IDENTIFICATION AND EVALUATION OF ANTAGONISTIC ACTIVITY FROM MARINE BACTERIA AGAINST PATHOGENIC Vibrio parahaemolyticus, CAUSATIVE AGENT OF ACUTE HEPATOPANCREATIC NECROSIS DISEASE (AHPND) IN SHRIMP FARMING  

In 2013 the shrimp culture in Mexico was devastated by a strong Vibriosis, with 60-80% of production losses. This condition was very similar to early mortality syndrome (EMS/AHPND) reported on 2009 in China. Diverse marine bacteria are able to produce antimicrobial compounds, or they also can be used as probiotics in aquaculture. The present study aims to isolate and evaluate antagonistic bacteria of pathogenic Vibrio parahaemolyticus (EMS/AHPND), from marine microbiota, as a first step in finding potential alternatives to control this disease in shrimp farming.

We sampled five points at coastal environment in the South of Sonora, Mexico, and isolated marine bacteria from wild shrimp, bivalves, marine algae, water and saline sediments. To evaluate the antagonistic activity against different Vibrio strains, selected isolates were tested in vitro using the cross streak method (CSM) and double layer (Dopazo). The candidate strains were further characterized by traditional and biochemical methods, and identified by partial sequence of 16S rRNA gene. A 10 days bioassay (5 treatments by duplicate, 8 shrimps per tank, 3 g average weight) was carried out to evaluate the pathogenicity of candidate strains in shrimp.

A total of 167 isolates were obtained, five demonstrated a strong antagonistic activity against Vibrio on the in vitro tests (Figure 1). The results indicated that four strains (30R, 36R, 42 and 13L) are Gram positive bacilli, catalase positive, glucose fermenting bacteria, and related to Bacillus spp. genera. 36Y strain is a Gram negative coccobacillus, oxidase positive, related to Pseudoalteromonas spp. genera. All the strains showed antibiotic sensitivity to florfenicol, enrofloxacin, and oxytetracycline. 13L and 36Y indicated good adhesion properties and biofilm production on microplate assay; OD 570 nm = 0.40 and 0.13, respectively. The bioassay results indicated that the five isolates individually added to aquatic environment (1.5-4x10⁵ CFU/ml) are not pathogenic for juvenile shrimp, obtaining good survival rates (94-99%). The marine bacterias isolated are considered good candidates for metabolite production and/or their use as probiotics in aquaculture, however more research is needed to characterize these bioactives and in vivo trials.