A pilot study on the effects of in-feed probiotic Lactobacillus rhamnosus ATCC 53103 (LGG) on vaccinated Atlantic salmon (Salmo salar): microbiomes and Aeromonas salmonicida challenge resilience.

The use of probiotics is an alternative approach to mitigate the proliferation of antimicrobial resistance in aquaculture. In our study, we examined the effects of Lactobacillus rhamnosus GG (ATCC 53103, LGG) delivered in-feed on the weight, length, skin mucus, and faecal microbiomes of Atlantic salmon. We also challenged the salmon with Aeromonas salmonicida 2004-05MF26 (Asal2004) and assessed the mortality. Our results showed no significant change (P > 0.05) in weight or length of Atlantic salmon or their resilience to Asal2004 infection after LGG feeding. Infection changed significantly the skin mucus and faecal microbiomes: Clostridium sensu stricto increased from 3.14% to 9.20% in skin mucus and 1.39% to 3.74% in faeces (P < 0.05). Aeromonas increased from 0.02% to 0.60% in faeces (P < 0.05). Photobacterium increased from not detected (0%) to 52.16% (P < 0.01) and Aliivibrio decreased from 67.21% to 0.71% in faeces (P < 0.01). After infection, Lactococcus (9.93%) and Lactobacillus (2.11%) in skin mucus of the LGG group were significantly higher (P < 0.05) than in the skin mucus from the rest of the groups (4.14% and 1.08%, respectively). In conclusion, LGG feeding did not further increase the resilience of vaccinated Atlantic salmon. Asal2004 infection had much greater impact on skin mucus and faecal microbiomes than LGG feeding.