Protective effectiveness of feeding phage cocktails in controlling Vibrio parahaemolyticus infection of sea cucumber Apostichopus japonicus

Vibrio parahaemolyticus is a common zoonotic bacterium that causes infections and disease in both sea cucumbers (Apostichopus japonicus) and humans, potentially greatly impacting both aquaculture and human health. In this study, two phages with the ability to control V. parahaemolyticus were isolated and designated as vB_VpaP_VP-ABTNL-1 (PVP1) and vB_VpaS_VP-ABTNL-2 (PVP2). Morphological analyses showed that PVP1 belonged to C3 phage group Podoviridae while PVP2 belonged to the family Siphoviridae. In vitro phage-treated cultures showed significant (P < 0.05) inhibition of growth for V. parahaemolyticus VP-ABTNL when compared to untreated groups. Subsequently, we determined that skim milk might be the best protective agent, as the dissolution time and moisture content were both significantly (P < 0.05) higher than other agents tested. We also explored the utility of using feedstuff mixed with freeze-dried phage powder as a form of protection for sea cucumbers against V. parahaemolyticus infection. The antibiotic group had a significant(P < 0.05)difference with MOI = 1, but not with the two phage treatments (MOI = 10, MOI = 100), indicating an increase of phage protection with MOI = 10 and MOI = 100 against V. parahaemolyticus VP-ABTNL. There were no significant differences in weight gain, wall-to-wall ratio, feeding rate or food conversion efficiency among sea cucumbers fed the two phage treatments compared with those fed the non-supplemented diet (P > 0.05). Hence, feeding phage cocktails might be another optimal therapeutic agents to treat V. parahaemolyticus infections in sea cucumber aquaculture.