Approaches to the mechanism of antifungal activity of Zuccagnia punctata-Larrea nitida bi-herbal combination

[Background] In our previous study the synergism of four combinations of Zuccagnia punctata (ZpE) and Larrea nitida (LnE) exudates with the reliable statistical-based MixLow method was assessed, and the markers of the most anti-C. albicans synergistic ZpE-LnE bi-herbal combination were quantified according to European Medicines Agency (EMA).[Purpose] To study the mechanisms of action as well as the cytotoxic properties of the ZpE-LnE most synergistic combination found in the previous work.[Materials and methods] Minimum Fungicidal Concentration (MFC) and rate of killing of ZpE-LnE were assessed with the microbroth dilution and the time-kill assays respectively. Morphological alterations were observed with both confocal and fluorescence microscopy on the yeast Schizosaccharomyces pombe. The ergosterol exogenous assay, the quantification of ergosterol, the sorbitol as well as glucan synthase (GS) and chitin synthase (ChS) assays were used to detect the effects on the fungal membrane and cell wall respectively. The capacity of ZpE-LnE of inhibiting Candida virulence factors was assessed with previously reported methods. The effect of ZpE-LnE and of ZpE or LnE alone on cell viability was determined on human hepatoma cells line Huh7.[Results] ZpE-Ln E was fungicidal killing C. albicans in a shorter time than amphotericin B and produced malformations in S. pombe cells. ZpE-LnE showed to bind to ergosterol but not to inhibit any step of the ergosterol biosynthesis. ZpE-LnE showed a low or moderate capacity of inhibiting GS and ChS. Regarding inhibition of virulence factors, ZpE-LnE significantly decreased the capacity of adhesion to eukaryotic buccal epithelial cells (BECs), did not inhibit the germ tube formation and inhibited the secretion of phospholipases and proteinases but not of haemolysins. ZpE-LnE demonstrated very low toxicity on Huh7 cells, much lower than that each extract alone.[Conclusion] The fungicidal properties of ZpE-LnE against C. albicans, its dual mechanism of action targeting the fungal membrane's ergosterol as well as the cell wall, its capacity of inhibiting several important virulence factors added to its low toxicity, make ZpE-LnE a good candidate for the development of a new antifungal bi-Herbal Medicinal Product.S.Z. and M.S. acknowledge Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT), PICT2014-1170 and PICT2016-1833 and National University of Rosario, Argentina (UNR) for funds. J.C.R. acknowledges MINECO (BIO2015-69958-P) and Junta de Castilla y León (CSI068P17), Spain for funds. A.D.Q. acknowledges ANPCyT (PICT2013-1440) and UNR for funds. E.B. acknowledges Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) for a doctoral fellowship. M.S., L.S. and A.D.Q. are members of the CONICET Researcher career; M.S., L.S., and E.B. belong to the teaching staff of Pharmacognosy area and M.S. belongs to the teaching staff of Mycological Area Faculty of Biochemical and Pharmaceutical Sciences, UNR.Peer reviewe