Bio-induced overproduction of heterocycloanthracin-like bacteriocin in Lysinibacillus macroides by Aspergillus austroafricanus: optimization of medium conditions and evaluation of potential applications.

Abstract

Background: Plants and microorganisms are at the forefront of natural exploitable bioresources for the discovery of novel bioactive compounds (BACs) to provide solutions to food and agricultural challenges. The present study aimed to produce a novel biotechnologically-relevant BAC from a mangrove sediment bacterium under optimized bioprocess medium conditions. The BAC-producing bacteria were isolated via the crowded plate technique, and medium optimization was performed via sequential statistics of response surface methodology (RSM). The RSM model predictions were optimized, validated, and scaled up in a 5-L bioreactor via submerged batch fermentation. The BAC was extracted with ethyl acetate, purified via silica gel column chromatography, and identified via semipreparative high-performance liquid chromatography using bioactive standards with known retention times. The biocontrol, antioxidant and biopreservation potential of the BAC were evaluated via standard methods.

Results: The results revealed that strain GKRMS-A9 produced the largest inhibition zone diameter (ZND) of 17 mm against the susceptible mould. The bacterium and its susceptible mould were identified as Lysinibacillus macroides and Aspergillus austroafricanus strains, respectively. Bioprocess medium optimization produced 9.6 g L^- 1 of the BAC with a ZND of 47.1 mm using 44.84% [v v^- 1] rice processing effluent, 8.58 gL^- 1 casamino acid, 1.39 g L^- 1 MgSO₄.7H₂O, 2.78 g L^- 1 CaCl₂.2H₂O, 16.94% [v v^- 1] inoculum volume, and 10.45 g L^- 1 Na₂HPO₄/NaH₂PO₄. The BAC concentration increased 48.7-fold in response to biological induction with susceptible mould. Silica gel chromatography revealed 9 bioactive fractions in the ethyl acetate extract, with fraction C (retention time of 9.02 min) eliciting the largest mean ZND of 38.1 ± 1.7 mm against Aspergillus austroafricanus. Fraction C was identified as a heterocycloanthracin-like class II bacteriocin with a molecular weight of 10.5 kDa.

Conclusion: The bacteriocin 'macroidin' is stable over a wide range of pH values and temperatures and has significant antimicrobial activity against Gram-positive food-borne and phytopathogenic strains of bacteria and moulds. Its antioxidant activities against DPPH and ABTS*⁺ radicals are comparable to those of ascorbic acid, making this biomolecule a promising agent for biopreservation and phytopathogen control applications in the food and agricultural sectors.