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Antibiotics 2018, 7, 76 5 of 12 The highest sensitivity of the P. cinnamomi isolate (MYC43) corresponded to the inclusion compound with gallic acid in DES (EC50 = 0.1 μg·mL−1), followed by the inclusion compounds with silymarin and ferulic acid, and finally by the one with curcumin, with EC50 values of 0.6, 0.6 and 8.9 μg·mL−1, respectively. 3. Discussion With a view to comparing the efficacy of the proposed nanocomposites verses other phenolic-based products against Phytophthora spp. discussed in the literature, it should be noted that Kim et al. [37] reported strong fungicidal activities of Curcuma longa L. rhizome-derived curcumin in ethyl acetate and hexane fractions against P. infestans with 100% and 84% control values at a concentration of 1000 μg·mL−1. Apart from the higher concentrations used in those experiments as compared to the ones presented herein, it should be noted that a contribution of the pronounced cytotoxic activities of the solvents could not be excluded. Pompimon et al. [38] assessed the anti-P. capsici activity of C. longa in acetone fraction, finding an inhibition of mycelial growth of ca. 90% at a concentration of 300 μg·mL−1, higher than the EC90 of 184 μg·mL−1 of the curcumin-based treatment in DES media reported in this study. The nanocomposites of the four polyphenols in DES medium would also be more active than, for instance, cuminic acid, which featured an EC50 value against mycelial growth of P. capsici of 19.7 μg· mL−1 (which in turn was lower than the EC50 value of other benzoic acid derivatives in previous reports) [39]. Other natural compounds, such as furanocoumarins (e.g., psoralen or isopsoralen) would require concentrations of 500 μg·mL−1 to attain 82–84% disease control against P. infestans [40]. An EC50 value of amphopolycarboxyglycinate-stabilized AgNPs against P. infestans of 3.1 μg·mL−1 was reported by Krutyakov et al. [41], i.e., a 30 and 6 times higher concentration than those obtained for the AgNPs combined with gallic acid, silymarin and ferulic acid inclusion compounds in DES, respectively. Banik and Pérez-de-Luque [42] found that the integration of copper nanoparticles (CuNPs) with non-nano copper like copper oxychloride, both at a 50 μg·mL−1 concentration, resulted in a 76% growth inhibition in vitro of the oomycete P. cinnamomi as compared to the control. Since in comparative assays between NPs the concentrations of AgNPs are usually 10 times higher than those CuNPs [43,44], the equivalent concentration of AgNPs to attain aforementioned effects should be 500 ca. μg·mL−1, four times higher than that required by the DES treatments to attain a comparable mycelial growth inhibition (ca. 90%). Chitosan has also been assayed against P. infestans [33], finding that concentrations of 500 μg· mL−1 would be required to fully inhibit mycelial growth, similar to those of the COS-based nanocomposites with gallic acid, ferulic acid and curcumin in this study. On the other hand, N-(6-carboxyl cyclohex-3-ene carbonyl) chitosan with different degrees of substitution achieved an EC50 of 298 μg·mL−1 for P. infestans [45], better than the ones for the COS composites based on the three aforementioned polyphenols (in the 450–490 μg·mL−1 range). The overall efficacy of the reported nanocomposites should be referred to the combination of the properties afforded by each of its constituents and their synergies. According to Kim et al. [46], nanosilver may exert an antifungal activity by disrupting the structure of the cell membrane and inhibiting the normal budding process due to the destruction of the membrane integrity. Silver nanoparticles antifungal action may also result from the release of silver ions into the intracellular matrix of the pathogen [47]. Reports on the mechanism of inhibitory action of silver ions on microorganisms have shown that upon treatment with Ag+, DNA loses its ability to replicate, resulting in inactivated expression of ribosomal subunit proteins, as well as certain other cellular proteins and enzymes essential to ATP production. It has also been hypothesized that Ag+ would affect the function of membrane-bound enzymes, such as those in the respiratory chain [4]. Apropos of the role of the stevioside, the improvement in the solubility and bioavailability of the polyphenolic compounds should be ascribed to the formation of a nanocomposite structure comprisingPDF Image | Silver Nano Polyphenol Inclusion Compounds Composites
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