Measuring bacteria activity with an optically trapped microparticle

Summary form only given. Analyzing bacteria activity is the key to gain insight on the viability and response of prokaryotic cells to changes in their environment. Classical methods to measure bacteria viability, however, are often time consuming or in the need of high bacterial densities for successful analysis. Here, we demonstrate how the movement of a single cell can be quantified by observing the interactions between an optically trapped bacterium and a silica bead, which are aligned next to each other in a dual beam optical tweezers configuration (Fig.1).The motion of a microparticle held in an optical trap is subject to thermal fluctuations as long as there is no external disturbance [1]. Non-equilibrium noise, however, leads to a slight, frequency dependent change of the particle position in the harmonic potential [2]. We utilize this basic principle to perform measurements on bacteria cells. Any vibrations generated as the bacterium tries to escape from the first optical trap in a dual beam setup are picked-up by the detector bead that is held in the second. The position trajectory of the trapped microparticle therefore renders information on the frequency and intensity of the activity of the whole bacteria cell. We used Bacillus Subtilis, a bacterium strain that holds peritrichous flagellation, as a model system to demonstrate the feasibility of this approach. The flagella of B. Subtilis are bundled together and rotate in clock-wise or counter-clock-wise direction in order to push the bacterium forward in liquid media [3]. We demonstrate that small, time-dependent changes of the flagellar rotation can be resolved to distinguish different states of bacteria activity without the requirement of staining, fluorescence labelling, or any further treatment of the bacteria cell.