Single bacterial cell detection with nonlinear rotational frequency shifts of driven magnetic microspheres*

Shifts in the nonlinear rotational frequency of magnetic microspheres, driven by an external magnetic field, offer a dynamic approach for the dynamic detection of single bacterial cells. We demonstrate this capability by measuring such frequency shifts when an \textit{Escherichia coli }attaches to the surface of a 2.0 micron magnetic microsphere, thereby affecting the drag of the system. From this change in drag, the nonlinear rotation rate was reduced, on average, by a factor of 3.8. Sequential bacterial attachments were also monitored using this approach.