Inorganic nitrite increases the susceptibility of tilapia (Oreochromis niloticus) leucocytes to Streptococcus agalactiae.

Deteriorating water quality, especially from high concentrations of nitrite, is currently largely blamed for disease outbreaks in farmed tilapia (Oreochromis niloticus). In this study, the underlying mechanism of nitrite on the susceptibility of tilapia leucocytes to Streptococcus agalactiae (S. agalactiae) was studied. We found that a high dose of heat-killed S. agalactiae decreased tilapia leucocytes cell viability, whereas nitrite decreased the cell viability of leucocytes exposed to a low dose of bacteria. Bacterial challenge increased the production of nitric oxide (NO), whereas nitrite and bacteria coexposure caused higher NO production than nitrite or bacterial exposure alone. Cell viability increased after elimination of NO, and negative correlations existed between cell viability and the NO content, suggesting that nitrite increased the susceptibility of the leucocytes against S. agalactiae was NO-dependent. For a more comprehensive understanding of the mechanism of nitrite affecting disease resistance in tilapia leucocytes, an RNA-Seq-based transcriptome was generated. The results showed that 6173 transcripts were differently expressed, and the differentially expressed transcripts (DETs) of the bacterial group, nitrite group and bacteria-nitrite co-treatment group compared to the control group were selected for GO and KEGG analyses. The DETs in the bacterial group and bacteria-nitrite cotreatment group were highly involved with the membrane component, signal transduction, and immune responses. KEGG analysis showed that the protein processing in the endoplasmic reticulum and the AMPK signaling pathway, which are related to autophagy, were significantly enriched in the cotreatment group but not in bacterial group. In addition, the mRNA expression of ten DETs and several autophagy and apoptosis related genes validated by q-PCR showed the high reliability of the RNA-seq. Taken together, the results of this study suggest that nitrite may increase the susceptibility of tilapia leucocytes to S. agalactiae by generating excess NO to affect the autophagy and apoptosis process.