Non-Optical, Label-free Imaging of Bacteria with Electrical Impedance Spectroscopy.

Optical imaging has provided insights into many fundamental aspects of bacterial colony growth, including expansion mechanisms, phenotypic pattern formation, and genotypic segregation. However using light to form images limits the range of environments and species we can investigate. Here we introduce a non-optical method to perform days-long timelapse imaging experiments on bacterial colonies over millimeter length scales in up to three dimensions. The method forms images by placing bacterial samples in contact with a transistor array that measures the electrical impedance of the material near each pixel. We demonstrate that impedance imaging can resolve spatial and temporal variation

in bacterial colonies, creating the foundation for distinguishing between bacterial species as well as phenotypic differences within species. Given the compact size and the ability to image without fluorescent labeling, our methods allow for experiments on species in environmental conditions previously inaccessible to light microscopy.