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Bacterial strains CF-20 (CECT5719) and C-148 (CECT5718), isolated from cultures of larvae of molluscs, are shown to produce substances 1-5 with strong antibiotic activity against Vibrio anguillarum (MIC: 0.03-0.07 mug/mL) and identified as the dd-diketopiperazines cyclo(d)-Pro-(d)-Phe (1), cyclo(d)-Pro-(d)-Leu (2), cyclo(d)-Pro-(d)-Val (3), cyclo(d)-Pro-(d)-Ile (4), and cyclo-trans-4-OH-(d)-Pro-(d)-Phe (5). Comparison with other stereoisomers indicates that inhibition of V. anguillarum is associated with the presence of at least one d-amino acid in the diketopiperazine system. This is the first time a series of dd-diketopiperazines has been isolated from a single natural source and their inhibitory activity against V. anguillarum described.

: A marine strain (BS107), identified as a Roseobacter species, was antagonistic to Vibrio species on agar plates. Results suggested that the inhibitory effect was displayed only in the presence of another bacterium. Quantification of the antibacterial activity showed that 48-hour-coculture supernatants from BS107 and another bacterial strain (V. anguillarum 408) reached the highest titers of bacterial inhibition. The antibacterial substance was also liberated when supernatants from V. anguillarum 408 were added to pure cultures of the inhibition-productive bacterium. The presence of a proteinaceous molecule may induce BS107 to display the inhibitory effect. The antibacterial substance was sensitive to trypsin (8000 U/ml) and stable at 100 degrees C. Cell extracts of the isolate BS107 (10(6) cells/ml) significantly enhanced scallop larval survival, thus being beneficial to the rearing process.

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