Simultaneous tracking of cell motility in liquid and at the solid-liquid interface

To better understand key behaviors of living cells, such as bacterial biofilm formation, they must be observed above surfaces and at the interface between the surface and liquid medium. We have established a methodology for label-free imaging and tracking of individual cells simultaneously at both the solid-liquid interface and within the bulk, utilizing imaging modes of digital holographic microscopy (DHM) in 3D, differential interference contrast (DIC) and total internal reflectance microscopy (TIRM) in 2D as well as analysis protocols using a bespoke software package. We illustrate the power of this method by making detailed single cell measurements of Pseudomonas aeruginosa in the first minutes of their interaction with a glass surface, focusing on the role of the flagella stators, motAB and motCD. Using this new method we have determined their relative contributions to bulk and near surface motion for populations of cells at the single cell level.