Agnese Seminara

Bacterial biofilm shows persistent resistance to liquid wetting and gas penetration

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.

Osmotic spreading of Bacillus subtilis biofilms driven by an extracellular matrix

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.

Dispersal of fungal spores on a cooperatively generated wind

Because of their microscopic size, the forcibly ejected spores of ascomycete fungi are quickly brought to rest by drag. Nonetheless some apothecial species, including the pathogen Sclerotinia sclerotiorum, disperse with astonishing rapidity between ephemeral habitats. Here we show that by synchronizing the ejection of thousands of spores, these fungi create a flow of air that carries spores through the nearly still air surrounding the apothecium, around intervening obstacles, and to atmospheric currents and new infection sites. High-speed imaging shows that synchronization is self-organized and likely triggered by mechanical stresses. Although many spores are sacrificed to produce the favorable airflow, creating the potential for conflict among spores, the geometry of the spore jet physically targets benefits of the airflow to spores that cooperate maximally in its production. The ability to manipulate a local fluid environment to enhance spore dispersal is a previously overlooked feature of the biology of fungal pathogens, and almost certainly shapes the virulence of species including S. sclerotiorum. Synchronous spore ejection may also provide a model for the evolution of stable, self-organized behaviors.

Cloud Droplet Growth by Condensation in Homogeneous Isotropic Turbulence

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.

Nutrient depletion in Bacillus subtilis biofilms triggers matrix production

Many types of bacteria form colonies that grow into physically robust and strongly adhesive aggregates known as biofilms. A distinguishing characteristic of bacterial biofilms is an extracellular polymeric substance (EPS) matrix that encases the cells and provides physical integrity to the colony. The EPS matrix consists of a large amount of polysaccharide, as well as protein filaments, DNA and degraded cellular materials. The genetic pathways that control the transformation of a colony into a biofilm have been widely studied, and yield a spatiotemporal heterogeneity in EPS production. Spatial gradients in metabolites parallel this heterogeneity in EPS, but nutrient concentration as an underlying physiological initiator of EPS production has not been explored. Here, we study the role of nutrient depletion in EPS production in Bacillus subtilis biofilms. By monitoring simultaneously biofilm size and matrix production, we find that EPS production increases at a critical colony thickness that depends on the initial

Droplet condensation in two-dimensional Bolgiano turbulence

The evolution of microdroplets transported by a turbulent flow is considered. Droplets surrounded by moist air are able to grow by diffusion of water vapour on their surface. A two-dimensional model of dry Boussinesq thermal convection is considered in which the turbulent velocity field is driven by a temperature gradient. The evolution of the velocity, temperature, vapour fields and of droplet trajectories and radii is analysed by means of high-resolution direct numerical simulations. Despite the fact that the environment becomes drier and drier, a mean growth of droplets is obtained. The mechanism identified is based on the presence of correlations between the vapour field and droplet trajectories. Besides, a spreading of size distribution is observed, with the formation of droplets with very different sizes. Improvements with respect to previous models are discussed.

Probing phenotypic growth in expanding Bacillus subtilis biofilms

We develop an optical imaging technique for spatially and temporally tracking biofilm growth and the distribution of the main phenotypes of a Bacillus subtilis strain with a triple-fluorescent reporter for motility, matrix production, and sporulation. We develop a calibration procedure for determining the biofilm thickness from the transmission images, which is based on Beer-Lambert’s law and involves cross-sectioning of biofilms. To obtain the phenotype distribution, we assume a linear relationship between the number of cells and their fluorescence and determine the best combination of calibration coefficients that matches the total number of cells for all three phenotypes and with the total number of cells from the transmission images. Based on this analysis, we resolve the composition of the biofilm in terms of motile, matrix-producing, sporulating cells and low-fluorescent materials which includes matrix and cells that are dead or have low fluorescent gene expression. We take advantage of the circular growth to make kymograph plots of all three phenotypes and the dominant phenotype in terms of radial distance and time. To visualize the nonlocal character of biofilm growth, we also make kymographs using the local colonization time. Our technique is suitable for real-time, noninvasive, quantitative studies of the growth and phenotype distribution of biofilms which are either exposed to different conditions such as biocides, nutrient depletion, dehydration, or waste accumulation.

A natural O-ring optimizes the dispersal of fungal spores

The forcibly ejected spores of ascomycete fungi must penetrate several millimetres of nearly still air surrounding sporocarps to reach dispersive airflows, and escape is facilitated when a spore is launched with large velocity. To launch, the spores of thousands of species are ejected through an apical ring, a small elastic pore. The startling diversity of apical ring and spore shapes and dimensions make them favoured characters for both species descriptions and the subsequent inference of relationships among species. However, the physical constraints shaping this diversity and the adaptive benefits of specific morphologies are not understood. Here, we develop an elastohydrodynamic theory of the spores ejection through the apical ring and demonstrate that to avoid enormous energy losses during spore ejection, the four principal morphological dimensions of spore and apical ring must cluster within a nonlinear one-dimensional subspace. We test this prediction using morphological data for 45 fungal species from two different classes and 18 families. Our sampling encompasses multiple loss and gain events and potentially independent origins of this spore ejection mechanism. Although the individual dimensions of the spore and apical ring are only weakly correlated with each other, they collapse into the predicted subspace with high accuracy. The launch velocity appears to be within 2 per cent of the optimum for over 90 per cent of all forcibly ejected species. Although the morphological diversity of apical rings and spores appears startlingly diverse, a simple principle can be used to organize it.

A universal growth limit for circular lichens

Lichens fix carbon dioxide from the air to build biomass. Crustose and foliose lichens grow as nearly flat, circular disks. Smaller individuals grow slowly, but with small, steady increases in radial growth rate over time. Larger individuals grow more quickly and with a roughly constant radial velocity maintained over the lifetime of the lichen. We translate the coffee drop effect to model lichen growth and demonstrate that growth patterns follow directly from the diffusion of carbon dioxide in the air around a lichen. When a lichen is small, carbon dioxide is fixed across its surface, and the entire thallus contributes to radial growth, but when a lichen is larger carbon dioxide is disproportionately fixed at the edges of an individual, which are the primary drivers of growth. Tests of the model against data suggest it provides an accurate, robust, and universal framework for understanding the growth dynamics of both large and small lichens in nature.