Investigating The Critical Dimensions Of Bacterial Transport

We investigate the transport of bacteria through sub-micron channels by using polydimethylsiloxane (PDMS) microfluidic devices. It is important to understand how bacteria move through confined spaces, toward certain chemicals and away from others, and whether they will continue to advance toward the source when the confinement becomes smaller than their own dimensions. The results obtained here will reveal more about bacterial behavior in soil environments where the surroundings resemble micro- and nano-pores.

To determine the critical dimension for bacterial migration, PDMS devices containing micro- and nanochannels were made using replica molding. Initial master molds were created using a combination of electron-beam and photolithography. The devices are very simple in their design. They include a series of sub-micron channels in various sizes and geometric shapes that connect large (approximately 10 x 100 micrometer) microfluidic channels to a central main channel. By varying constriction geometries it is possible to control where bacteria go in the devices. The bacteria are loaded into the main channel while fresh food is loaded into the side channels, creating a gradient along the sub-micron constrictions.

In this study, we used fluorescently labeled Bacillus subtilis and Escherichia coli, which are prevalent in soil and studied extensively in research laboratories. A chemotactic response was imposed on the cells by filling the main channel containing bacteria with phosphate buffered saline while the side channels were filled with Lysogeny Broth (LB). Images taken with fluorescent microscope show that bacteria quickly move toward the food source and form biofilms when the dimensions of the constrictions approach the diameter of the bacteria cells. When the cross-sectional area of the constrictions is significantly smaller than the cell dimensions, cells attach themselves at the constriction entrance. An interesting observation is that the cells were able to deform and squeeze through constrictions where only one dimension is smaller than their diameter, for example 0.7 x 1.5 micrometer.

These simple devices can be used to elucidate and understand the complex interactions between microbial species in the environment. We will next investigate the chemotactic response of microbes to other chemical compounds.