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We monitor the rotation of a flagellated bacterium in a single laser beam optical trap. Bacillus subtilis, a rod shaped bacterium shows both run and tumble sequences as a free swimmer, the swimming aided by flagellar rotations. We detect and characterize the changes in flagellar and cell body rotations of the bacterium through detection of the forward scattered signal using a quadrant photo-detector(QPD). Simultaneously, the rotations in trap are visualized in time via video microscopy and the rotational frequency is measured through power spectral analysis . While the body rotation results in the appearance of a peak at lower frequencies (approx. 3 Hz) against the background Lorentzian spectrum, flagella rotations result in a broad higher frequency peak (approx. 88 Hz) in the power spectrum. The resultant peaks are modeled by a solution to the Langevin equation that takes into account the drag forces acting on the system. By monitoring the flagellar rotation speed’s variation with time, through continuous frequency measurement, we are able to determine the extent of photodamage to the cell and thereby, the time it takes to completely stop the rotations. We observe periodic fluctuations in the rotation frequency of the flagella that varies between a relatively higher and a lower value, with each of them gradually decreasing with time, until no further rotations are seen. In further, measurements, look for effects on the rotating bacterium in a fluid environment with altered pH. We observe a significant increase in time before the rotation of the flagella completely stops when the pH of the suspension media is lowered. Thus, direct monitoring of the optically trapped bacterium and onset of photodamage, is enabled through sequential power spectrum recording and this can clearly reveal the response of the bacterium to environmental changes. The experimental setup offers a simple and convenient way to confining and studying a single bacterium’s response to different fluid environments in real time.
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