Knowledge of the location of adhesion and/or translocation of pathogenic bacteria to and 46 within the enterocytes of various gut segments in fish larvae

27 The location and cell damage caused by Vibrio anguillarum, the causative agent of classical 28 vibriosis, within the developing gut of the newly hatched sea bass (Dicentrarchus labrax L.) 29 is unknown. A gnotobiotic sea bass model was used to investigate the early interactions of V. 30 anguillarum with sea bass larvae. In the present study, germ-free sea bass larvae were orally 31 exposed to a V. anguillarum HI-610 pathogen labelled with the Green Fluorescent Protein 32 (GFP-HI-610) and sampled at regular intervals. Pathogenic colonisation of gut enterocytes 33 was observed 2 hrs post exposure (p.e.) and onwards, whereas bacteria within the swim 34 bladder were visualized 48 hrs p.e and onwards. Ultrastructural findings demonstrated direct 35 bacterial contact with the host cell in the oesophageal mucosa and putative attachment to 36 microvilli of midand hindgut enterocytes. The present findings form a starting point for 37 studies assessing the impact of potential candidates (probiotics, prebiotics, antimicrobial 38 peptides) to mitigate bacterial virulence. 39 40