Dynamic Detection of a Single Bacterium: Nonlinear Rotation Rate Shifts of Driven Magnetic Microspheres

We report on a new technique which was used to detect single Escherichia coli that is based on the changes in the nonlinear rotation of a magnetic microsphere driven by a magnetic field. The presence of one Escherichia Coli bacterium on the surface of a 2.0 micron magnetic microsphere (with an aluminum ``nanocap'' that indicates the microsphere's orientation) caused an easily measurable change in the drag of the system and, therefore, in the nonlinear rotation rate. The straight-forward measurement uses standard microscopy techniques and the observed average shift in the nonlinear rotation frequency changed by a factor of $\sim$3.8 (Arxiv preprint cond-mat/0610144). Further miniaturization will allow for dynamic detection of viruses and potentially even biomolecules in fluidic environments.