The wells of

the bottom chambers were filled with 200 μL of mucus (mucus test) or HBSS (negative control). Polycarbonate membranes (Nucleopore, Pleasontan, CA) with a diameter of 13 mm and a pore size of 0.8 μm were carefully placed on the top of the bottom chambers with the shiny side up. Following assembly of the chambers, 200 μL of an F. columnare cell preparation was placed in the wells of the top chambers. Triplicate chambers were used for each assay. Following incubation at room temperature for 1 h, the chambers were disassembled and the membranes were removed carefully using a PenVacuum with a 3/8″ probe (Ted Pella, Redding, CA). The contents of the bottom wells were mixed and 100-μL samples were removed and placed buy Oligomycin A in flat-bottom microtiter 96-well plates (Thermo-Scientific, Milfort, MA). Each mucus test or HBSS alone was also added to the 96-well plate (100 μL) to determine the background absorbance due to the sample alone. Positive controls consisting of 100 μL of the adjusted F. columnare culture diluted 1 : 5 in HBSS were also added to the 96-well plates. To each test well that contained either mucus, positive or negative controls, 20 μL of the combined MTS/PMS [Celltiter 96 Aqueous Non-Radioactive Cell Proliferation Assay (Promega,

Madison, WI) was added and mixed. The plate was covered by an aluminum foil to protect from light and incubated for 4 h at 28 °C. The A490 nm was recorded using a Model 680 microplate reader (Bio-Rad, Pirfenidone research buy Hercules, CA). The absorbance values of the mucus samples or HBSS alone were subtracted from mucus test samples and HBSS control to correct the absorbance values of mucus sample or HBSS control alone. Three independent assays were carried out using the pooled mucus sample. To quantify the F. columnare chemotactic response in CFU mL−1, the corrected absorbance values for the cell concentrations were plotted against the corresponding numbers of viable F. columnare CFU mL−1. Linear regression

was performed using graphpad prism (version 2.01, GraphPad Software, San Diego, CA) to determine the correlation between the corrected A490 nm and the number viable CFU mL−1. To assess the effect of sodium metaperiodate (Sigma) on chemotaxis, bacteria were prepared in HBSS as described above and treated at concentrations of 0.5, 1.0, 1.5, 2.0 and 2.5 mM for 1 h in the dark at 28 °C. The treatments were Silibinin stopped by adding three to five drops of 10% ethylene glycol. The bacteria were then washed once in HBSS, resuspended in HBSS and assayed for their chemotaxis capacity. To evaluate the effect of 50 mM of carbohydrates (Sigma) on chemotaxis, bacteria were prepared as described above and incubated with 50 mM of d-galactosamine, d-glucosamine, d-sucrose, d-fructose, l-fucose, N-actyl-d-glucosamine, N-acetyl-d-galactosamine, d-glucose or d-mannose for 1 h in the dark at 28 °C. The effect of 50 mM d-mannose alone on the chemotactic response of F. columnare to mucus samples from 24 individual catfish was also determined.