Vibrio species are among the primary disease-causing agents of cultured fish and shrimp. Vibrio harveyi and Vibrio parahaemolyticus are two species reported to cause significant losses in penaeid shrimp hatchery and grow-out cultures. The use of probiotics has been documented as a promising strategy in preventing and controlling vibriosis infection in shrimp aquaculture.
The present study evaluated the effect of prefeeding a Bacillus probiotic (Rescue Zeigler USA) on growth and pathogenicity of V. harveyi and V. parahaemolyticus in juvenile white shrimp Penaeus vannamei. To evaluate gut colonization, Rescue was fed to shrimp at a dose of 0, 100, 1000, and 5000 mg kg-1 feed followed by periodic bacillus agar counts. Bacillus colony numbers in shrimp gut fed 1000 and 5000 mg kg-1 feed reached 6.39 × 105 and 5.69 × 105 cfu ml-1 respectively after 144 hours. To evaluate the effect of probiotics on vibrio pathogenicity, shrimp were pre-fed a commercial shrimp feed top coated with Rescue at a dose of 1000 mg kg-1 feed for 6 days prior to the challenge. Shrimp continued to receive Rescue top-coated feed during the 21-day challenge phase. From the seventh day forward, V. parahaemolyticus and V. harveyi were administered by top-coating feed at a predetermined LD50 of 1×107 cfu ml-1 once daily for the duration of the trial. The challenge trials were repeated twice, with vibrio challenge doses top coated on morning or afternoon feedings. Vibrio numbers in shrimp guts were determined daily by TCBS plate counts.
Although in vitro testing demonstrated significant bacteriocidal activity, in vivo results showed variation in Vibrio colony numbers over time, with no clearly significant differences between treatment groups. Mortality curves indicated that V. harveyi exposed shrimp fed Rescue probiotic exhibited a 33% to 44% improvement (morning 66%, afternoon 60%) in survival as compared to the control group (morning 44%, afternoon 46%). Groups exposed to pathogenic V. parahaemolyticus s howed that the treated group receiving the Rescue probiotic exhibited 28% to 40% improvement in survival (morning 51%, afternoon 66%) as compared to the control group (morning 40%, afternoon 47%). It may be hypothesized that the significant improvement in survival of the Rescue treated groups following V. harveyi and V. parahaemolyticus exposure may be attributed to a reduction in the virulence of these bacterial shrimp pathogens in vivo . Further research will be needed to better elucidate the mode of action. Pond trials with 3 replicates resulted in a 31 % reduction on average total vibrio count in shrimp’s gut . Collectively, supplementation of P. vannamei feeds with Rescue probiotics at a dose of 1000 mg/kg diet significantly reduced mortalities following infection with pathogenic V.harveyi and V. parahaemolyticus.