Fig 1. Protocol used in this study to raise and infect or re-conventionalize germ-free (GF) trout larvae.

Eyed eggs were sterilized 5 days before hatching and kept in sterile, autoclaved mineral water at 16°C in Petri dishes until hatching. Once hatched (0 day post hatching = dph), rainbow trout larvae were transferred into vented cap cell culture flasks for the duration of the experiment. Larvae were fed every 2 days with sterile powder food from 21 dph until the end of the experiment; water was renewed 30 minutes after feeding. To test the protective effect of potential probiotic strains, larvae were re-conventionalized by one or several commensal bacteria diluted in water at 22 dph. Pathogenic bacteria were added to the water at 24 dph for 24 h and then larvae were washed with fresh sterile water. Survival after infection was monitored twice per day.