Among metallic nanoparticles, silver nanoparticles (AgNPs) have numerous applications in the field of nanobiotechnology due to their unique antimicrobial efficiency as growth inhibitors, killing agents or antibiotic carriers (Hamidi et al., 2019). AgNPs have widely attracted attention for the food, cosmetics and biomedical applications (Sondi and Salopek-Sondi, 2004). In the last few years, different chemical and physical methods had been included in AgNPs synthesis. These incorporated methods produced contaminated, toxic AgNPs in low yields. So, scientific researchers went to the biological synthesis of AgNPs using microorganisms (Wang et al., 2019). Through microbial biosynthesis, numerous scientists used bacterial strains in AgNPs biosynthesis due to their rapid growth rate and highly efficient enzymatic system (Galvez et al., 2019). The use of bacterial crude metabolites was embedded as reducing agents of silver ions into safe and ecofriendly AgNPs that called extracellular biosynthesis (De Souza et al., 2019). The extracellular production is more prioritized than the intracellular which requires extraction and purification of AgNPs from the microbial growth. In addition, the extracellular production was confirmed to include high amounts of proteins which acted as capping agents (Annamalai and Nallamuthu, 2016). One of the mechanistic aspects for AgNPs biosynthesis is the secreted enzymes by bacteria that act as reducing agents for silver ions (Quinteros et al., 2016). The shape and size of the biosynthesized nanoparticles (NPs) could be handled throughout controlling the production parameters such as concentration of metal ions, temperature, incubation period, pH and effect of solar irradiation (Sumitha et al., 2019). AgNPs have a strong bactericidal effect against a broad spectrum of bacteria such as Pseudomonas sp., Acinetobacter sp., Escherichia sp., Vibrio sp. and Salmonella sp. (Paul and Londhe, 2019). Furthermore, the biosynthesized AgNPs showed significance antifungal potential against Aspergillus flavus, A. nomius and A. parasiticus, Alternaria alternata, Fusarium sp., Candida tropicalis and C. albicans was reported (Bocate et al., 2019). This makes the AgNPs a potential candidate as a new generation of antifungal agents. The present work aimed to obtain a potent bioreductant bacterium possessing the ability to synthesize AgNPs extracellularly with efficient antimicrobial activity.

The fungus–substrate interaction on the antioxidant activity of a solid state fermentation system (SSFS) was investigated employing two basidiomycete fungi: Pleurotus columbinus and P. floridanus and two ascomycetes: Aspergillus fumigatus and Paecilomyces variotii on powdered peels of banana, pomegranate and orange, empty pea pods and rice straw. The oven-dried substrates were moistened at 90% water holding capacity, inoculated with the test fungi and incubated at 25 °C for appropriate time. Culture extracts were tested for the enzymatic and non-enzymatic antioxidant activity. The effect of substrate on the antioxidant activity of the SSFS was stronger than that of the fungal species. Peroxidase (POX) activity was higher in the basidiomycetes than ascomycetes and achieved its maximum in P. floridanus versus complete absence in A. fumigatus. By contrast, catalase (CAT) activity was higher in the ascomycetes, particularly P. variotii on banana peel, than in the basidiomycetes. Phenolics and flavonoids were highest in pomegranate peels but lowest in banana peels and rice straw, and they were subjected to severe consumption by the basidiomycetes versus mild production by the ascomycetes. The reducing power (RP) and DPPH scavenging activity were higher in the peels of pomegranate, orange and banana relative to rice straw and empty pea pods, and the fungal effect was limited and independent of the fungal taxonomic group. Orange peel is the appropriate substrate for production of fungal CAT and POX, which activities were mutually exclusive; but pomegranate peel is more suitable for production of phenolics and flavonoids.

No abstract

No abstract

No abstract