Beta galactosidase assay

Promoter activity of csgD was assayed with different csgD promoter constructs [25, 36]. Expression of adrA was analyzed with a chromosomal MudJ transcriptional fusion in adrA [37]. Strains were grown on LB without salt plates supplemented with appropriate antibiotics and inducer. Samples were collected after growth for 24 h at 28°C. β-galactosidase activity was the read out for promoter activity [38] after adjustment of bacterial suspension to OD₆₀₀ = 0.4 for pUGE13 and to 0.1 for pUGE5, pUGE7 and pUGE19. Normalized β-galactosidase activity was calculated using the formula: Miller units = 1000 {[OD420 ‐ (1.75 × OD₅₅₀)]/(t × V × OD₆₀₀)} with t = reaction time in min; V = volume of cell suspension. All β-galactosidase measurements were done in duplicates using at least three technical replicates. Statistical analysis was performed applying an unpaired t-test with two-tailed p-value (*** is p < 0.0001) using Prism 5 (GraphPad Software).