Vibrionaceae dominates the microflora antagonistic towards Listonella anguillarum in the intestine of cultured Atlantic cod (Gadus morhua L.) larvae

The intestinal microbiota in 27 Atlantic cod larvae (Gadus morhua L.) from five rearing groups was screened for antagonistic activity against Listonella anguillarum. A total of 98 antagonistic isolates were phenotypically characterized and belonged to 12 clusters at the 80% similarity level. The 16S rDNA of representative isolates was partially sequenced and compared to sequences in the GenBank database. The majority of antagonistic bacterial isolates had high sequence similarity to the γ-Proteobacteria, and the rest were members of the α-Proteobacteria. Antagonistic isolates with sequence similarity to Vibrionaceae were found in larvae from all five rearing groups, and this was the most numerous group of antagonistic isolates. The other antagonistic bacteria had sequence similarities to Hyphomicrobium, Marinomonas, Pseudoalteromonas, Roseobacter and Shewanella. The composition of the antagonistic bacterial flora varied both between individual cod larvae from the same rearing group and between larvae from the five different rearing groups. Also, the average percentage of antagonistic isolates was significantly different in one of the five rearing groups.