Biofilm formation of F. columnare isolates in a microfluidic chamber. Biofilm formation (B) of the highly virulent (HV, upper channel) and low virulent (LV, lower channel) F. columnare isolate are compared in parallel at 24 h pi. Bacterial cell suspensions were introduced into each channel via the bacterial inlet at 1 µL/min for 1 h, after which bacterial inflow was stopped. For the HV isolate, bacterial cell clusters completely fill the channel and are even encountered upstream (arrowheads), while the latter is not observed for the LV isolate. This microfluidic experiment was performed in singlefold merely to grasp whether both F. columnare isolates could form biofilm formation in the microfluidic chambers. Scale bar 20 µm.