In vitro biofilm formation assays were performed as described previously ( Wei and Zhang 2006). Briefly, test strains were grown to saturation in LB media and then diluted 1:1000 in fresh LB media. The diluted culture (0.5 ml) was transferred to a polyvinyl chloride (PVC) plastic Eppendorf tube and incubated without shaking for 24 h at 30 °C. The resulting biofilm was stained with 0.1% (w/v) crystal violet for 20 min, and then unattached cells and residual dye were removed. The dye was dissolved in 95% ethanol, and A570 nm of the dissolved dye was determined.