We study the effect of bacterial motion on micron-scale beads in a freely suspended soap film. Given the sizes of bacteria and beads, the geometry of the experiment is quasi-two-dimensional. Large positional fluctuations are observed for beads as large as 10 microm in diameter, and the measured mean-square displacements indicate superdiffusion in short times and normal diffusion in long times. Though the phenomenon is similar to Brownian motions of small particles, its physical origin is different and can be attributed to the collective dynamics of bacteria.

An experimental study of Rayleigh-Be'nard convection in helium gas at roughly 5 K is performed in a cell with aspect ratio 1. Data are analysed in a ' hard turbulence ' region (4 x 10' < Ra < 6 x 10l2) in which the F'randtl number remains between 0.65 and 1.5, The main observation is a simple scaling behaviour over this entire range of Ra. However the results are not the same as in previous theories. For example, a classical result gives the dimensionless heat flux, Nu, proportional to R d while experiment gives an index much closer to 5. A new scaling theory is described. This new approach suggests scaling indices very close to the observed ones. The new approach is based upon the assumption that the boundary layer remains in existence even though its Rayleigh number is considerably greater than unity and is, in fact, diverging. A stability analysis of the boundary layer is performed which indicates that the boundary layer may be stabilized by the interaction of buoyancy driven effects and a fluctuating wind.

We investigate swimming and chemotactic behaviors of the polarly flagellated marine bacteria Vibrio alginolyticus in an aqueous medium. Our observations show that V. alginolyticus execute a cyclic, three-step (forward, reverse, and flick) swimming pattern that is distinctively different from the run-tumble pattern adopted by Escherichia coli. Specifically, the bacterium backtracks its forward swimming path when the motor reverses. However, upon resuming forward swimming, the flagellum flicks and a new swimming direction is selected at random. In a chemically homogeneous medium (no attractant or repellent), the consecutive forward t f and backward t b swimming times are uncorrelated. Interestingly, although t f and t b are not distributed in a Poissonian fashion, their difference Δt ¼ jt f − t b j is. Near a point source of attractant, on the other hand, t f and t b are found to be strongly correlated, and Δt obeys a bimodal distribution. These observations indicate that V. alginolyticus exploit the time-reversal symmetry of forward and backward swimming by using the time difference to regulate their chemotactic behavior. By adopting the three-step cycle, cells of V. alginolyticus are able to quickly respond to a chemical gradient as well as to localize near a point source of attractant.bacterial chemotaxis | bacterial swimming pattern E nteric bacteria, such as Escherichia coli, swim by rotating a set of flagella that forms a bundle when the flagellar motors turn in the counterclockwise (CCW) direction (1-3). The bundle falls apart when one or more motors turns in the clockwise (CW) direction, and the bacterium tumbles (4). A new swimming direction is selected upon resuming the CCW rotation of the flagellar motors. By modulating the CCW and CW intervals according to external chemical cues, the cells are able to migrate toward attractants or away from repellents (5, 6).Certain bacterial species possess a single polar flagellum with a bidirectional motor similar to E. coli. Being single polarly flagellated, low Reynolds number (Re) hydrodynamics dictates that, aside from random thermal motions, the bacterium can only backtrack its trajectory when the motor reverses. This raises an interesting question concerning how this type of cells performs chemotaxis. Studies of motility patterns of single polarly flagellated bacteria Pseudomonas citronellolis showed that the bacteria change the swimming direction by a brief reversal between two long runs. From the published trajectories (7), each reversal typically results in a small change in cell orientation, and thus several reversals appear to be necessary for a significant change in the swimming direction. Backtracking was also observed in a number of single flagellated marine bacteria such as Shewanella putrefaciens, Pseudoalteromonas haloplanktis, and Vibrio alginolyticus, which execute the so-called run-reverse steps when following attractants released from porous beads and from algae (8-10). A pioneering experiment in V. alginolyticus revealed that the timereversal s...

We use measurements of swimming bacteria in an optical trap to determine fundamental properties of bacterial propulsion. In particular, we directly measure the force required to hold the bacterium in the optical trap and determine the propulsion matrix, which relates the translational and angular velocity of the flagellum to the torques and forces propelling the bacterium. From the propulsion matrix, dynamical properties such as torques, swimming speed, and power can be obtained by measuring the angular velocity of the motor. We find significant heterogeneities among different individuals even though all bacteria started from a single colony. The propulsive efficiency, defined as the ratio of the propulsive power output to the rotary power input provided by the motors, is found to be Ϸ2%, which is consistent with the efficiency predicted theoretically for a rigid helical coil.bacterial flagellum ͉ bacterial propulsion ͉ propulsion matrix B acteria swim by rotating helical propellers called flagellar filaments. For Escherichia coli (E. coli), these filaments are several micrometers in length and 20 nm in diameter, organized in a bundle of four or five. Each flagellar filament is driven at its base by a reversible rotary engine, which turns at a frequency of Ϸ100 Hz (1). Many important properties of the swimming bacteria, such as their average swimming speed, the rotation rate of the flagellar bundle, and the torque generated by the molecular motor, have been determined (1)(2)(3)(4)(5)23). Other properties such as the translational and rotational drag coefficients of flagellar bundles, however, are difficult to measure, especially for intact cells. These parameters are significant for quantitative understanding of bacterial propulsion and are the subject of extensive mathematical analysis and computer simulations (6-10). In this work, we investigate the fundamental swimming properties of intact E. coli by using optical tweezers and an imposed external flow. We directly measure the force required to hold the bacterium and the angular velocities of the flagellar bundle and the cell body as a function of the flow velocity. The propulsion matrix, which relates the translational and angular velocity of the flagella to the forces and torques propelling the bacterium, can thus be determined one bacterium at a time. We find that the population-averaged matrix elements are in reasonable agreement with the resistive force theory for helical propellers (7), but there is a large variability even among bacteria of similar length grown from a single colony.The propulsion matrix also allows us to determine the propulsive efficiency , which is defined as the ratio of the propulsive power output (the part of the power used to push the cell body forward) to the rotary power input (the power used to rotate the flagellar bundle). We find the propulsive efficiency is strongly dependent on growth conditions but is not very sensitive to cell-body size. Despite the flexibility and internal friction between the filaments in the flagellar ...

We investigate a new form of collective dynamics displayed by Thiovulum majus, one of the fastest-swimming bacteria known. Cells spontaneously organize on a surface into a visually striking two-dimensional hexagonal lattice of rotating cells. As each constituent cell rotates its flagella, it creates a tornadolike flow that pulls neighboring cells towards and around it. As cells rotate against their neighbors, they exert forces on one another, causing the crystal to rotate and cells to reorganize. We show how these dynamics arise from hydrodynamic and steric interactions between cells. We derive the equations of motion for a crystal, show that this model explains several aspects of the observed dynamics, and discuss the stability of these active crystals.