including ionic liquids (ILs) has recently attracted great attention. The present work is aimed at evaluating how the addition of halogens and hydrophobic substituents on alkylimidazolium units of ILs as well as the increase in their chain lengths affects the antimicrobial properties of such ILs. After their synthesis, the antibacterial activities of these compounds against Pseudomona aeruginosa, Escherichia coli, and Staphylococcus aureus are determined by measuring their minimal inhibitory concentrations (MICs). Key features in ILs-membrane interactions are also studied using long-term all-atom molecular dynamics simulations (MDs). The results show that these ILs have good antibacterial activity against S. aureus, E. coli, and P. aeruginosa, with MIC values range from <7.81 to 62.50 μM. The antimicrobial property of tert-butyl N-methylphenolimidazolium salts (denoted as 8b and 8c) is particularly better with MIC values of < 7.81 μM. The antibacterial efficacy is also found to depend on the alkyl chain length and substituents on the phenolic ring. Finally, MDs done for ILs in a phosphatidylcholine (POPC) bilayer show key features in the mechanism of IL-induced membrane disruption, where the ILs are inserted as clusters into one side of the bilayer until saturation is reached. This insertion increases "leaflet strain" up to critical threshold, likely triggering the morphological disruption of the membranes in the microbes.

Variability of the antibacterial potential among analogue diterpenes against Gram-positive bacteria: considerations on the structure–activity relationship

The search for new antibacterial agents and a better comprehension of substances with antimicrobial behavior is mandatory nowadays due to the serious public health problem of infection diseases. In the present work, 30 diterpenes were studied, with 2 natural derivatives, named ent-16-kauren-19-oic acid and ent-pimara-8(14),15-dien-19-oic acid, and 28 semi-synthetic derivatives. The natural diterpenes were isolated from Mikania glomerata and Viguiera arenaria, respectively. All diterpenes were submitted to antimicrobial assays against six different Gram-positive microorganisms to better understand the structure–activity relationship of antimicrobial diterpenes. The semi-synthetic derivatives were all obtained from the two natural derivatives by structural modifications, mainly esterification reactions. Both natural derivatives, together with the derivative ent-8(14)-pimaren-19-oic acid, displayed the most relevant antibacterial activities, with minimal inhibitory concentration (MIC) values that were less than 10 μg mL–1 for most pathogens; thus, they were considered promising antimicrobial agents. Moreover, in light of the hypothesis of Urzúa and colleagues, several considerations about the structure–activity relationship of antimicrobial diterpenes could be stated.

Antifungal activity against Botrytis cinerea of labdane-type diterpenoids isolated from the resinous exudate of Haplopappus velutinus Remy (Asteraceae)

Two labdane diterpenoids were isolated, from the resinous exudate of Haplopappus velutinus Remy (Asteraceae); the main compound was identified as 7,13-(E)-labdadien-15,18-dioic-acid-18-methyl ester (1) and the minor compound identified as 7-labden-15,18-dioic-acid-18-methyl ester (2). Their structures were obtained using FTIR, MS, HRMS and NMR data: 1D NMR (¹H, ¹³C and DEPT-135), 2D homonuclear NMR (COSY and NOESY) and heteronuclear NMR (HSQC and HMBC). The trans stereochemistry of the decalin moiety of compounds 1 and 2 was established through NOESY experiments of the reduction product of 1; 7-labden-15,18-diol (1a). Diterpenoids 1 and 1a are described for the first time and showed antifungal activity, inhibiting approximately 40% mycelial growth of Botrytis cinerea.