Abstract: Contact-dependent inhibition plays an important role in bacterial competition and host colonization. However, its application in the control of fish bacterial pathogens and the underlying molecular mechanisms remain poorly understood. This study aimed to evaluate the contact-dependent inhibitory activity of Pseudomonas mosselii (P. mosselii) against Aeromonas veronii (A. veronii) and its protective effect on the host, as well as to preliminarily investigate the molecular basis of this antagonistic activity. Bacterial competition assays were performed to assess the contact-dependent inhibitory activity of P. mosselii against Escherichia coli (E. coli) and A. veronii. A zebrafish infection model was employed to evaluate the protective effect of P. mosselii against A. veronii infection. Furthermore, multiple mutants lacking different virulence secretion systems were constructed by using homologous recombination technology for analyzing the role of these secretion systems in contact-dependent inhibition. The results showed that P. mosselii exhibited significant contact-dependent inhibitory activity against both E. coli and A. veronii. In the zebrafish infection model, P. mosselii significantly increased host survival following A. veronii infection. When E. coli was used as the target strain, type VI secretion system (T6SS) played an important role in contact-dependent inhibition. In contrast, when A. veronii was used as the target strain, no significant difference in contact-dependent inhibitory activity was observed between the wild-type strain and mutants lacking type I～VI virulence secretion systems. This study provides an experimental basis for the development of a new strategy for controlling fish bacterial pathogens based on contact-dependent antibacterial activity.
Keywords: Pseudomonas mosselii; Aeromonas veronii; contact-dependent antibacterial activity; virulence secretion systems; gene knockout
（Acta Laser Biology Sinica, 2026, 35(4): 319-325）