Detection of Bacterial Movements in a Picoliter-sized Droplet Using a Microcantilever

Atomic Force Microscope (AFM) cantilevers have recently emerged as a promising tool for detecting nanomechanical movements of bacteria. In this approach, bacteria are adhered onto the surface of a microcantilever, and their metabolic activity results in excess low-frequency fluctuations of the microcantilever. Our work here is inspired by dip-pen nanolithography: we load liquid media onto a microcantilever from a bacteria (E. coli) solution and detect the excess microcantilever fluctuations due to the captured bacteria. We measure both the resonance frequency and the low-frequency fluctuations of the microcantilever before and after loading the liquid. Resonance frequency shifts allow us to estimate the mass and volume of the loaded droplet. Typical droplet sizes of 1-40 pL contain approximately 100 bacteria, which generate an observable increase in the microcantilever fluctuations. Our approach circumvents some of the disadvantages encountered in AFM measurements in liquids due to viscous damping and may allow for increased detection sensitivity.