Flagellated bacterial motility in polymer solutions

Abstract

It is widely believed that the swimming speed, v, of many flagellated bacteria is a nonmonotonic function of the concentration, c, of high-molecular-weight linear polymers in aqueous solution, showing peaked v(c) curves. Pores in the polymer solution were suggested as the explanation. Quantifying this picture led to a theory that predicted peaked v(c) curves. Using high-throughput methods for characterizing motility, we measured v and the angular frequency of cell body rotation, Ω, of motile Escherichia coli as a function of polymer concentration in polyvinylpyrrolidone (PVP) and Ficoll solutions of different molecular weights. We find that nonmonotonic v(c) curves are typically due to low-molecular-weight impurities. After purification by dialysis, the measured v(c) and Ω(c) relations for all but the highest-molecular-weight PVP can be described in detail by Newtonian hydrodynamics. There is clear evidence for non-Newtonian effects in the highest-molecular-weight PVP solution. Calculations suggest that this is due to the fast-rotating flagella seeing a lower viscosity than the cell body, so that flagella can be seen as nano-rheometers for probing the non-Newtonian behavior of high polymer solutions on a molecular scale. T he motility of microorganisms in polymer solutions is a topic of vital biomedical interest. For example, mucus covers the respiratory (1), gastrointestinal (2), and reproductive (3) tracks of all metazoans. Penetration of this solution of biomacromolecules by motile bacterial pathogens is implicated in a range of diseases, e.g., stomach ulcers caused by Helicobacter pylori (4). Oviduct mucus in hens provides a barrier against Salmonella infection of eggs (5). Penetration of the exopolysaccharide matrix of biofilms by swimming bacteria (6) can stabilize or destabilize them in vivo (e.g., the bladder) and in vitro (e.g., catheters). In reproductive medicine (human and veterinary), the motion of sperms in seminal plasma and vaginal mucus, both non-Newtonian polymer solutions, is a strong determinant of fertility (3), and polymeric media are often used to deliver spermicidal and other vaginal drugs (7).Microorganismic propulsion in non-Newtonian media such as high-polymer solutions is also a hot topic in biophysics, soft matter physics, and fluid dynamics (8). Building on knowledge of propulsion modes at low Reynolds number in Newtonian fluids (8), current work seeks to understand how these are modified to enable efficient non-Newtonian swimming. In particular, there is significant interest in a flapping sheet (9, 10) or an undulating filament (11) (modeling the sperm tail) and in a rotating rigid helix (modeling the flagella of, e.g., Escherichia coli) (12, 13) in non-Newtonian fluids.An influential set of experiments in this field was performed 40 years ago by Schneider and Doetsch (SD) (14), who measured the average speed, v, of seven flagellated bacterial species (including E. coli) in solutions of polyvinylpyrrolidone (PVP, molecular weight given as M = 360 kDa) and in methyl cellulose (MC, M...

Osmotaxis in Escherichia coli through changes in motor speed

“…At our load, a 0.5 µm bead attached to the motor via a short filament stub, the motor is still expected to operate in the high-load, 'plateau' region of the torque-speed curve [Inoue et al, 2008, Lo, 2007 with the estimated full stator number [Lo, 2007]. In addition, we observe an average 30 Hz increase in motor-speeds even at our lowest osmotic shocks where the viscosity of the solution hardly changes (it increases by 1.057 times) and even free-swimming cells with the motor operating in the lineartorque regime do not increase the swimming speed at viscosity we use in our experiments [Martinez et al, 2014]. Therefore, it is less likely that additional stator incorporation or adaptive motor remodeling are sole explanations for the speed increases we observe.…”

“…In DDM experiments, the mean swimming speed was calculated from DDM movies as an average over ∼10 4 cells/ml. Details of the image processing and data analysis were as before [Wilson et al, 2011,Martinez et al, 2012,Martinez et al, 2014,Schwarz-Linek et al, 2016. DDM allows the measurement of an advective speed, simultaneously, over a range of spatialfrequency q, where each q defines a length-scale L=2*pi/q (for more details, see [Wilson et al, 2011, Martinez et al, 2012).…”

“…The systematic error of our measurements is then calculated as the standard deviation of the mean, and falls within ∼5% of the mean value (here not plotted for clarity). The traces shown have been viscosity corrected as well, as free-swimming cells with the motor operating in liner-torque regime do not increase the swimming speeds with viscosities we used in our experiments [Martinez et al, 2014]. (C) Viscosity corrected single motors speeds calculated as 3 min averages corresponding to a section between t=17 and t=20 min in A.…”

Bacteria exploit a polymorphic instability of the flagellar filament to escape from traps

“…Screw Formation Is Triggered by a Polymorphic Filament Instability at CW Rotation. The effects of Ficoll are to slow down the motion of the flagellum and to increase the drag forces (32,34). We therefore used Ficoll-supplemented medium on agarose surfaces to reduce the speed of the rotating flagellar filament, allowing us to obtain sufficient temporal resolution to monitor its motion and morphology during formation and release of the large helix around the cell body (Fig.…”

Following fluctuating signs: Anomalous active superdiffusion of swimmers in anisotropic media

“…By and large, most artificial microswimmers considered so far are embedded in a simple Newtonian fluid at low Reynolds number [1][2][3][4][5]. Many microorganisms in their natural environment, however, are exposed to much more complex media, which are more appropriately described by complex non-Newtonian fluids [6,7]. Examples range from the motion of cilia and spermatozoa in mucus [8,9] to bacteria in the host tissue [10] and nematodes migrating though soil [11].…”