Production of Antimicrobial and Antioxidant Metabolites by Penicillium crustosum Using Lemon Peel as a Co-Substrate in Submerged Fermentation

Fungal secondary metabolites are valuable sources of natural antioxidants and antimicrobials. This study evaluated the submerged fermentation of Penicillium crustosum OR889307 supplemented with lemon peel as a co-substrate to enhance the production of bioactive compounds. Lemon peel was selected for its phenolic precursors and sustainable availability as an agro-industrial byproduct. Crude extracts, aqueous and organic fractions, and molecular-weight partitions were assessed for antioxidant activity using the DPPH assay and for antimicrobial activity against Escherichia coli, Staphylococcus aureus, methicillin-resistant S. aureus (MRSA), Pseudomonas aeruginosa, and Candida albicans. Semi-purified extracts from co-substrate fermentations exhibited enhanced bioactivity, showing MIC values of 185 µg/mL against P. aeruginosa and 225 µg/mL against MRSA, along with strong ABTS radical-scavenging capacity (238.95 ± 2.17 µmol TE). RP-HPLC-ESI-MS profiling revealed phenolic acids, flavanones, flavonols, and lignans, including ferulic acid 4-O-glucoside, bisdemethoxycurcumin, secoisolariciresinol, and quercetin 3-O-xylosyl-glucuronide. These findings demonstrate that lemon peel supplementation promotes the biosynthesis of antimicrobial and antioxidant metabolites by P. crustosum. This approach supports sustainable agro-waste valorization and offers a promising strategy for obtaining natural bioactive compounds with potential applications in food preservation and health-related formulations.