| Summary: | Probiotics are being developed as water additives to stimulate the immune system of fish, and to improve water quality by affecting the microbial communities in the fish and the culture environment. In the present study, the effect of the probiotics Bacillus cereus NY5 and Alcaligenes faecalis Y311 on water quality and microbiota in Nile tilapia tanks, and on the fish themselves, were investigated. The probiotics (1.0 × 104 colony-forming units /mL) were added separately to the experimental tanks every 7 days over a 3-month period. The water quality, environmental and fish microbiota, and immune enzyme activities in the intestine, skin, and gill of tilapia were analyzed. Both probiotics significantly decreased the total phosphorus levels in tank water compared with the control (P < 0.05, n=4, one-way ANOVA). The total superoxide dismutase activities in the skin from the B. cereus group increased by 19.09 % (P < 0.05, n=12, one-way ANOVA). The alkaline phosphatase activities in the intestine and gill in the A. faecalis group increased by 57.22 % (P < 0.05, n=12, one-way ANOVA) and 59.15 % (P < 0.05, n=12, one-way ANOVA), respectively. Illumina sequencing revealed that the main bacteria in the three sample types were Sphingomonas (53.72–59.44 %, relative abuandance), Methylobacterium (6.31–8.88 %), Acinetobacter (2.27–2.82 %), Escherichia-Shigella (2.02–2.46 %), Pseudomonas (1.23–1.67 %), and Streptococcus (0.87–2.16 %), and their abundance was not affected by probiotics. The relative abundance of some low-abundance genera in skin, gill and gut of tilapia was affected by the probiotics. The abundance of beneficial bacteria, including Cetobacterium, Methylobacterium, and Tepidimonas, in tilapia tissues increased upon probiotic treatment. The relative abundance of potential pathogens, such as Acinetobacter, was decreased in the intestine upon probiotic treatment. The findings of the current study indicate that the abundance of beneficial microbiota and immune enzyme activities in the mucosal immune tissues of tilapia can be improved by using probiotics as water additives.
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