In this letter we propose a kinematic model to explain how collisions with a surface and rotational Brownian motion give rise to accumulation of micro-swimmers near a surface. In this model, an elongated microswimmer invariably travels parallel to the surface after hitting it from an oblique angle. It then swims away from the surface, facilitated by rotational Brownian motion. Simulations based on this model reproduce the density distributions measured for the small bacteria E. coli and Caulobacter crescentus, as well as for the much larger bull spermatozoa swimming between two walls.Swimming aids the function of microorganisms, such as enhancing the formation of biofilms on surfaces [1]. Swimming also helps transport sperms toward eggs for fertilization [2]. The density of cells as a function of distance from a surface has been measured for swimming E. coli [3] and bull spermatozoa [4], showing interestingly in both cases values much higher near the surface than far away. This near surface accumulation has mainly been attributed to the hydrodynamic attraction between the cells and the surface [4,5]. Recently, Berke et al. [3] combined the effects of the hydrodynamic attraction and the translational Brownian motion of the cells to predict the distribution of E. coli as a function of distance. As noted by the authors [3], however, this interpretation is not applicable to cells within 10 μm from the surface, where the cell density is the highest. The hydrodynamic interaction among the microswimmers has been shown to be important only at high cell concentrations [6,7].In this letter we present a different account for the near surface accumulation. We ignore the hydrodynamic attraction but emphasize the role of collision with a surface at a low Reynolds number [8], an interaction that deflects the swimming direction, and the role of rotational Brownian motion of individual microswimmers in a confined environment. We show that a typical microswimmer with an elongated shape tends to swim parallel to a surface after hitting it at an oblique angle and therefore accumulate near the surface. Rotational Brownian motion [9] then relaxes the accumulation by randomly changing the swimming direction so that the cells have chances to swim away from the surface. In the extreme case of no rotational Brownian motion, all the cells would end up swimming in close proximity with the surface. In the opposite extreme of very fast rotational Brownian motion, the cells will quickly change to any possible swimming direction and subsequently would be found anywhere with equal probability. In reality, a microswimmer randomly changes its swimming direction with a finite rotational diffusion constant, resulting in a distribution in between the two extremes, that is, more cells stay near the surface and fewer far away.We used the bacterium C. crescentus strain CB15 SB3860 to examine the near surface swimming and accumulation. Swarmer cells of this mutant swim forward only and do not *Jay_Tang@brown.edu. . We noted that although this ...

Summary The attachment of bacteria to surfaces provides advantages such as increasing nutrient access and resistance to environmental stress. Attachment begins with a reversible phase, often mediated by surface structures such as flagella and pili, followed by a transition to irreversible attachment, typically mediated by polysaccharides. Here we show that the interplay between pili and flagellum rotation stimulates the rapid transition between reversible and polysaccharide-mediated irreversible attachment. We found that reversible attachment of Caulobacter crescentus cells is mediated by motile cells bearing pili and that their contact with a surface results in the rapid pili-dependent arrest of flagellum rotation and concurrent stimulation of polar holdfast adhesive polysaccharide. Similar stimulation of polar adhesin production by surface contact occurs in Asticcacaulis biprosthecum and Agrobacterium tumefaciens. Therefore, single bacterial cells respond to their initial contact with surfaces by triggering just-in-time adhesin production. This mechanism restricts stable attachment to intimate surface interactions, thereby maximizing surface attachment, discouraging non-productive self-adherence, and preventing curing of the adhesive.

The adhesion of bacteria to surfaces plays critical roles in the environment, disease, and industry. In aquatic environments, Caulobacter crescentus is one of the first colonizers of submerged surfaces. Using a micromanipulation technique, we measured the adhesion force of single C. crescentus cells attached to borosilicate substrates through their adhesive holdfast. The detachment forces measured for 14 cells ranged over 0.11 to 2.26 N, averaging 0.59 ؎ 0.62 N. Based on the calculation of stress distribution with the finite element analysis method (dividing an object into small grids and calculating relevant parameters for all of the elements), the adhesion strength between the holdfast and the substrate is >68 N͞mm 2 in the central region of contact. To our knowledge, this strength of adhesion is the strongest ever measured for biological adhesives.adhesive strength ͉ Caulobacter crescentus ͉ cell mechanics ͉ holdfast ͉ micromanipulation

Monodisperse 11 nm indium tin oxide (ITO) nanocrystals (NCs) were synthesized by thermal decomposition of indium acetylacetonate, In(acac)(3), and tin bis(acetylacetonate)dichloride, Sn(acac)(2)Cl(2), at 270 °C in 1-octadecene with oleylamine and oleic acid as surfactants. Dispersed in hexane, these ITO NCs were spin-cast on centimeter-wide glass substrates, forming uniform ITO NC assemblies with root-mean-square roughness of 2.9 nm. The assembly thickness was controlled by ITO NC concentrations in hexane and rotation speeds of the spin coater. Via controlled thermal annealing at 300 °C for 6 h under Ar and 5% H(2), the ITO NC assemblies became conductive and transparent with the 146 nm-thick assembly showing 5.2 × 10(-3) Ω·cm (R(s) = 356 Ω/sq) resistivity and 93% transparency in the visible spectral range--the best values ever reported for ITO NC assemblies prepared from solution phase processes. The stable hexane dispersion of ITO NCs was also readily spin-cast on polyimide (T(g) ~360 °C), and the resultant ITO assembly exhibited a comparable conductivity and transparency to the assembly on a glass substrate. The reported synthesis and assembly provide a promising solution to the fabrication of transparent and conducting ITO NCs on flexible substrates for optoelectronic applications.

Brownian motion influences bacterial swimming by randomizing displacement and direction. Here, we report that the influence of Brownian motion is amplified when it is coupled to hydrodynamic interaction. We examine swimming trajectories of the singly flagellated bacterium Caulobacter crescentus near a glass surface with total internal reflection fluorescence microscopy and observe large fluctuations over time in the distance of the cell from the solid surface caused by Brownian motion. The observation is compared with computer simulation based on analysis of relevant physical factors, including electrostatics, van der Waals force, hydrodynamics, and Brownian motion. The simulation reproduces the experimental findings and reveals contribution from fluctuations of the cell orientation beyond the resolution of present observation. Coupled with hydrodynamic interaction between the bacterium and the boundary surface, the fluctuations in distance and orientation subsequently lead to variation of the swimming speed and local radius of curvature of swimming trajectory. These results shed light on the fundamental roles of Brownian motion in microbial motility, nutrient uptake, and adhesion.Caulobacter ͉ adhesion ͉ Derjaguin-Landau-Verwey-Overbeek theory ͉ hydrodynamics B rownian motion, the random movement of microscopic objects in fluid caused by constant thermal agitation, is of fundamental significance in life science (1), particularly in the microbial world (2). The motility of microbes in aqueous environments is substantially altered by Brownian motion. In the widely read book entitled Random Walks in Biology, Howard Berg discusses the strong influence of Brownian motion on the swimming trajectory of peritrichously flagellated Escherichia coli. (2). A monotrichous bacterium, however, would not be able to vary its swimming direction and seek food efficiently without rotational Brownian motion, because it cannot tumble to change direction as an E. coli does (3). Therefore, Brownian motion may be especially important for the chemotaxis of a monotrichous bacterium that uses its single polar flagellum to swim back and forth (4). Indeed, it has been shown by computer simulations that rotational Brownian motion significantly increases the ability of singly-flagellated marine bacteria to stay with falling marine snow particles, which are rich in nutrients (5, 6).The commonly recognized influence of Brownian motion on a swimming microbe is the random deviation of its swimming trajectory from a straight path. The deviations are caused by collisions between the microbe and its surrounding water molecules in thermodynamic equilibrium. We show here that Brownian motion has an additional and even stronger influence when it is coupled to the hydrodynamic interaction between a swimming bacterium and a fluid boundary. In this situation, the hydrodynamic interaction depends sensitively on the distance of the bacterium to the boundary surface (7,8). Brownian motion causes that distance to vary randomly, and via coupling with the hydro...

We measured the distribution of a forward swimming strain of Caulobacter crescentus near a surface using a three-dimensional tracking technique based on dark field microscopy and found that the swimming bacteria accumulate heavily within a micrometer from the surface. We attribute this accumulation to frequent collisions of the swimming cells with the surface, causing them to align parallel to the surface as they continually move forward. The extent of accumulation at the steady state is accounted for by balancing alignment caused by these collisions with the rotational Brownian motion of the micrometer-sized bacteria. We performed a simulation based on this model, which reproduced the measured results. Additional simulations demonstrate the dependence of accumulation on swimming speed and cell size, showing that longer and faster cells accumulate more near a surface than shorter and slower ones do.

The aquatic bacterium Caulobacter crescentus attaches to solid surfaces through an adhesive holdfast located at the tip of its polar stalk, a thin cylindrical extension of the cell membrane. In this paper, the elastic properties of the C. crescentus stalk and holdfast assembly were studied by using video light microscopy. In particular, the contribution of oligomers of N-acetylglucosamine (GlcNAc) to the elasticity of holdfast was examined by lysozyme digestion. C. crescentus cells attached to a surface undergo Brownian motion while confined effectively in a harmonic potential. Mathematical analysis of such motion enabled us to determine the force constant of the stalk-holdfast assembly, which quantifies its elastic properties. The measured force constant exhibits no dependence on stalk length, consistent with the theoretical estimate showing that the stalk can be treated as a rigid rod with respect to fluctuations of the attached cells. Therefore, the force constant of the stalk-holdfast assembly can be attributed to the elasticity of the holdfast. Motions of cells in a rosette were found to be correlated, consistent with the elastic characteristics of the holdfast. Atomic force microscopy analysis indicates that the height of a dried (in air) holdfast is approximately one-third of that of a wet (in water) holdfast, consistent with the gel-like nature of the holdfast. Lysozyme, which cleaves oligomers of GlcNAc, reduced the force constant to less than 10% of its original value, consistent with the polysaccharide gel-like nature of the holdfast. These results also indicate that GlcNAc polymers play an important role in the strength of the holdfast.Caulobacter crescentus is a gram-negative bacterium ubiquitous in fresh water, soil, and seawater (1, 2, 14, 23-25). These environments are often very dilute in nutrients, such as the essential nutrient inorganic phosphate. C. crescentus exhibits a dimorphic life cycle (Fig. 1) that probably provides an advantage in such competitive environments. The hallmark of the dimorphic life cycle is the ordered synthesis of polar structures visible by microscopy, which allows developmental stages to be easily defined. Approximately one-third of the C. crescentus life cycle is spent in an obligatory free-swimming dispersal mode known as the swarmer phase. The most notable structure that defines the swarmer cell phase is the single polar flagellum. Flagellar motility allows the swarmer cell to explore new microenvironments where nutrients may be more plentiful. While in the swarmer phase, C. crescentus cannot initiate DNA replication or cell division. After the obligatory swarmer phase, the cell differentiates into a stalked cell by initiating DNA replication, releasing the flagellum, and synthesizing a stalk, a thin cylindrical extension of the cell membrane (24). Synthesis of the adhesive holdfast occurs early during swarmer cell differentiation (11) at the same pole as the stalk, resulting in its positioning at the tip of the stalk. The stalked cell elongates, initiates cell di...

We determined the torque of the flagellar motor of Caulobacter crescentus for different motor rotation rates by measuring the rotation rate and swimming speed of the cell body and found it to be remarkably different from that of other bacteria, such as Escherichia coli and Vibrio alginolyticus. The average stall torque of the Caulobacter flagellar motor was approximately 350 pN nm, much smaller than the values of the other bacteria measured. Furthermore, the torque of the motor remained constant in the range of rotation rates up to those of freely swimming cells. In contrast, the torque of a freely swimming cell for V. alginolyticus is typically approximately 20% of the stall torque. We derive from these results that the C. crescentus swarmer cells swim more efficiently than both E. coli and V. alginolyticus. Our findings suggest that C. crescentus is optimally adapted to low nutrient aquatic environments.