Acoustofluidic Medium Exchange for Preparation of Electrocompetent Bacteria Using Channel Wall Trapping

Transformation, i.e. reprogramming of bacteria by delivering exogenous genetic material (such as DNA) into the cytoplasm, is a key process in molecular engineering and modern biotechnology in general. Transformation is often performed by electroporation, i.e. creating pores in the membrane using electric shocks in a low conductivity environment. However, cell preparation for electroporation can be cumbersome as it requires the exchange of growth medium (high-conductivity) for low-conductivity medium, typically performed via multiple time-intensive centrifugation steps. To simplify and miniaturize this step, we developed an acoustofluidic device capable of trapping the bacterium Escherichia coli non-invasively for subsequent exchange of medium, which is challenging in acoustofludic devices due to detrimental acoustic streaming effects.
With an improved etching process, we were able to produce a thin wall between two microfluidic channels, which, upon excitation, can generate streaming fields that complement the acoustic radiation force and therefore can be utilized for trapping of bacteria. Our novel design robustly traps Escherichia coli at a flow rate of 10 µL minute^-1 and has a cell recovery performance of 47 ± 3 % after washing the trapped cells.
To verify that the performance of the medium exchange device is sufficient, we tested the electrocompetence of the recovered cells in a standard transformation procedure and found a transformation efficiency of 8∙10⁵ CFU per µg of plasmid DNA. Our device is a viable low-volume alternative to centrifugation-based methods and opens the door for miniaturization of a plethora of microbiological and molecular engineering protocols.