Using particle-tracking techniques, the translational and rotational diffusion of paralyzed E. coli with and without flagella are studied experimentally. The position and orientation of the bacteria are tracked in the lab frame and their corresponding mean-square displacements are analyzed in the lab frame and in the body frame to extract the intrinsic anisotropic translational diffusion coefficients as well as the rotational diffusion coefficient for both strains. The deflagellated strain is found to show an anisotropic translational diffusion, with diffusion coefficients that are compatible with theoretical estimates based on its measured geometrical features. The corresponding translational diffusion coefficients of the flagellated strain have been found to be reduced as compared to those of the deflagellated counterpart. Similar results have also been found for the rotational diffusion coefficients of the two strains. Our results suggest that the presence of flagella --even as a passive component-- has a significant role in the dynamics of E. coli, and should be taken into account in theoretical studies of its motion.

19Bacteria propel and change direction by rotating long, helical filaments, called flagella. The 20 number of flagella, their arrangement on the cell body and their sense of rotation 21 hypothetically determine the locomotion characteristics of a species. The movement of the 22 most rapid microorganisms has in particular remained unexplored because of additional 23 experimental limitations. We show that magnetotactic cocci with two flagella bundles on 24 one pole swim faster than 500 µm·s -1 along a double helical path, making them one of the 25 fastest natural microswimmers. We additionally reveal that the cells reorient in less than 5 26 ms, an order of magnitude faster than reported so far for any other bacteria. Using 27 hydrodynamic modeling, we demonstrate that a mode where a pushing and a pulling bundle 28 cooperate is the only possibility to enable both helical tracks and fast reorientations. The 29 advantage of sheathed flagella bundles is the high rigidity, making high swimming speeds 30 possible. 31 32 Introduction 33The understanding of microswimmer motility has implications ranging from the 34 comprehension of phytoplankton migration to the autonomously acting microbots in 35 medical scenarios [1, 2]. The most present microswimmers in our daily lives are bacteria, 36 most of which use flagella for locomotion. Well-studied examples of swimming 37 microorganisms include the peritrichous (several flagella all over the body surface) 38Escherichia coli with an occasionally distorted hydrodynamic flagella bundling [3] and the 39 monotrichous (one polar flagellum) Vibrio alginolyticus, which are pushed or pulled by a 40 flagellum and exploit a mechanical buckling instability to change direction [4, 5]. The 41 3 swimming speeds of so far studied cells are in the range of several 10 µm s -1 and their 42 reorientation events occur on the time scale of 50-100 ms [5, 6]. 43Magnetococcus marinus (MC-1) is a magnetotactic, spherical bacterium that is capable of 44 swimming extremely fast [7][8][9][10]. MC-1 as well as the closely related strain are 45 equipped with two bundles of flagella on one hemisphere (bilophotrichous cells). The 46 bacterium also features a magnetosome chain, which imparts the cell with a magnetic 47 moment ('magnetotactic' cell). They are assumed to swim with the cell body in front of both 48 flagella, which synchronously push the cell forward [7]. This assumption leads to helical 49 motion in the presence of a strong magnetic field, which exerts a torque on the cell's 50 magnetic moment, as seen in hydrodynamic simulations [12]. In the absence of a magnetic 51 field, this model predicts rather straight trajectories. 52Our observations disagree with the above-mentioned model, indicating that an 53 understanding of the physics of their swimming is still missing, even though proof of concept 54 biomedical applications of these bacteria have already emerged [2]. Here we show that MC-55 1 not only reach speeds of over 400 µm s -1 but that this speed is recorded along an 56 unexplored double he...

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Maximum trapping efficiency in optical tweezers occurs close to the coverslip because spherical aberration owing to a mismatch in the refractive indices of the specimen (water) and the immersion oil dramatically decreases the trap efficiency as the trap depth increases. Measuring the axial trap efficiency at various tube lengths by use of an oil-immersion objective has shown that such an aberration can be balanced by another source of spherical aberration, leading to a shift in the position of the maximum efficiency in the Z direction. For a 1.1 microm polystyrene bead we could achieve the maximal efficiency at a depth of 70 microm, whereas the trap was stable up to a depth of 100 microm.

Bacteria propel and change direction by rotating long, helical filaments, called flagella. The number of flagella, their arrangement on the cell body and their sense of rotation hypothetically determine the locomotion characteristics of a species. The movement of the most rapid microorganisms has in particular remained unexplored because of additional experimental limitations. We show that magnetotactic cocci with two flagella bundles on one pole swim faster than 500 µm·s−1 along a double helical path, making them one of the fastest natural microswimmers. We additionally reveal that the cells reorient in less than 5 ms, an order of magnitude faster than reported so far for any other bacteria. Using hydrodynamic modeling, we demonstrate that a mode where a pushing and a pulling bundle cooperate is the only possibility to enable both helical tracks and fast reorientations. The advantage of sheathed flagella bundles is the high rigidity, making high swimming speeds possible.