Growth, Vibriosis, and Streptococcosis Management in Shrimp-Tilapia Polyculture Systems, and the Role of Quorum Sensing Gene cqsS in Vibrio harveyi Virulence

Abstract

Tilapia culture in Indonesia was started with the Mozambique Tilapia (Oreochromis mossambicus) in the 1930’s, and the Nile Tilapia (Oreochromis niloticus) the 1960’s. The genetic improvement program of the Nile Tilapia, has led Indonesia to be one of the main tilapia producers in the world. On the other hand, shrimp aquaculture in the country was not started until the 1960’s, it became more popular after the eye ablation technology for broodstock maturation was developed in the early 1980’s. The first experimental study was conducted to investigate the feasibility of low salinity shrimp farming in a polyculture system with tilapia. Polyculture increased the survival for shrimp (77% compared to 62%), but at the same time decreased the survival of tilapia (87% compared to 97%). Together, the data on survival, specific growth rates, and feed conversion ratios showed that the shrimp performed well at low salinity. The second experimental study investigated the feasibility of brackishwater shrimp farming in a polyculture system with tilapia. Polyculture increased the survival for shrimp (82% compared to 65%), and had higher survival for the tilapia (60% compared to 43%). The Red hybrid Tilapia strain used in the study experienced mortalities after one month, suggesting the need for a salt tolerant strain. The presence of tilapia stimulated the growth of microalgae (Chlorella dominance), promoted higher numbers of heterotrophic bacteria in the water, and had lower presumptive vibrios on TCBS agar. A challenge study was conducted by mixing pathogenic luminescent Vibrio harveyi UAZ-651 into shrimp and tilapia feed. The survival of shrimp in monoculture were significantly lower (20%) compared to in polyculture systems (75 - 95%). Mortality was not found in tilapia. Based on 16S rRNA gene sequence, shrimp monoculture water was dominated by marine Vibrio spp., while the polyculture system had Bacillus spp. and Vibrio spp. with high homology to V. cholerae. The presence of Bacillus spp. which produce a lactonase enzyme AiiA, seems to inhibit vibrio growth. While providing advantages, shrimp-tilapia polyculture might also contribute to streptococcosis transmission. Injecting shrimp with Streptococcus iniae and S. agalactiae resulted in mortalities. S. iniae caused higher mortality in the shrimp cultured in 20 ppt (40%) compared to 10 ppt (20%), and no mortality in 5 ppt. S. agalactiae caused higher mortality in 5 ppt (40%) compared to 10 ppt (20%) and 20 ppt (20%). Quorum sensing (QS) is a density dependent cell to cell communication process in bacteria. Based on challenge studies in shrimp, the luminescent Vibrio harveyi BB120 wild-type strain caused 75-90 % mortality through injection of 106 CFU/shrimp. The mortality patterns in the QS mutants suggest that QS defined, when specific virulence genes were expressed or repressed. As QS in V. harveyi consists of three different circuits, further experiments deployed six mutants lacking either a synthase or a receptor for each circuit. The highest survival in the CqsS (a receptor for CAI-1 circuit) mutant group indicates that the CAI-1 circuit is the most crucial for virulence, followed by the AI-2 and HAI-1 cascades. Chitin acquisition and oxygen scavenging may be two reasons for luminescence in V. harveyi evolution and why they infect shrimp.