Antibacterial activity and pH-dependent loss of efficacy of diffusible naphthoquinone-rich pigments secreted by marine filamentous fungus Fusarium sp. SRD3

Naturally occurring fungal metabolites are ideal candidates for combating multidrug-resistant pathogens in clinical settings. However, bacterial susceptibility and conditions affecting the inhibition of bacterial growth remain poorly explored. Here we investigated diffusible naphthoquinone-rich extract (SRD3nQ) originating from a fungal strain SRD3 for antioxidant properties and antibacterial activity toward multiple methicillin-resistant Staphylococcus aureus (MRSA) and drug-resistant Enterococcus faecium , Klebsiella pneumonia , and Acinetobacter baumannii . The sequencing of the D1/D2 region of large subunit (LSU) rRNA gene and associated phylogenetic analysis revealed SRD3 as a strain of Fusarium , which grew well on potato dextrose agar (PDA) in the presence of 0%–10% NaCl, pH 3–10, and at 30°C. SRD3nQ secretion occurred strongly on PDA at pH 3, 0% NaCl, and 6 to12 days of incubation. SRD3nQ exhibited a minimum inhibitory concentration (MIC) of 1.56 μg/mL and fractional inhibitory concentration (FIC) and ∑FIC of 0.125 and 0.61, respectively, in combination with ciprofloxacin, versus Staphylococcus aureus ATCC BAA-44, suggesting a potential additive interaction. Thin-layer chromatography resolved three distinct colored SRD3nQ, designated as lower purple (LP), middle orange (MO), and top purple (TP), with differential antimicrobial and antioxidant activities. The major fraction MO, exhibiting the highest levels of antimicrobial and bactericidal activities, has been putatively annotated as a dimeric naphthoquinone, aurofusarin, through LC–MS/MS analysis, while LP and TP remained unidentified. SRD3nQ retained antibacterial activity following heat treatment, although chemical stability was not directly assessed. SRD3nQ was active against multiple clinical strains of MRSA, particularly under acidic–near-neutral pH. Antibacterial activity was reduced under alkaline conditions, suggesting alkalinity-driven loss of activity. This study provides preliminary evidence that Fusarium sp. SRD3 produces diffusible naphthoquinone metabolites, which are active against drug-resistant pathogens, and further investigation is required to determine the mechanism underlying the pH-dependent activity.