Use of Enterococcus faecium to improve common sole (Solea solea) larviculture

The potential of common sole Solea solea (Linnaeus, 1758) for consistent production in recirculating systems is limited by the huge losses usually observed during the early stages of larval development, mainly due to pathogen infections and insufficient nutritional balance. In the hope that use of a suitable probiotic strain may counter these difficulties, in this study we isolated Enterococcus faecium IMC 511 from common sole brood stock and utilized it as probiotic candidate during fish feeding. After a daily administration (twice a day) to sole larvae through live feeds, the intestinal presence of E. faecium IMC 511 was estimated on days 10, 30, and 50 post hatch, and the effects on intestinal microbial load, fish survival, welfare, and growth were evaluated by morphometric and molecular approaches by examining relative body weight, total length, and gene expression of myostatin. In particular, the effect on animal welfare was assessed through analyses of cortisol levels and 70 kDa Heat Shock Protein gene expression. The results showed that while the Vibrio populations of the intestine of S. solea larvae were reduced in a significant way by the probiotic E. faecium IMC 511, survival was not affected. 70 kDa Heat Shock Protein and myostatin gene expression were significantly reduced in probiotic fed larvae with respect to the control group. It is of note that E. faecium IMC 511 seemed to be predominant at day 50 post hatching, probably due to stabilization of the intestinal microbiota, a factor that, together with the lower levels of HSP70, could improve animal well-being and growth. Cortisol levels significantly increased after 10 days of treatment in probiotic fed larvae, while on days 30 and 50 no significant differences were observed among all experimental groups. Considered together, the microbiological data and the molecular and morphometric results suggest that 50 days of probiotic treatment can improve common sole larval growth. Also, it is hoped that these results offer a useful starting point for elucidating the pathways involved in S. solea growth fostered by E. faecium IMC 511 administration.