Bacterial composition and activity determines host gene-expression responses in gnotobiotic Atlantic cod (Gadus morhua) larvae.

In this study the effects of different live versus dead bacteria on the survival and gene-expression patterns in Atlantic cod (Gadus morhua) larvae were examined. Seventeen different gnotobiotic conditions were tested in a 5-day experiment, and the effects of these conditions on the expression of selected genes involved in immune response, xenobiotic breakdown and nutrition (C3, Gpx, Cyp1a1 and Fdps) were investigated. For some of the conditions where dead bacteria were added, we observed significantly higher survival compared to the axenic control. Microbial specificity of host-responses was observed for all four genes, for both live and dead bacteria, although expression of Gpx and Fdps was more influenced by active bacteria. Live bacteria generally resulted in higher transcript levels of all the genes investigated, indicating that these host-responses are partly dependent on microbial activity, and not just due to interactions with bacteria as particles. The observed microbial specificity toward dead bacteria highlights the need to take into account the role of bacterial components when investigating host-microbe relationships.