Abstract
Flagella and cilia are cellular appendages that inherit essential functions of microbial life including sensing and navigating the environment. In order to propel a swimming microorganism they displace the surrounding fluid by means of periodic motions, while precisely timed modulations of their beating patterns enable the cell to steer towards or away from specific locations. Characterizing the dynamic forces, however, is challenging and typically relies on indirect experimental approaches. Here, we present direct in vivo measurements of the dynamic forces of motile Chlamydomonas reinhardtii cells in controlled environments. The experiments are based on partially aspirating a living microorganism at the tip of a micropipette force sensor and optically recording the micropipette’s position fluctuations with high temporal and sub-pixel spatial resolution. Spectral signal analysis allows for isolating the cell-generated dynamic forces caused by the periodic motion of the flagella from background noise. We provide an analytic, elasto-hydrodynamic model for the micropipette force sensor and describe how to obtain the micropipette’s full frequency response function from a dynamic force calibration. Using this approach, we measure the amplitude of the oscillatory forces during the swimming activity of individual Chlamydomonas reinhardtii cells of 26 ± 5 pN, resulting from the coordinated flagellar beating with a frequency of 49 ± 5 Hz. This dynamic micropipette force sensor technique generalizes the applicability of micropipettes as force sensors from static to dynamic force measurements, yielding a force sensitivity in the piconewton range. In addition to measurements in bulk liquid environment, we study the dynamic forces of the biflagellated microswimmer in the vicinity of a solid/liquid interface. As we gradually decrease the distance of the swimming microbe to the interface, we measure a significantly enhanced force transduction at distances larger than the maximum extent of the beating flagella, highlighting the importance of hydrodynamic interactions for scenarios in which flagellated microorganisms encounter surfaces.
Highlights
Microbial motility governs a variety of phenomena in microbiology, biophysics, as well as in many other fields of research, and has been studied intensively for decades [1,2]
We resolve the forces generated by the beating flagella of C. reinhardtii as a function of their distance to a solid interface, elucidating the nature of wall interaction mechanisms of puller-type microswimmers
We extend our experiments towards dynamic force measurements on living C. reinhardtii cells in the vicinity of a solid wall in order to study the effect of flagella/ wall interactions and, in particular, characterize the presence and magnitude of hydrodynamic interactions between beating flagella and the surface of the wall
Summary
Microbial motility governs a variety of phenomena in microbiology, biophysics, as well as in many other fields of research, and has been studied intensively for decades [1,2]. Since biophysical techniques enabling truly direct force measurements are lacking to date, a conclusive picture of the mechanics and coordination of cilia and flagella, which control microbial motility, still remains elusive [27] This includes an ongoing debate whether hydrodynamic or steric contact forces dominate the interaction of puller-type microswimmers with interfaces, as well as their impact on microbial navigation in confined spaces [28,29]. We establish a versatile method for a full calibration of the micropipette’s frequency response function and show that signal analysis in Fourier space allows for isolating the signal caused by the beating flagella of C. reinhardtii from external vibrations and background noise fluctuations This novel experimental approach enables direct measurement of dynamic forces in the piconewton range, generated by flagellated cells at high frequencies. We resolve the forces generated by the beating flagella of C. reinhardtii as a function of their distance to a solid interface, elucidating the nature of wall interaction mechanisms of puller-type microswimmers
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