demand. This proposal aims to unravel the interplay between the bacterial fish pathogen Flavobacterium columnare and the stress response mediated via the hypothalamus-pituitaryinterrenal (HPI) axis of its host under acute and chronic stress conditions. Hereby a marked contribution will be made to i) the use of the cortisol content and glucocorticoid (GC) profile of scales as a reflection of the fish’s exposure to chronic stress as this will enable the aquaculture industry to detect chronic stress and manage its mitigation more efficiently; and ii) the elucidation of the enigma of the interplay between bacterial pathogens as stressors GCs and the HPI axis. To achieve this, at first, we will seek how a bacterial infection may influence the stress response via the HPI axis and lead to a possible build-up of GCs, cortisol in particular, in the scales of common carp (Cyprinus carpio). Secondly, we will investigate under laboratory conditions the direct effect of different GCs, including precursors and metabolites of cortisol, in varying concentrations on several F. columnare isolates with traits associated with harmfulness to the fish as adopted parameters. Thirdly, the effect of the GCs on adhesion, colonization, proliferation and cluster formation of F. columnare on explanted gill tissue will be examined by means of the semi in vivo gill perfusion model.