Journal of Applied Microbiology 1997, 82, 729–734 Determination of bacteria associated with reared turbot (Scophthalmus maximus) larvae A.R. Blanch, M. Alsina, M. Simo ´ n and J. Jofre Department of Microbiology, University of Barcelona, Barcelona, Catalonia, Spain 5796/06/96: received 10 June 1996, revised 25 October 1996 and accepted 28 October 1996 A.R. BLANCH, M. ALSINA, M. SIMO ´ N AND J. JOFRE. 1997. In order to extend our knowledge of the presence of bacteria in hatcheries and their influence on rearing performance, the aerobic and facultative bacterial flora associated with farmed turbot larvae were studied in relation to the microflora of the water and diets. A settlement of specific groups of bacterial populations was found in the gut of the larvae. A clear succession of bacterial phenotypes was also observed from day 1 to day 90 post-hatching. Oxidative Gram-negative rods were predominant at the initial stages, whereas some phenotypes of Vibrio were frequent at the final stages. A high heterogeneity of Vibrio species was observed in the intermediate period when the highest mortalities of turbot larvae occur. INTRODUCTION 1977; Tanasomwang and Muroga 1988). However, others report that the bacterial flora of farmed fish was not consistent The source, the characteristics and the biological significance with that of water or the fish diet (Campbell and Buswell of the microbial flora in the intestine of marine fish are not 1983; Muroga et al. 1987; Nicolas et al. 1989). Bacteria of the well known. An understanding of the features and the role of genus Vibrio seem to constitute the dominant flora of the the indigenous microflora of marine fish larvae may help to intestine of marine fish (Sakata et al. 1980; Onarheim and improve feeding and other conditions for the intensive rearing Raa 1990), independently of the bacterial content in the water of fish. Several functions have been assigned to the resident or in the food ingested (Campbell and Buswell 1983; Muroga gut microflora. These include positive effects on nutrition et al. 1987; Nicolas et al. 1989). Quantitative studies have been (Fong and Mann 1980; MacDonald et al. 1986; Rimmer performed to evaluate the influence of associated bacteria on and Wiebe 1987) and possible prevention of colonization by the survival of turbot larvae (Nicolas et al. 1989; Westerdahl bacterial pathogens (Onarheim and Raa 1990; Westerdahl et et al. 1991). However, the diversity of the bacterial population al. 1994; Bergh 1995). The latter may involve inactivation in water, food and larvae at different stages of rearing has not of pathogenic bacterial toxins or metabolites, production of been considered. We present a study performed in a turbot bacteriocins or inhibitory factors, changes in pH and redox hatchery, in which aerobic and facultative bacteria have been potential, competition with pathogens for nutrients and isolated and identified from these different origins. The adhesion sites and stimulation of non-specific immunity (Jong results may facilitate the prediction of some of the charac- 1993). Aquaculture of marine fish has resulted in intensive teristics of the microbiota that most frequently colonize the systems for egg incubation and hatching. In such systems, intestinal tract of turbot larvae in intensive rearing. newly hatched larvae, which for a short time have a sterile intestinal tract (Wood 1967), are exposed to an environment, with variable bacterial populations (Hansen and Olafsen MATERIALS AND METHODS 1989). Moreover, such bacterial populations may be different to those of the natural environment. Some authors report Fish larvae that the intestinal microflora of fish reflects the bacterial Turbot larvae were sampled at Tinamenor Hatchery (San- content of ingested food and of the water (Seki 1969; Horsley tander, Spain) from day 1 after eclosion of eggs to day 100, when they were ready to leave the hatchery. Samples were Correspondence to: Dr Anicet R. Blanch, Department of Microbiology, taken from the production lots kept under the usual rearing University of Barcelona, Diagonal 645, 08028 Barcelona, Catalonia, Spain (e- mail: anicet@porthos.bio.ub.es). conditions for this hatchery. Six thousand-litre tanks main- © 1997 The Society for Applied Bacteriology