Biofilm formation of HV F. columnare isolate stimulated with cortisol. Biofilm formation of the highly virulent (HV) isolate cultivated without (upper channel) and with (lower channel) a high dose of cortisol is compared in parallel at 4 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 cortisol supplemented bacterial cells display a more organized cell clustering lining-up at one-third of the microfluidic chamber (white arrows). 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 HV F. columnare isolate supplemented with and without cortisol could for biofilm formation in the microfluidic chambers. Scale bar 20 µm.