19 research outputs found
How bacterial cells and colonies move on solid substrates
Many bacteria rely on active cell appendages, such as type IV pili, to move
over substrates and interact with neighboring cells. Here, we study the motion
of individual cells and bacterial colonies, mediated by the collective
interactions of multiple pili. It was shown experimentally that the substrate
motility of Neisseria gonorrhoeae cells can be described as a persistent random
walk with a persistence length that exceeds the mean pili length. Moreover, the
persistence length increases for a higher number of pili per cell. With the
help of a simple, tractable stochastic model, we test whether a tug-of-war
without directional memory can explain the persistent motion of single
Neisseria gonorrhoeae cells. While the persistent motion of single cells indeed
emerges naturally in the model, a tug-of-war alone is not capable of explaining
the motility of microcolonies, which becomes weaker with increasing colony
size. We suggest sliding friction between the microcolonies and the substrate
as the missing ingredient. While such friction almost does not affect the
general mechanism of single cell motility, it has a strong effect on colony
motility. We validate the theoretical predictions by using a three-dimensional
computational model that includes explicit details of the pili dynamics, force
generation and geometry of cells.Comment: 25 pages, 17 figure
Aggregation controlled by condensate rheology
Biomolecular condensates in living cells can exhibit a complex rheology, including viscoelastic and glassy behavior. This rheological behavior of condensates was suggested to regulate polymerization of cytoskeletal filaments and aggregation of amyloid fibrils. Here, we theoretically investigate how the rheological properties of condensates can control the formation of linear aggregates. To this end, we propose a kinetic theory for linear aggregation in coexisting phases, which accounts for the aggregate size distribution and the exchange of aggregates between inside and outside of condensates. The rheology of condensates is accounted in our model via aggregate mobilities that depend on aggregate size. We show that condensate rheology determines whether aggregates of all sizes or dominantly small aggregates are exchanged between condensate inside and outside on the timescale of aggregation. As a result, the ratio of aggregate numbers inside to outside of condensates differs significantly. Strikingly, we also find that weak variations in the rheological properties of condensates can lead to a switch-like change of the number of aggregates. These results suggest a possible physical mechanism for how living cells could control linear aggregation in a switch-like fashion through variations in condensate rheology
Pili mediated intercellular forces shape heterogeneous bacterial microcolonies prior to multicellular differentiation
Microcolonies are aggregates of a few dozen to a few thousand cells exhibited
by many bacteria. The formation of microcolonies is a crucial step towards the
formation of more mature bacterial communities known as biofilms, but also
marks a significant change in bacterial physiology. Within a microcolony,
bacteria forgo a single cell lifestyle for a communal lifestyle hallmarked by
high cell density and physical interactions between cells potentially altering
their behaviour. It is thus crucial to understand how initially identical
single cells start to behave differently while assembling in these tight
communities. Here we show that cells in the microcolonies formed by the human
pathogen Neisseria gonorrhoeae (Ng) present differential motility behaviors
within an hour upon colony formation. Observation of merging microcolonies and
tracking of single cells within microcolonies reveal a heterogeneous motility
behavior: cells close to the surface of the microcolony exhibit a much higher
motility compared to cells towards the center. Numerical simulations of a
biophysical model for the microcolonies at the single cell level suggest that
the emergence of differential behavior within a multicellular microcolony of
otherwise identical cells is of mechanical origin. It could suggest a route
toward further bacterial differentiation and ultimately mature biofilms.Comment: 29 pages, 5 figures, supplementary information attache
Multiscale modeling of bacterial colonies: how pili mediate the dynamics of single cells and cellular aggregates
Neisseria gonorrhoeae is the causative agent of one of the most common sexually transmitted diseases, gonorrhea. Over the past two decades there has been an alarming increase of reported gonorrhea cases where the bacteria were resistant to the most commonly used antibiotics thus prompting for alternative antimicrobial treatment strategies. The crucial step in this and many other bacterial infections is the formation of microcolonies, agglomerates consisting of up to several thousands of cells. The attachment and motility of cells on solid substrates as well as the cell–cell interactions are primarily mediated by type IV pili, long polymeric filaments protruding from the surface of cells. While the crucial role of pili in the assembly of microcolonies has been well recognized, the exact mechanisms of how they govern the formation and dynamics of microcolonies are still poorly understood. Here, we present a computational model of individual cells with explicit pili dynamics, force generation and pili–pili interactions. We employ the model to study a wide range of biological processes, such as the motility of individual cells on a surface, the heterogeneous cell motility within the large cell aggregates, and the merging dynamics and the self-assembly of microcolonies. The results of numerical simulations highlight the central role of pili generated forces in the formation of bacterial colonies and are in agreement with the available experimental observations. The model can quantify the behavior of multicellular bacterial colonies on biologically relevant temporal and spatial scales and can be easily adjusted to include the geometry and pili characteristics of various bacterial species. Ultimately, the combination of the microbiological experimental approach with the in silico model of bacterial colonies might provide new qualitative and quantitative insights on the development of bacterial infections and thus pave the way to new antimicrobial treatments
Quantitative Measurement of Fluorescent Layers with Respect to Spatial Thickness Variations and Substrate Properties
Imaging fluorescence spectroscopy proves to be a fast and sensitive method for measuring the thickness of thin coatings in the manufacturing industry. This encouraged us to systematically study, theoretically and experimentally, parameters that influence the fluorescence of thin layers. We analyzed the fluorescence signal as a function of the scattering and reflectance properties of the sample substrate. In addition, we investigated effects of the layer properties on fluorescence emission. A ray-tracing software is used to describe the influence of these parameters on the fluorescence emission of thin layers. Experiments using a custom-made system for imaging fluorescence analysis verify the simulations. This work shows a factor five variation of fluorescence intensity as a function of the reflectance of the sample substrate. Simulations show variations by a factor of up to eight for samples with different surface roughness. Results on tilted samples indicate a significant increase of the detected fluorescence signal, for fluorescent droplets on reflective substrates, if illuminated and coaxially observed at angles greater than 25°. These findings are of utmost relevance for all applications which utilize the fluorescence emission to quantify thin layers. These applications range from in-line lubricant monitoring in press plants to monitoring of functional coatings in medical technology and the detection of filmic contaminations
Paving the Way for Low Breakage Rates in Industrial Production of N.I.C.E.-Wire Modules
Significant improvements in module performance are possible via implementation of multi-wire electrodes. This is economically sound as long as the mechanical yield of the production is maintained. While flat ribbons have a relatively large contact area to exert forces onto the solar cell, wires with round cross section reduce this contact area considerably – in theory to an infinitively thin line. Therefore, the local stresses induced by the electrodes might increase to a point that mechanical production yields suffer unacceptably.
In this paper, we assess this issue by an analytical mechanical model as well as experiments with an encapsulant-free N.I.C.E. test setup. From these, we can derive estimations for the relationship between lay-up accuracy and expected breakage losses. This paves the way for cost-optimized choices of handling equipment in industrial N.I.C.E.-wire production lines
The multi-busbar design : an overview
The demand for highly efficient photovoltaic modules at low costs leads to new solar cell designs. For enhanced module efficiency the cell efficiency has to be optimized regarding later operation under module conditions. This implies that the interconnected solar cell structure has to be assessed. Commonly the solar cell itself is optimized separately.In this work an easy to implement cell design was investigated where the number of busbars was varied to decrease the total series resistance of the interconnected solar cell. For this study a simulation program based on the two-diode model was applied to determine the optimal efficiency of the device. Furthermore, the simulations revealed that a device with multiple busbars has a high potential in cost savings due to a reduction in metal consumption for the front side metallization. For an optimized cell structure the amount of Ag paste needed for a sufficient front side metallization could be reduced to 7 mg Ag paste for a 6 inch solar cell. In the same time the efficiency can be increased. A detailed simulation of a screen printed and stringed rear side of a multi-busbar solar cell revealed the amount of rear side pads necessary for a sufficient interconnection leading to low series resistances