Biofilm formation of LV F. columnare isolate stimulated with cortisol. Biofilm formation of the low virulent (LV) isolate cultivated without (upper channel) and with (lower channel) a high dose (HD) of cortisol is compared in parallel at 12 h pi. The flow rate of the bacteria was kept constant at 1 µL/min for the entire duration of the experiment (12 h) to prevent the chambers from clogging. The bacterial cells of the LV isolate with HD of cortisol form aggregates filling the channel up to the middle and moreover biofilm formation upstream from the bacterial inlet (white arrows) is seen, whereas this was not the case for the non-supplemented LV isolate. In the latter, only downstream biofilm formation is seen. As the bacterial inflow remained constant during the whole trial to prevent the chamber from clogging, the thickness of biofilm formation was smaller compared to the one formed in the first experiment (Figure 1) in which the complete channel was filled. This microfluidic experiment was performed in singlefold merely to grasp whether the LV F. columnare isolate supplemented with and without cortisol could for biofilm formation in the microfluidic chambers. Scale bar 20 µm.