The fermentative product from Bacillus siamensis HU603 (6'-phospho macrolactin B) inhibits Micropterus salmoides rhabdovirus infection in vitro and in vivo.

Micropterus salmoides rhabdovirus (MSRV) poses a substantial challenge to the health and sustainability of largemouth bass aquaculture. It causes huge economic losses, particularly in juvenile fish. However, there are still no effective therapeutic strategies. Here, we evaluated the anti-MSRV activity of 6'-phospho macrolactin B (23 TH), a phosphorylated metabolite from Bacillus siamensis HU603 fermentation. In Epithelioma papulosum cyprini (EPC) cells, 23 TH exhibited low cytotoxicity and, at 40 mg/L, engendered a remarkable 98.11% inhibition of the MSRV nucleoprotein (N) expression. Further studies showed that 23 TH effectively suppressed virus-induced cytopathic effects and attenuated virus-induced apoptosis by preserving nuclear integrity and mitochondrial membrane potential. Mechanistically, 23 TH directly reduced viral infectivity following pre-incubation with MSRV, significantly inhibiting viral binding and internalization. Time-of-addition and removal assays showed that 23 TH acts mainly during the early stage of viral replication. In vivo, 23 TH treatment significantly improved survival rates of MSRV-infected fish (up to 76%), reduced viral load, and alleviated histopathological damage. Additionally, 23 TH markedly up-regulated antiviral-related genes level, including interferon-γ (IFN-γ), interferon regulatory factor 3 (IRF3), and IRF7, by 5.38-, 7.37-, and 6.05-fold, respectively. In summary, these findings confirm the antiviral potential of 23 TH and highlight its application as a promising therapeutic agent for controlling MSRV infection in aquaculture.