Quantifying Bacterial Cell Motility

A capillary-based assay (Palleroni, 1976 (link)) was performed to provide quantitative measurement of the bacterial cell motility, as described previously (Sun et al., 2009 (link)). Briefly, 10 ml cultures grown to an OD of approximately 0.4–0.5 (mid-log) were spun down at ~5500 × g for 8 min at room temperature and resuspended in an equal volume of phosphate buffered saline (PBS). Each channel of the Palleroni chamber was filled with 550 μl of resuspended cells. The capillary (32 mm length, 1.1 mm inner diameter) was filled with one of the following solutions: 30 mM sulfate, 60 mM lactate or 1 × PBS (control) and placed horizontally into the Palleroni chamber. After the 15 min incubation period, contents from the capillary were dispensed into 135 μL of 1 × PBS. The micro-bicinchoninic acid (micro-BCA) assay (Pierce, Rockford, IL, USA) was used as per manufacturer's instruction to measure the protein from the cells, and served as a measure of the cell mass that entered the capillary during the assay. Absorbance was measured by the SpectraMax Pro microplate reader (Molecular Devices, Sunnyvale, CA). Dilutions of bovine serum albumin in 1 × PBS were used to prepare a standard curve.