Evaluation on the effects of phage cocktail in preventing Aeromonas veronii infection in Gibel carps (Carassius auratus gibelio)

Members in Aeromonas genus especially for Aeromonas veronii isolates are ubiquitous in aquaculture environments and have increasingly implicated in diseases of fish, and many of them have multidrug resistance. In this work, we used a phage cocktail as an alternative strategy to combat the infection of A. veronii 1810 in gibel carps. Three bacteriophages were isolated and designated as pAEv1810, pAEv1812 and pAEv1818. Electron microscopy observation revealed that all three phages belonged to the order of Caudovirales. Host spectrum assay showed that all three phages were lytic to A. veronii 1810 and pAEv1812 had a relatively broad host range. The cocktail combined with the three phages performed a significant inhibition (P < 0.05) of growth of A. veronii 1810 compared with the untreated cultures. During the 60 days of cultivation time in the in vivo trial, our results revealed that feeding the diet containing the phage cocktail cannot affect the growth performance and digestive enzyme activities of gibel carps. At the end of the 12-day observation in the challenge phase, the survival rate of gibel carps was calculated as 80% for the phage cocktail treatment, which was almost caught up with the rate of antibiotic treatment. For the digestive enzyme activities, our results indicated that the oral administrated phages helped the main enzymes including amylase, lipase and proteinase return to normal. Additionally, certain numbers of phages steadily existed in the cultured water, gills and intestines of gibel carps during the 60 days of phage feeding time. Subsequently, the intestinal microbiota of gibel carps in the group of CAbc (blank control), CABT (Bacterial challenge treatment) and CApt (phage cocktail treatment) were analyzed at the genus level. The Aeromonas abundance in the CApt group was significantly decreased compared to that of CABT (P < 0.05) and the intestinal microbiota in the CApt group presented high similarity compared to that of the CAbc group, suggesting the applied phages could effectively protect the structure and bacterial communities in the intestinal tracts of gibel carps. Together, the overall results of this experiment indicated that using a phage cocktail could be effective in combating A. veronii infections in gibel carps.