Characterisation of haemolytic bacteria isolated from turbot, Scophthalmus maximus (L.), larvae

Aquaculture of turbot, Scophthalmus maximus (L.), has developed over the last 20 years but there is a major bottleneck in turbot aquaculture due to limited production of juvenile fish. This is due to hatcheries experiencing high mortalities, or 'crashes', during larval rearing, the main cause of which is thought to be haemolytic bacteria introduced with live food organisims. This thesis describes the characterisation of haemolytic bacteria isolated from turbot larvae and their food, Artemia, in order to identify pathogenic bacteria which may be introduced via the Artemia. The main method of characterisation was single carbon usage using Biolog GN microplates, but simple biochemical tests, such as Kovac's oxidase, detection of catalase, growth on TCBS medium, and utilisation of sucrose were also used. Selected strains were also tested for possible pathogenicity against turbot larvae. Representitive strains were also identified using partial DNA sequence analysis of 16S rRNA genes, and RPLP analysis of 16S rRNA genes used to group the phenons produced from the biochemical data. A total of 158 isolates were characterised from both hatcheries and this produced two major phenons. Phenon 1 was comprised of Vibrio alginolyticus type bacteria and phenon 2 consisted of Vibrio splendidus of both biovars 1 and 2. The RFLP analysis of V. splendidus biovar 1 revealed that even though different strains had similar or identical phenotypes their RFLP patterns could be different, and those strains with the same RFLP profile could be phenotypically different. The RFLP patterns of V. splendidus biovar 1 and biovar 2 were distinct from each other. Minor strains also detected included V. tubiashi, Marinamonas vaga, and Roseobacter gallaeciensis. All pathogenic strains belonged to K splendidus biovar 1 group and were found in high concentration in a batch of larvae that suffered high mortality, although they were also present at a low concentration in larval batches with high survival. The identification of these turbot larvae pathogens allows the development of alternative methods of bacterial control, which would reduce antibiotic usage which tends to select for resistant strains