Development of a gnotobiotic growth system for turbot (Psetta maxima)

Dissertação mest., Aquacultura e Pescas, Universidade do Algarve, 2007Turbot (Psetta maxima), is one of the most important marine fin fishes for aquaculture in Northern Europe because of its relative rapid grow and high market value. However, disease outbreaks are being recognized as a significant constraint to aquaculture production and evidence points towards bacteria as a major cause of the problem. Antibiotics have been applied in aquaculture production to control diseases, but the malpractice is increasing bacteria resistance, reducing the efficacy of fish disease control, transferring resistance to human pathogens, as well as adverse ecological impacts. The use of probiotics provides an ecological alternative to the use of antimicrobial compounds, for disease prevention in aquaculture. The aim of this work was to develop a methodology to culture turbot under gnotobiotic conditions using glutaraldehyde, ampicillin, rifampicin, tetracycline, and kanamycin as well as evaluate the effect of the LVS3-RP4 transconjugant (Aeromonas hydrophila-LVS3 and Escherichia coli-RP4) as a probiont, in these treatments. Two experiments were done using turbot eggs and larvae in a starvation experiment. The experiments were conduced at 16ºC and the eggs were previously treated with glutaraldehyde 200 mg/l [50% (v/v) in water] for 5 minutes, afterwards different combinations of antibiotics and the application of 105 CFU/ml of LVS3-RP4 transconjugant were added. The disinfection efficiency and hatchability of the turbot eggs and the survival of the larvae were evaluated. Experiments were conduced to check the LVS3-RP4 transconjugant development within the treatments. The egg quality has influenced the experiment results on hatching and survival. The LVS3-RP4 transconjugant has demonstrated its probiotic effect in the larvae survival experiment although it entered a viable but non-culturable (VBNC) state. It became also clear that only using glutaraldehyde for egg surface disinfection can lead to a lower larvae survival, because this treatment probably kills the majority of the bacteria unbalancing the natural flora from the egg surface, allowing the opportunistic pathogens to grow. Subsequently, through the transference of bacteria from the egg surface to the hatched larvae, some reminiscent opportunistic bacteria grew and caused adverse effects on larvae. The subsequent use of ampicillin also was not efficient probably because of the large number of bacteria resistant to this antibiotic