Largemouth bass (Micropterus nigicans) gut microbiota in response to immersion vaccination.

Recent data has indicated increased largemouth bass (LMB; Micropterus nigricans) production as culturists aim to capitalize on the high market value. As one of the leading causes of mortality in warmwater culture, motile Aeromonas septicemia (MAS; including Aeromonas hydrophila and Aeromonas veronii) poses a significant threat to production facilities. Three experimental vaccination periods of 0-, 2-, and 4-months were studied to determine the optimal timeframe for vaccinating LMB fingerlings. At time 0 fish were 5 g. For each experimental time point, formalin-killed cultures of A. veronii (ARS-LMB-32-2018) and A. hydrophila (ARS-LMB-2022-09) were administered via a 30-minute static immersion bath. The LMB were monitored for 30 days post-vaccination (DPV) to allow the specific immune response to be generated. Thereafter, distal intestines were sampled at 0-, 15-, and 30 DPV to characterize microbial gut communities. At 30 DPV, LMB were challenged using an intraperitoneal injection or a standardized fin-clip methodology and subjected to a 1 h static bath at 2x107 CFU mL-1. Cumulative percent mortality was monitored incrementally through the 7-day challenge duration. Alpha- and beta-diversity indices also showed no negative impact on microbial communities between vaccine treatments, while days post-vaccination showed changing communities within sham and Aeromonas spp. vaccinations. With this long-term study, we aimed to improve the understanding of LMB fingerling response to formalin-killed immersion bath vaccinations of A. veronii & A. hydrophila and the potential for mortality reduction.