Bacterial biofilms are organized communities of cells living in association with surfaces. The hallmark of biofilm formation is the secretion of a polymeric matrix rich in sugars and proteins in the extracellular space. In Bacillus subtilis, secretion of the exopolysaccharide (EPS) component of the extracellular matrix is genetically coupled to the inhibition of flagella-mediated motility. The onset of this switch results in slow expansion of the biofilm on a substrate. Different strains have radically different capabilities in surface colonization: Flagella-null strains spread at the same rate as wild type, while both are dramatically faster than EPS mutants. Multiple functions have been attributed to the EPS, but none of these provides a physical mechanism for generating spreading. We propose that the secretion of EPS drives surface motility by generating osmotic pressure gradients in the extracellular space. A simple mathematical model based on the physics of polymer solutions shows quantitative agreement with experimental measurements of biofilm growth, thickening, and spreading. We discuss the implications of this osmotically driven type of surface motility for nutrient uptake that may elucidate the reduced fitness of the matrix-deficient mutant strains.collective motility | gel swelling | surface translocation | bacterial biofilm | polymeric secretion B acterial biofilms are heterogeneous populations of differentiated bacteria that live in association with surfaces and exhibit a remarkable degree of spatio-temporal organization (1-3). The formation of a mature biofilm occurs in several stages, starting from the attachment of a single cell to a solid substrate. When cells commit to the surface, a protein-and sugar-rich polymeric extracellular matrix (ECM) is secreted in the extracellular space and holds the community together. Several different functions have been attributed to the ECM, ranging from protection to mechanical integrity and reserve of nutrient (4, 5). At the same time, flagella are downregulated, and most cells lose their individual motility. For the Gram-positive soil bacterium Bacillus subtilis the loss of flagella-mediated motility is genetically coupled to the production of extracellular matrix (6-10). This switch results in a slow kind of surface motility that allows the biofilm to spread outward on the substrate. Although spreading has been attributed to a qualitative concept of "pushing" associated with biomass growth, the physical force generating mechanism that drives biofilm expansion outward across a surface is not known. One intriguing possibility is the potential contribution of the ECM to biofilm growth; the ECM is a highly visco-elastic and sticky substance, and it might be expected to hinder expansion rather than facilitate it (4, 5). However, the ECM clearly plays a crucial role in biofilm, and its possible effect in the actual expansion of the biofilms has never been investigated.Here, we demonstrate that, in the first 24 h of biofilm development, extracellular matrix product...

Most of the world's bacteria exist in robust, sessile communities known as biofilms, ubiquitously adherent to environmental surfaces from ocean floors to human teeth and notoriously resistant to antimicrobial agents. We report the surprising observation that Bacillus subtilis biofilm colonies and pellicles are extremely nonwetting, greatly surpassing the repellency of Teflon toward water and lower surface tension liquids. The biofilm surface remains nonwetting against up to 80% ethanol as well as other organic solvents and commercial biocides across a large and clinically important concentration range. We show that this property limits the penetration of antimicrobial liquids into the biofilm, severely compromising their efficacy. To highlight the mechanisms of this phenomenon, we performed experiments with mutant biofilms lacking ECM components and with functionalized polymeric replicas of biofilm microstructure. We show that the nonwetting properties are a synergistic result of ECM composition, multiscale roughness, reentrant topography, and possibly yet other factors related to the dynamic nature of the biofilm surface. Finally, we report the impenetrability of the biofilm surface by gases, implying defense capability against vapor-phase antimicrobials as well. These remarkable properties of B. subtilis biofilm, which may have evolved as a protection mechanism against native environmental threats, provide a new direction in both antimicrobial research and bioinspired liquid-repellent surface paradigms.antimicrobial resistance | microcomputed tomography | biofilm hydrophobicity | liquid repellency | nonwettability

Abstract

The growth of cloud droplets by diffusion of water vapor in a three-dimensional homogeneous isotropic turbulent flow is considered. Within a simple model of advection and condensation, the dynamics and growth of millions of droplets are integrated. A droplet-size spectra broadening is obtained and it is shown to increase with the Reynolds number of turbulence by means of two series of direct numerical simulations at increasing resolution. This is a key point toward a proper evaluation of the effects of turbulence for condensation in warm clouds, where the Reynolds numbers typically achieve extreme values. The obtained droplet spectral broadening as a function of the Reynolds number is shown to be consistent with dimensional arguments. A generalization of this expectation to Reynolds numbers not accessible by direct numerical simulation (DNS) is proposed, yielding upper and lower bounds to the actual size spectra broadening. It is argued that the lower bound is the relevant limit at high Reynolds numbers. A further DNS matching the large scales of the system suggests consistency of the picture drawn. The assumptions underlying the model are expected to hold up to spatial scales on the order of 100 m; no direct comparison with in situ measures is possible. Additional effort is needed to evaluate the impact of this effect for condensation in more realistic cloud conditions.

SUMMARY The ability to shift between multiple decision-making strategies during natural behavior allows animals to strike a balance between flexibility and efficiency. We investigated odor-guided navigation by mice to understand how decision making strategies are balanced during a complex natural behavior. Mice navigated to odor sources in an open arena using naturally fluctuating airborne odor cues as their positions were recorded precisely in real time. When mice had limited prior experience of source locations, their search behavior was consistent with a gradient ascent algorithm that utilized directional cues in the plume to navigate to the odor source. Gradient climbing was effective because the arena size allowed animals to conduct their search mainly within the odor plume, with frequent odor contacts. With increased experience, mice shifted their strategy from this flexible, sensory-driven search behavior to a more efficient and stereotyped foraging approach that varied little in response to odor plumes. This study demonstrates that mice use prior knowledge to adaptively balance flexibility and efficiency during complex behavior guided by dynamic natural stimuli.

Abstract. -The problem of droplet growth by condensation in a turbulent flow of nearly saturated vapour is addressed theoretically and numerically. We show how the presence of an underlying turbulent velocity field induces a correlation between droplet trajectories and supersaturation. This leads both to the enhancement of the droplet growth rate and to a fast spreading of the droplet size distribution.Introduction. -The evolution of microdroplets in a turbulent environment is an issue of great interest for a variety of applications ranging from health care [1], to engineering, to atmospheric sciences [2][3][4]. In the latter context, microdroplet growth by condensation/evaporation is a phenomenon of paramount importance for the early stages of cloud evolution. Warm clouds are essentially a polydisperse aerosol of water droplets suspended in a moist air. The smallest droplets are created by condensation onto sub-micron solid particles (cloud condensation nuclei), whereas raindrops typically exceed 1 mm in radius. This observation naturally motivates one to investigate droplet growth, which eventually leads droplets to fall under gravitational force. Different stages follow droplets formation. First, they grow by condensation of vapour molecules on their surface. Second, upon having reached a radius of the order of 20 µm, they begin to coalesce to form bigger drops. Here we focus on the first stage of the growth by condensation. Experimental measurements of droplet radii (see, e.g., [5,6]) in fair weather clouds show a broad distribution in the range 1-20 µm. The presence of droplets with very different sizes can significantly enhance the efficiency of successive collisions and thus contribute to a fast initiation of the precipitation process. However, up to now this effect has not been reproduced by classical models of the condensation stage [7]. Their basic ingredient is the assumption that droplets are essentially confined to a small portion of the cloud, dubbed "fluid parcel", where they experience the same value of humidity. Many efforts have been made to evaluate the influence of fine scale turbulence on the macroscopic properties of clouds (see [8] for a review). Recent numerical simulations of a turbulent ascending moist air parcel show that resolving the fluctuations below the scale of the parcel itself does not result in a significant spectral broadening [9][10][11]. The theory assuming Brownian random walk of an air

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The phospholipid phosphatidylinositol-4-phosphate [PI(4)P], generated at the Golgi and plasma membrane, has been implicated in many processes, including membrane traffic, yet its role in cell morphology changes, such as the budding to filamentous growth transition, is unknown. We show that Golgi PI(4)P is required for such a transition in the human pathogenic fungus Candida albicans. Quantitative analyses of membrane traffic revealed that PI(4)P is required for late Golgi and secretory vesicle dynamics and targeting and, as a result, is important for the distribution of a multidrug transporter and hence sensitivity to antifungal drugs. We also observed that plasma membrane PI(4)P, which we show is functionally distinct from Golgi PI(4)P, forms a steep gradient concomitant with filamentous growth, despite uniform plasma membrane PI-4-kinase distribution. Mathematical modeling indicates that local PI(4)P generation and hydrolysis by phosphatases are crucial for this gradient. We conclude that PI(4)P-regulated membrane dynamics are critical for morphology changes.