EARLY WEANING OF SEA-BASS LARVAE, DICENTRARCHUS-LABRAX - THE EFFECT ON MICROBIOTA, WITH PARTICULAR ATTENTION TO IRON SUPPLY AND EXOENZYMES

Microbiota was studied in 20-day old seabass larvae after 10 days of f eeding compound diets, in comparison with a control group kept live fo od. This early weaning caused bacterial overload in the larvae fed the normal compound diet (10(9) CFU per g of body weight). The bacterial load was significantly limited with a diet lacking ferrous salt in the mineral premix (3 X 10(8) CFU g(-1)). A similar range of associated f lora was observed before weaning at day 10, but only 10(7) CFU g(-1) w ere counted in the larvae fed Artemia till day 20. The bacterial surge in weaned larvae was due to one strain of Vibrio, especially dominant in the iron-supplemented diet. In spite of its opportunism, the strai n did not seem pathogenic to starving seabass larvae challenged from d ay 5 to day 10. This Vibrio produced siderophore, protease, phospholip ase and amylase. These extracellular products were also produced by 25 to 60% of the diversified flora of the Artemia-fed larvae. Lipase was the main extracellular product observed in the latter flora (80% of t he isolates), whereas it was seldom produced in the weaned groups (les s than 10%). The omission of dietary ferrous salt had no detrimental e ffect on survival and growth rates of the larvae, but limited bacteria l load. It is concluded that diet formulation should take into account the effects on microbiota, based on this first example with iron limi tation. (C) 1997 Elsevier Science B.V.