Isolation and characterization of bacteria associated with a Penaeus stylirostris disease (Syndrome 93) in New Caledonia

Since 1993, significant mortality outbreaks of Penaeus stylirostris usually during winter, have been reported from grow out ponds in New Caledonia. During the mortality peaks in 1994 and 1995, moribund and apparently healthy shrimps were collected from all New Caledonian farms for bacteriological and histological studies in order to describe the 'Syndrome 93' disease. Hemolymph was collected aseptically and spread on to Marine agar 2216E and TCBS agar (Difco). Hemolymph from apparently healthy shrimps was generally sterile. Each contaminated hemolymph contained various bacteria, most of the time Vibrionaceae, but also Pseudomonas spp., Flavobacterium spp., Acinetobacter spp., Cytophaga spp., Moraxella spp., Flexibacter spp., Alteromonas spp. and Bacillus spp. In contrast, moribund shrimps were always infected with Vibrionaceae and sometimes a combination of Vibrionaceae and Pseudomonas sp. Preliminary identification studies based on morphological, physiological and biochemical characteristics including testing for the utilization of 100 sources of carbon, show that one particular group of Vibrio sp. was present and predominant in shrimps collected throughout farms in New Caledonia. Molecular chemistry techniques like ribotypic patterns and DNA-DNA hybridization were applied, as a complement to conventional identification methods, to 18 selected strains from moribund shrimps randomly selected from nine mortality peaks. They showed the presence of two different groups of Vibrio sp., provisionally called 'Noumea 1' and 'Noumea 2' for the purpose of this study. The Vibrio belonging to the first group have been identified as Vibrio penaeicida. Identification of the second group is now in progress, and it is already clear that these bacteria belong to a different species. Toxin production capacity and experimental pathogenicity of these species are currently being studied. Some identified V. penaeicida strains demonstrated a very high virulence. The etiological role of these bacteria is also being investigated, particularly through the search for a suitable experimental model.