Effect of host-derived Enterococcus faecium L6 on growth performance, intestinal health and antibacterial activities of juvenile grass carp
The various diseases outbreaks and infections caused by pathogens have brought great losses to the aquaculture industry of grass carp. Probiotics are one of the important ways in water quality management and inhibition of bacterial diseases. Thus, more and more attention is being paid to potential indigenous probiotics. The present study isolated a probiotic strain of Enterococcus faecium ( E. faecium ) L6 from the intestinal content of healthy grass carp, and evaluated the probiotic effect in vitro. Subsequently, a feeding trial of 56 days was performed to evaluate the effect of E. faecium (1 × 10 8 CFU/g) on growth performance in juvenile grass carp and infection resistance to Aeromonas hydrophila ( A. hydrophila ). Our data showed that diet supplementation with E. faecium L6 for 56 days significantly increased final body weight (FBW), weight gain rate (WGR), specific growth rate (SGR) of juvenile grass carp and decreased feed conversion ratio (FCR) compared to that of basal diet group without probiotic supplemented ( P < 0.05). The expression of inflammatory factors TNF-α , IL-6 and IL-1β in the liver, spleen and kidney showed an early increase and later decrease, and ultimately restored a nearly normal level on 56 days of feeding, and tight junction proteins genes zonula occludens-2 ( ZO-2 ), zonula occludens-3 ( ZO-3 ), occludin and claudin were significantly up-regulated from 21 days after feeding E. faecium L6 supplementary diet ( P < 0.05). Following a challenge by A. hydrophila infection, there was an increase of 26.67 % in the survival rate of diet supplementation of E. faecium L6 group, and the bacterial load in the liver, spleen and kidney was significantly reduced after E. faecium L6 feeding ( P < 0.01). High-throughput sequencing of 16S rRNA gene revealed that E. faecium L6 feeding increased the relative abundance of the phylum Fusobacteriota and Bacteroidota, and Cetobacterium and Bacteroides at genus level in the intestinal microbiota composition, as well as improved the functional pathways related to metabolism of purine, biotin, cysteine, methionine, d -Glutamine and D-glutamate, and biosynthesis of aminoacyl-tRNA and lipopolysaccharide. Analysis of microbial community-level phenotypes revealed that aerobic, contains mobile elements, facultatively anaerobic, biofilm-forming, Gram-negative, potential-pathogenicity and stress-tolerance phenotypes were remarkably decreased in E. faecium L6 supplemented diet group, while anaerobic and Gram-positive phenotypes were highly increased. Collectively, E. faecium strain L6 as a prospective probiotic can be used in the healthy breeding of grass carp production.
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