Kojic acid: a River Nile-derived fungal secondary metabolite with preservative potential against Listeria monocytogenes in milk

Objective
This study aims to assess the antibacterial potential of kojic acid produced by Trichoderma harzianum, a fungus isolated from the River Nile, and its potential application as a natural food preservative against Listeria monocytogenes.

Methods
A comprehensive chemical analysis of the ethyl acetate extract of T. harzianum was conducted using chromatographic and spectroscopic techniques. The isolated compounds were subsequently evaluated for their antibacterial activity and cytotoxic effects.

Results
Three major secondary metabolites were identified and purified from T. harzianum: kojic acid, di-(2-ethylhexyl) phthalate, and phomaligol A. Among these, only kojic acid demonstrated moderate antibacterial activity against L. monocytogenes, with negligible cytotoxicity against liver and colorectal cancer cell lines (IC₅₀: 41.34 ± 1.7 μM and 17.35 ± 0.8 μM, respectively). In situ studies revealed a dose-dependent antibacterial effect of kojic acid against L. monocytogenes in skimmed milk. Further evaluation across different dairy products indicated that its efficacy was influenced by fat content, leading to a bacterial count reduction of up to 4.17 log units. Additionally, kojic acid effectively inhibited bacterial growth under both low and high contamination levels. Notably, it preserved milk stored at room temperature by preventing bacterial overgrowth for up to four days.

Conclusion
These findings suggest that kojic acid possesses significant potential as a natural milk preservative due to its selective antibacterial activity against L. monocytogenes and favorable safety profile. As a result, kojic acid could serve as a viable alternative to synthetic chemical preservatives in dairy products.