Relationships between disease outbreak in cultured tiger shrimp ( Penaeus monodon) and the composition of Vibrio communities in pond water and shrimp hepatopancreas during cultivation

Tiger shrimp ( Penaeus monodon) larvae were first cultivated for 1 month in the same pond and then at postlarval 43 were transferred into three different culture ponds (E2, E4, and W4). Vibrio spp. from the pond water and shrimp hepatopancreas were isolated and identified using thiosulfate-citrate-bile salt-sucrose (TCBS) agar and fatty acid methyl esters (FAME) analysis. For the initial 60 days after transfer, the composition of the Vibrio community in the pond water remained fairly diverse, but subsequently decreases in species diversity were observed in all three ponds, suggesting that the culture system was under some kind of stress. However, no clear relationship between this reduction in species diversity, and the subsequent occurrence of disease to the changes in total viable and Vibrio counts were observed. Although the Vibrio community in pond E4 remained relatively diverse, in water samples taken from E2 and W4 between Day 73 and 89, the dominant species according to FAME were Vibrio cholerae and, apparently, Vibrio furnissii/ Aeromonas spp., respectively. By Day 95, at which time all the shrimps, including those in pond E4, were dead, the microbial communities in all these ponds appeared to be dominated by Aeromonas spp. and V. furnissii. When thirty-eight of these Aeromonas spp. and V. furnissii isolates were randomly selected and re-identified using Biolog GN plates, the majority of them (58%) were reassigned either to Vibrio parahaemolyticus or the Vibrio harveyi/ Vibrio carchariae group. Vibrios were only detected once in the hepatopancreas of shrimp from pond E2. In W4 shrimp, more than 10 4 CFU of vibrios/g were detected in samples taken on Day 62 and thereafter. The average weight of shrimp collected on Day 89 from ponds W4 and E2 was 4.91 and 7.96 g, respectively, which suggests that the presence of a large number of vibrios in the hepatopancreas may be associated with growth retardation in shrimps. The Biolog system identified 53.3% of the vibrios isolated from the hepatopancreas of shrimps collected from pond W4 on Day 62, as V. harveyi and 20% as V. parahaemolyticus. One month later, immediately before mass mortality of the shrimp, Biolog results suggested that the W4 hepatopancreatic Vibrio communities were dominated by V. parahaemolyticus.