Synergistic antifungal activity of catechin and silver nanoparticles on Aspergillus niger isolated from coffee seeds

In this work, silver nanoparticles (AgNPs) were synthesized solely using catechin (Ct), a natural polyphenol, then their effects on the growth and metabolism of Aspergillus niger GM13 were investigated using 7.5 and 15 ppm of nanoparticles. AgNPs-Ct, induced physio-metabolic changes activating different stress compensation mechanisms that affect the fungal growth and survival. In particular, 15 ppm of AgNPs-Ct leads to a reduction of 81% of the mycelia biomass. The stress responses in the presence of AgNPs-Ct were further determined by overall cell changes through the production of reactive oxygen species (ROS) by the fungal hyphae. The increase in ROS production confirmed cellular oxidative stress which, together with membrane damage, can be considered to be the main antifungal effect. Adaptative fungal responses during AgNPs-Ct treatment were linked to a high accumulation of trehalose, glycerol, citric and oxalic acid, and several important enzymes involved in the catabolism of nutrients for cell growth, such as acid phosphatase, Naphthol-ASBI phosphohydrolase. The exposure to AgNPs-Ct also triggers changes in the production of VOC derivating from the α- or β-oxidation of fatty acids as alcohols, aldehydes, alkanes, and acids. Herein, the antifungal effect of AgNPs-Ct is demonstrated, elucidating the impact on its physio-metabolic profile.