Can an entomopathogenic nematode serve, as proxy for strongyles, in assessing the anthelmintic effects of phenolic compounds?

As gastro-intestinal nematodes (GINs) become increasingly resistant to chemical anthelmintics, and because consumers scrutinize chemical residues in animal products, the use of herbal anthelmintics and in particular, phenolic compounds, has become attractive. Most life stages of GINs cannot be grown in the lab as they are obligatory parasites, which limits our understanding of the effects of phenolic compounds on their parasitic stages of life. We hypothesized that a species phylogenetically close to GINs and grown in vitro, the insect-parasitic nematode Heterorhabditis bacteriophora (Rhabditida; Heterorhabditiade), when fed with Photorhabdus luminescens exposed to plant phenolics, can serve, as proxy for strongyles, in assessing the anthelmintic effects of phenolic compounds. We compared the development of H. bacteriophora infective juveniles (IJ) and the exsheathment rate of L3 larvae of the strongyle Teladorsagia circumcincta and Trichostrongylus colubriformis when exposed to catechin, rutin, chlorogenic and gallic acids, and myricetin. Gallic acid had the highest impact in terms of IJ mortality but the highest impairment of IJ development to adulthood was imposed by myricetin. The studied compounds were not lethal to GINs stricto sensu but we consider that the practical implications of total exsheathment inhibition and mortality on GIN populations are similar. Catechin and rutin had similar effects on rhabditid and strongyles: they imposed ca. 90% lethality of IJs at concentrations higher than 1200 ppm and the remaining live IJs did not develop further, and they also totally inhibited strongyle L3 exsheathment in a dose-response fashion. Gallic acid was 100% lethal to IJs exposed above 300 ppm and chlorogenic acid caused 87% mortality above 1200 ppm, with no development for the surviving IJs but for all lower concentrations, all the IJs developed to adult stages. Likewise, gallic and chlorogenic acids did not affect the exsheatment of GIN L3 larvae. Therefore, a discrepancy between the effects of gallic and chlorogenic acids on the development of rhabditid IJs and exsheathment of GIN L3 larvae was found only when they were exposed to high concentrations. A dose-response of IJ lethality to myricetin was found, with no IJ development between 150 and 2400 ppm; but contrary to the other compounds, myricetin also impaired IJ development of IJs above 10 ppm in a dose-response manner and showed dose-responses in the L3 exsheathment. Apart for the high rates of lethality imposed on IJs by gallic and chlorogenic acids at high concentration, these results suggest that H. bacteriophora fed P. luminescens exposed to phenolics shows potential to serve as model in studies of the anthelmintic effects of phenolics in GIN.

Toxicity of phenolic compounds to entomopathogenic nematodes: A case study with Heterorhabditis bacteriophora exposed to lentisk (Pistacia lentiscus) extracts and their chemical components.

Insects show adaptive plasticity by ingesting plant secondary compounds, such as phenolic compounds, that are noxious to parasites. This work examined whether exposure to phenolic compounds affects the development of insect parasitic nematodes. As a model system for parasitic life cycle, we used Heterorhabditis bacteriophora (Rhabditida; Heterorhabditiade) grown with Photorhabdita luminescens supplemented with different concentrations of plant phenolic extracts (0, 600, 1200, 2400 ppm): a crude ethanol extract of lentisk (Pistacia lentiscus) or lentisk extract fractionated along a scale of hydrophobicity with hexane, chloroform and ethyl acetate; and flavonoids (myricetin, catechin), flavanol-glycoside (rutin) or phenolic acids (chlorogenic and gallic acids). Resilience of the nematode to phenolic compounds was stage-dependent, with younger growth stages exhibiting less resilience than older growth stages (i.e., eggs < young juveniles < young hermaphrodites < infective juveniles < mature hermaphrodites). At high concentrations, all of the phenolic compounds studied were lethal to eggs and young juveniles. The nematodes were able to survive in the presence of medium and low concentrations of all studied compounds, but very few of those treatments allowed for reproduction beyond the infective juvenile stage and, at low concentrations, the crude 70% ethanol extract, chloroform and hexane extracts, and myricetin were associated with some impaired reproduction. The ethyl-acetate fraction and gallic acid were extremely lethal to the young stages and allowed almost no development beyond the infective juvenile stage. We conclude that exposure of infective juveniles to phenolics before they infect insects and post-infection exposure of other nematode developmental stages may affect the initiation of the infection, suggesting that the chemistry of dietary phenolics may limit H. bacteriophora's infection of insects.

Probing binding-mode diversity in guanidinium-oxoanion host-guest systems.

An attempt to experimentally estimate the role of binding-mode diversity (structural fuzziness) on the molecular recognition seen in the prominent guanidinium-oxoanion host-guest pair is described. The global heat response as measured by isothermal titration calorimetry in acetonitrile, which was obtained from the interaction of five different but structurally closely related guanidinium hosts with three rigid phosphinate guests of decreasing accessibility of their binding sites, is correlated to provide a trend analysis. All host-guest associations of 1:1 stoichiometry in this series are strongly enthalpy-driven. The change in complexation entropy can be related to the tightness of the mutual fit of the host-guest partners, which approaches a minimum limit and is interpreted as the unique lock-and-key binding mode. The ordinary host-guest complexation in this ensemble features substantial positive entropy changes that correlate inversely with the binding interface area. This finding excludes desolvation effects as the major cause of entropy production, and provides evidence for the existence of a broad variety of complex configurations rather than a single binding mode to represent the associated host-guest pair. This result bears on the molecular design of systems that vitally depend on structural fidelity, such as nanoassemblies or homogeneous catalysis.