Abstract
Vibrio infection poses a threat to the health of juvenile sea cucumber (Apostichopus japonicus) farming. In the present study, three phages, PVA1, PVc1 and PVS3, are respectively lytic to Vibrio alginolyticus, V. cyclitrophicus and V. splendidus and were used to make a phage cocktail with a ratio of 1:1:1 (V/V/V). Bioinformatic analyses showed that gene related toxins and virulence proteins were not detected in the genome of the three phages. Based on these results, a small pilot study was carried out to measure the effectiveness of a three-phage cocktail on sea cucumber farming. During the 48-day cultivation time, the total Vibrio counts in phage treatment groups decreased and phage particles were detected in the coelomic fluids of sea cucumbers over the cultivation time. In-vitro studies of those incubated with phages showed no significant cell viability loss (P > 0.05) compared to the control group. The level of superoxide dismutase and lysozyme in the phage immersion group remained lower than the control group (P < 0.05), while the activities of nitric oxide synthase and acid phosphatase remained stable. At the end of 48-day cultivation time, the total weight gain of the sea cucumbers in the phage cocktail immersion treatment group increased significantly (P < 0.01) compared to the control group, while remained lower than the antibiotic treatment group (P < 0.05). No significant differences were detected in the viscera body wall ratio and feed conversion among the three groups (P > 0.05). After the metagenomic analysis of sea cucumber intestinal tracts, the Vibrio abundance in the phage cocktail group decreased compared to control group, while there was no effect on the abundance of the other genera. Hence, all results of this small pilot study indicate that using a phage cocktail is a safe and feasible way to combat Vibrio infections in juvenile sea cucumber farming.
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