ACTIVITY OF PROBIOTIC Paenibacillus BACTERIA AND THEIR ANTIMICROBIALS AGAINST SALMON PATHOGENS  

Vaccination and improved qualities of water and diet have reduced but not eliminated the need for antibiotics in salmon aquaculture. To help further reduce antibiotic use, this research aims to evaluate the activity of probiotic bacteria and the antimicrobials they produce against salmon pathogens, and their application as feed additives to prevent disease in salmon aquaculture. The present study focuses on two candidate probiotic species of interest: the Gram-positive spore-forming bacteria Paenibacillus polymyxa and Paenibacillus terrae. P. polymyxa strain NRRL B-30507 produces the antimicrobial lipopeptides polymyxin B and tridecaptin B1, and P. terrae strain NRRL B-30644 produces the lantibiotic paenicidin B and the lipopeptide tridecaptin A1.

We have performed in vitro tests with the live probiotic organisms, with the antimicrobials harvested from the culture supernatant of these organisms, and with synthetic or purified forms of tridecaptin A1 and polymyxin B. These tests have shown antimicrobial activity against the salmon pathogens Aeromonas salmonicida, Vibrio anguillarum, Yersinia ruckeri, Flavobacterium psychrophilum, and Renibacterium salmoninarum.

In preparation for in vivo studies, we have determined that the number of viable bacteria in feed treated with P. polymyxa or P. terrae is relatively stable over time when the feed is stored at 4°C. During short feeding trials in several species of Atlantic and Pacific salmon, no differences in feeding behavior were observed between control feeds and feeds treated with live probiotic organisms or synthetic tridecaptin A1. No morbidity or mortality was observed in any treatment group and probiotic bacteria were absent from kidney cultures in fish fed with the live probiotic diets. These results indicate no evidence of palatability or safety concerns when using these organisms or their antimicrobials as feed additives.

We are currently performing in vivo studies to investigate the therapeutic potential of P. polymyxa and P. terrae. In these studies, salmon are being fed with diets containing live probiotic bacteria, culture supernatants, or synthetic variants of the identified antimicrobials to assess their impact on fish health. The effect of these dietary treatments on mortality following exposure to virulent V. anguillarum will be tested to determine their efficacy in disease prevention. Furthermore, effects on intestinal microbiota composition, innate immune parameters, and gene expression in intestinal and immune-relevant tissues will be investigated to provide insight into possible mechanisms of action of the probiotic organisms and the antimicrobials they produce.