in $43 billion in global losses to the shrimp aquaculture industry since its emergence in 2009 and has resulted in decreased production in affected regions by nearly 40%. The causative agent is a strain of Vibrio parahaemolyticus, which produces a toxin that damages the hepatopancreas of shrimp, resulting in mass mortalities up to 100% within weeks following the transfer of post larval shrimp to grow-out ponds.
To determine if using a bacteriocin-infused feed additive would increase the survival of AHPND challenged shrimp, pathogen free juvenile shrimp Penaeus (Litopenaeus) vannamei were fed a commercial shrimp diet infused with a bacteriocin additive for 7 days prior to oral pathogen exposure. The bacteriocin formulation was combined with shrimp premix (Rangen Inc., Buhl, Idaho) at 0.84 mg/g, 8.4 mg/g, and 84 mg/g. The pathogen challenge was two 10 ml aliquots of 1.2x109 cfu/ml of a AHPND-causing V. parahaemolyticus strain. Fifteen 90 L aquaria, each with 20 shrimp, were then divided into (i) 3 negative controls (no bacteriocin pre-treatment/no pathogen), (ii) 3 positive controls (no pre-treatment and pathogen added), and (iii) 9 treatment tanks (bacteriocin pre-treatment and pathogen addition). The study was terminated after 16 days, with three survivors per diet group preserved for histology; mortalities were frozen at -20°C for histology. A Kaplan-Meier plot was generated and logrank test was performed (Figure 1).
The negative control group had a survival rate of 98%, while the positive control group had an expected drop in survival (35%) due to AHPND infection. The treatment groups had an average survival of 96%, with the lowest and highest concentrations yielding the highest survival rates. These results show that bacteriocin additives can be used to prevent or mitigate AHPND infection in shrimp. Incorporation rates of 0.84 mg/g and 84 mg/g were shown to produce survival rates equivalent to shrimp not challenged with AHPND. Histological examination revealed no observable impacts of the treatment. Additional assays have been done to assess the thermal stability of the bacteriocin, which shows no decreased activity after 15 min at 100C, a temperature commonly employed in creating pelleted shrimp feed. Additional assays have also been done to determine the specificity of the bacteriocin treatment, which reveals that this particular bacteriocin has high specificity towards several Vibrio pathogens, with limited activity against beneficial members of the shrimp gut microbiome (data not shown). These results suggest that, compared to treatment with conventional antibiotics, a bacteriocin treatment can more specifically eliminate shrimp pathogens while leaving the shrimp microbiome relatively intact and thus enhancing shrimp health.