Antibacterial Activity of the B Subunit of PirAB vp Toxin Secreted by Vibrio parahaemolyticus Causing Acute Hepatopancreatic Necrosis Disease in Shrimp

In vitro bacterial competition assays were performed to evaluate the ecological and antibacterial role of the PirABvp toxin complex produced by Vibrio parahaemolyticus Vp278. When co-cultured with V. cholerae, the PirABvp producing strains exhibited competitive dominance, eliminating V. cholerae after 19-24 h, whereas non-producing strains coexisted with V. cholerae throughout the incubation period. Quantification and immunodetection analyses confirmed a time-dependent increase in PirBvp levels during competition, reaching up to 350 ng/mL, while non-producer strains showed no toxin production. Recombinant PirBvp displayed direct antibacterial activity, causing up to 66.6% reduction in V. cholerae growth and a 5.2-log decrease in CFU at 20 μg/mL. Similarly, A. fischeri exhibited dose-dependent inhibition of bioluminescence and viability, with cell counts reduced from 3.0 × 10⁹ to 5.2 × 10⁵ CFU/mL at 20 μg/mL. The inhibitory effect was markedly reduced when PirBvp was preincubated with IPTG and the glycoprotein Fetuin, suggesting that the jacalin-like lectin domain, which binds to amino sugars in the bacterial membrane, is essential for its antibacterial action. These findings demonstrate that PirBvp contributes to interspecific competition and ecological dominance of V. parahaemolyticus through direct, lectin-mediated antibacterial activity.