Antibacterial Activity of the B Subunit of <scp> PirAB ^<i>vp</i> </scp> Toxin Secreted by <i>Vibrio parahaemolyticus</i> Causing Acute Hepatopancreatic Necrosis Disease in Shrimp

In vitro bacterial competition assays were performed to evaluate the ecological and antibacterial role of the PirAB^vp toxin complex produced by Vibrio parahaemolyticus Vp278. When co-cultured with V. cholerae, the PirAB^vp 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 PirB^vp levels during competition, reaching up to 350 ng/mL, while non-producer strains showed no toxin production. Recombinant PirB^vp 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 PirB^vp 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 PirB^vp contributes to interspecific competition and ecological dominance of V. parahaemolyticus through direct, lectin-mediated antibacterial activity.