Probiotic bacteria are a recognized alternative to classical methods of disease prophylaxis and therapy. We tested the effects of their application on the immune reaction in juvenile turbot. To prevent digestion of the probiotics, rectal administration was applied to maximise colonization, by-passing digestion in the stomach. The application of Psychrobacter nivimaris and Psychrobacter faecalis showed beneficial effects on the inflammatory response and disease resistance after infection with the common pathogen Tenacibaculum maritimum. Treatment with P. nivimaris and P. faecalis resulted in 0% and 8% mortalities post-infection, while in the treatment control, an elevated mortality of 20% was observed. In the challenge controls (no infection), no mortalities were observed during the entire experimental period. After an experimental infection, mRNA expression of selected immune markers (mhc II α, il-1β, tcr, tgf β and tnf α) were determined by RT-QPCR at 0, 1 and 5 days post-infection (dpi). At 0 dpi, gene expression was comparable between the treatments and the treatment control, suggesting that probiotics did not act via immune stimulation of the host. At 1 dpi, all genes were up-regulated in the treatment control but not in the probiotic groups, indicating that the infection in probiotic-treated fish developed at a less severe level. At 5 dpi, mRNA expression returned to baseline levels. As a conclusion, the native probiotic candidates P. nivimaris and P. faecalis improved survival, whereas, in the control, mortality increased and expression of the immune markers was up-regulated post infection. This highlights a potential application of P. nivimaris and P. faecalis in disease prophylaxis, but further research is needed.

Probiotic bacteria have been widely studied in the past few decades as an efficient alternative to the existing methods of preventing disease outbreaks. Most commercial probiotics used in animal husbandry are non-native species that may fail to colonize the target habitat of the host. After rectal administration to maximise colonization by-passing digestion in the stomach, the beneficial effects of two native probiotics Psychrobacter nivimaris and Psychrobacter faecalis on the immune status, inflammatory response, and disease resistance against Tenacibaculum maritimum were studied in juvenile turbot. After an experimental infection with T. maritimum (bath of 2.65 × 107 CFU mL− 1), selected immune markers (mhc II α, il-1β, tcr, tgf β and tnf α) were determined by RT-QPCR at 0, 1 and 5 days post infection (dpi). Treatment with P. nivimaris and P. faecalis resulted in 0% and 8% mortalities (n = 3). In the treatment control, an elevated mortality of 20% was observed. In the challenge controls (no infection), no mortalities were observed during the experimental period. At 0 dpi, gene expression was comparable between the treatments and the treatment control, suggesting that probiotics did not act via immune stimulation of the host but rather via competitive exclusion of the pathogen. At 1 dpi, gene expression of all genes was up-regulated only in the treatment control, showing that the infection in probiotic-supplemented fish progressed in a milder way. At 5 dpi, gene expression returned to baseline levels, and no significant differences were observed between groups. After 1 dpi, no mortalities were observed. As a conclusion, the native probiotic candidates P. nivimaris and P. faecalis improved survival, whereas, in the control, mortality increased and expression of selected immune markers was up-regulated upon experimental infection with T. maritimum. This highlights a potential application of P. nivimaris and P. faecalis in disease prophylaxis.