L-amino acids modulate biofilm formation, UV resistance, and insecticidal activity in Bacillus thuringiensis formulations: Mechanisms and optimization

Enhancing the field persistence of Bacillus thuringiensis (Bt) biopesticides is crucial for efficient pest management. This study presents the first systematic investigation into the mechanisms by which all 20 L-amino acids influence Bt biofilm formation, ultraviolet (UV) resistance, and insecticidal activity. The results demonstrate that amino acids functioned as interfacial modulators: non-polar hydrophobic types (e.g., L-leucine) significantly increased biofilm biomass in wettable powder through hydrophobic adhesion, whereas basic types (e.g., L-arginine, l-lysine) enhanced the stability of aqueous suspension concentrate via electrostatic stabilization. Through statistical optimization (Plackett-Burman design and response surface methodology), a synergistic formulation composed of L-arginine, l-lysine, and L-leucine was identified. This formulation increased the biofilm yield of the commercial Kernel 32,000 IU/mg wettable powder by 3.8-fold and improved survival rate by approximately 50 % after 4 h of UV irradiation. At the physio-toxicological level, this amino acid combination reduced the LC50 against second-instar Plutella xylostella larvae by 8.12-fold. Transmission electron microscopy revealed complete microvilli shedding and mitochondrial cristae breakage in the treated midgut, indicating that the synergistic mechanism stems from the disruption of the midgut epithelial barrier. This study provides the first direct link between amino-acid-mediated interfacial interactions and midgut pathological damage, offering molecular–physiological evidence for next-generation, low-dose, highly stable Bt formulation with manageable resistance risks.