Screening of local Bacillus thuringiensis isolates for melanin synthesis and enhanced UV resistance

Bacillus thuringiensis (Bt) is a Gram-positive, spore-forming bacterium widely applied as a microbial pesticide because of its insecticidal δ-endotoxins. These toxins, produced in crystalline protoxin form during sporulation, are activated in the insect gut, bind to midgut receptors, and cause larval mortality. Despite its effectiveness and environmental safety, the field performance of Bt-based biopesticides is limited due to rapid inactivation under sunlight, particularly ultraviolet (UV) radiation. Melanin, a dark polymeric pigment generated by the oxidation of L-tyrosine through a tyrosinase-mediated pathway, exhibits strong UV absorption and photoprotective functions. In this study, six Bt isolates, were screened for their ability to synthesize melanin under high-temperature induction (42 °C) with L-tyrosine supplementation. Pigment yield, cell density, and spore counts were quantified to evaluate strain performance. Among the isolates, SMa-1 exhibited the highest melanin yield (37.13 mg/L), along with superior cell and spore counts, identifying it as the most promising strain. The findings indicate that melanin biosynthesis provides an intrinsic strategy to improve UV tolerance and field persistence of Bt formulations, contributing to the development of more reliable and cost-effective crop protection solutions.