Carnobacterium sp. as probiotics for Atlantic salmon and rainbow trout

The authors of this review look into the effect which Carnobacterium sp. has when used as an additive to the diet of Atlantic salmon and rainbow trout, in order to establish the benefits of the bacteria for use as a probiotic in salmonid aquaculture.

Diets were prepared by adding formulations of fish oil containing live probiotic (Carnobacterium sp.), to specific rainbow trout and Atlantic salmon diets, to create a viable dose equivalent to 5 x 10⁷ cells g^-1 of feed.

To determine the antagonism of fish pathogens to Carnobacterium sp., a cross streaking method was completed at right angles across inocula of the probiotic on TNA plates or amended Cytophaga agar, and incubated at 15-22^oC for 7 days. Antagonism was indicated by overgrowth or interruption in the growth of the pathogen.

The fish experiments were undertaken on rainbow trout and Atlantic salmon which originated from Scottish fish farms as fry (1-2g) and fingerlings (15-20g) were given a 2 week acclimation period before trials began.

Groups of 20 fish from each species were used to examine for any harmful effects of the probiotic. These fish were injected with saline solution containing Carnobacterium sp., and controls injected with pure saline.

Groups of 200 fry and 50 fingerlings were used to assess the ability of the probiotic to colonise the gut tissue.

5 infected fish with either A. salmonicida, V. anguillarum, V. ordalii, or Y. ruckeri were added to experimental groups of 25 fish. The groups were monitored for up to 28 days, during which they were fed control diets devoid of Carnobacterium sp.. Dead fish and survivors were examined for evidence of the disease and presence of the probiotic.

The authors found no evidence to suggest that the Carnobacterium sp. was harmful to the fish. They also discovered that the longevity of Carnobacterium sp. as a probiotic in diets was over 6 months and even had data suggesting that the bacteria were still present after 2 years storage.

The experiments exposed a constant increase in the abundance of probiotic cells in the digestive tract during the trial. However, after the feeding of the fish with the probiotic diet the abundance of Carnobacterium sp. declined rapidly, until extinct from the gut at 6 days.

The percentage survival of the fish in every case of cohabitation with infected fish was improved when fish were administered the probiotic; with the greatest differences shown by the rainbow trout, but none the less great increase in the survival percentage of Atlantic salmon also.

This experiment made apparent the benefits of Carnobacterium sp. to salmonids and suggests its usefulness as a probiotic for use in aquaculture. It does however consider further research around the mechanisms that make it a good probiotic.

Robertson, P.A.W., O’Dowd, C., Burrells, C., Williams, P., Austin, B., 2000. Use of Carnobacterium sp. as a probiotic for Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss, Walbaum). Aquaculture. 185, 235-243.