Nitrite, a common pollutant from nitrogenous waste in fish culture, compromises fish nasal and gut microbiomes, increasing susceptibility to nasal pathogens. Also, nitrite is linked to an increase in mortality by Edwardsiella ictaluri infections in catfish intensive farming. The nose and gut are the two main portals for E. ictaluri that causes enteric septicemia of catfish (ESC) by Edwardsiella ictaluri infections. Thus, we hypothesize that nitrite compromises the catfish nasal and gut microbiomes increasing susceptibility to nasal pathogen infections.
To test our hypothesis, Channel catfish (Ictalurus punctatus ) were exposed to 0mM (control), 0.2 mM and 2mM concentrations of nitrite. Catfish were kept for 30 days in 500 L tanks per treatment, with three replicate tanks per treatment containing nine fish per tank. Exposure to 2mM sodium nitrite was 90% lethal within 24 hours of exposure. A subsample of fish from the 0mM and 0.2mM treatments were obtained at day 0, 5 and 30. The surviving fish from 2mM treatment were sampled after 24 hours. All fish were sampled for nose, gut, muscle, gill, brain, blood, liver and kidney for either biochemical, microbial or histological analysis. Fish exposed to 0.2 mM and 2mM accumulated nitrites in several tissues with values higher from those in water. Fish from the 2 mM exhibited a marked methemoglobinemia due to nitrite-induced oxidation of hemoglobin. Histological analysis showed an oxidative damage in all organs of individuals exposed to 2 mM and a moderate effect in the nose, gill and gut of fish from the 0.2 mM treatment. Nose and gut microbiomes will be sequenced using whole genome shotgun sequencing to determine the impacts of nitrite on the microbiome. It is expected to see a degeneration of the olfactory and gut mucosa, coupled with a decreased nasal and digestive microbiome.
Our long term goal of this study is to develop nasal probiotics that can decrease the negative impacts from nitrite which would lead to better survival from infections. Future experiments will include identifying potential nasal and gut probiotic strains with protective effect against E. ictaluri and testing the efficacy in catfish chronically exposed to nitrite. Additionally, loss of olfactory function and bacteria detection will be screened by electro-olfactograms. Combining microbiological, electrophysiological, neurological and clinical approaches, our experiments will reveal nasal mucosa as a target of fish wellbeing. The outcomes of this proposal will produce novel probiotic treatments and will describe delivery pathways for bacterial infections in fish.