Long-term colonization dynamics of probiotic Aliivibrio spp. in Atlantic Salmon (Salmo salar) following bath administration

Ulcerative conditions present a major challenge in Norwegian salmon farming. Probiotic Aliivibrio species have previously been demonstrated to provide health benefits in both Atlantic salmon and lumpfish. This study aimed to investigate whether these bacteria could colonize Atlantic salmon following bath administration, determine the tissue tropism, and assess the duration of colonization. We examined the host microbiota using culture-based methods, qPCR and immunohistochemistry techniques specifically designed to target the applied Aliivibrio strains. Our findings reveal that the probiotic bacteria can successfully colonize Atlantic salmon and persist for at least nine months post-administration. We identified the administered strains in the skin and underlying tissue with all three methods. The probiotics were also identified in the distal intestine and the visceral organs. Additionally, we isolated the probiotic Aliivibrio species from mixed cultures in ulcerated areas. While viable bacteria were recoverable from recently euthanized fish, tissue decay promoted bacterial recovery of the administered species across all experiments. Given prior evidence on ulcer reduction associated with these probiotics, competitive exclusion appears to be a plausible mechanism of action, though further investigation is warranted. • Long-time colonization of two probiotic Aliivibrio species previously documented to reduce ulcer prevalence. • Colonization of several ecological niches within Atlantic salmon, including ulcers. • Atlantic salmon can also be naturally colonized in seawater. • Transient microbiota may affect the degree of colonization.