Lectin-Producing Bacteria from Silkworm (Bombyx mori) Gut Microbiota: Isolation, Characterization, and Biofunctional Analysis

Lectins are carbohydrate-binding proteins with antimicrobial, antioxidant, and immunomodulatory properties. Microbial lectins are increasingly recognized as sustainable substitutes for plant-derived lectins because of their straightforward culture and scalable synthesis; yet lectin-producing bacteria inside the gut microbiome of insects, especially in the silkworm (Bombyx mori), remain largely unexplored. This study aimed to isolate, characterize, and evaluate the bio-functional properties of lectin-producing bacteria from the gut of fifth-instar B. mori larvae. Gut-associated isolates were evaluated for lectin production through a hemagglutination assay, resulting in two lectin-positive isolates identified by 16S rRNA sequencing as Enterococcus faecium NBRC 100486 and Enterococcus faecium DSM 20477, both exhibiting 99.13% similarity to reference strains. The partially purified lectins were obtained by ammonium sulfate precipitation, with the best yield at 65% saturation, at protein concentrations of 2.69 mg/mL and 2.08 mg/mL. Hemagglutination activity was highest at 25 °C and decreased at higher temperatures, suggesting intermediate thermal stability. The carbohydrate inhibition assay showed high carbohydrate specificity, with the highest inhibition by lactose, followed by sucrose, glucose, and fructose. Crude extracts had no antibacterial action alone, but metal ions considerably boosted it, especially against Bacillus subtilis, suggesting a metal-dependent change of lectin function. The lectins demonstrated significant antioxidant activity, with DPPH radical-scavenging efficiencies of 68.8% and 73.8%. The results obtained in this study have shown that E. faecium isolated from the gut of silkworms can be a potential source of bioactive lectins that show carbohydrate-binding properties, antioxidants, and metal-enhanced antibacterial properties relevant to biomedical and biotechnological applications.