Efficiency of phage cocktails in the inactivation of Vibrio in aquaculture

Aquaculture is one of the fastest growing sectors in the world, having a very important role in the economy. However, the losses associated with bacterial infections such as vibriosis, lead to huge economic costs. The regular use of antibiotics in aquaculture has resulted in the development of resistant strains, which have contributed to the inefficacy of antibiotics. To reduce the risk of the development and spreading of microbial resistance and to control the fish diseases in aquaculture, alternative strategies must be developed. Phage therapy can be an eco-friendly alternative to prevent and control pathogenic bacteria in aquaculture. However, phage bacterial resistance is already well documented but the use of phage cocktails can overcome this drawback. The aim of this study was to evaluate the efficiency of cocktails of two and three phages of Vibrio parahaemolyticus (VP-1, VP-2 and VP-3) to control Vibrio in aquaculture. All phages were effective against V. parahaemolyticus, however, the VP-3 phage was the most efficient one (additional reduction of more 2 log when compared with the other two phages). The use of cocktails with two and three phages was significantly more effective (reductions of 4 log after 2 h) than the use of VP-1 and VP-2 phages alone (reductions of 0.8 log after 2 h), however, the efficiency of VP-3 phage was similar for the phage alone and for the phage in the cocktails (reduction of 3.8 log and 4.2 log for VP-3 phage alone and in cocktails, respectively, after 8 and 6 h). All phages remained viable for a long time (at least 5-7 months) in marine water. The VP-3 phage presented a larger burst size and a shorter latent period (42 and 40 min, respectively) than the other two phages (9 and 120 min and 15 and 90 min, respectively, for VP-1 and VP-2 phages). Overall, the use of phage cocktails of two or three phages increased the efficiency of phage therapy against Vibrio (more efficient and faster bacterial inactivation), delaying the development of resistance by the bacteria and the use of Vibrio phages with high burst sizes and short lytic cycles also increases the efficiency of phage therapy.