Condensed tannins act as anthelmintics by increasing the rigidity of the nematode cuticle.

Tannins and tanniferous plant extracts have been discussed as sustainable means for helminth control in the past two decades in response to a dramatic increase of resistances towards standard anthelmintics. While their bioactivities have been broadly investigated in vitro and in vivo, less is known about their mode of action in nematodes, apart from their protein binding properties. In the current study we therefore investigated the impact of a phytochemically well characterized plant extract from Combretum mucronatum, known to contain procyanidins as the active compounds, on the model organism Caenorhabditis elegans. By different microscopic techniques, the cuticle was identified as the main binding site for tannins, whereas underlying tissues did not seem to be affected. In addition to disruptions of the cuticle structure, molting defects occurred at all larval stages. Finally, an increased rigidity of the nematodes' cuticle due to binding of tannins was confirmed by force spectroscopic measurements. This could be a key finding to explain several anthelmintic activities reported for tannins, especially impairment of molting or exsheathment as well as locomotion.