Krister Wiklund

The Shaft of the Type 1 Fimbriae Regulates an External Force to Match the FimH Catch Bond

Type 1 fimbriae mediate adhesion of uropathogenic Escherichia coli (UPEC) to host cells. It has been hypothesized that fimbriae can, by their ability to uncoil under exposure to force, reduce fluid shear stress on the adhesin-receptor interaction by which the bacterium adheres to the surface. In this work we develop a model that describes how the force on the adhesin-receptor interaction of a type 1 fimbriae varies as a bacterium is affected by a time dependent fluid flow mimicking in vivo conditions. The model combines in vivo hydrodynamic conditions with previously assessed biomechanical properties of the fimbriae. Numerical methods are used to solve for the motion and adhesion force under the presence of time dependent fluid profiles. It is found that a bacterium tethered with a type 1 pilus will experience significantly reduced shear stress for moderate to high flow velocities and that the maximum stress the adhesin will experience is limited to ~120 pN, which is sufficient to activate the conformational change of the FimH adhesin into its stronger state but also lower than the force required for breaking it under rapid loading. Our model thus supports the assumption that the type 1 fimbriae shaft and the FimH adhesin-receptor interaction are optimized to each other, and that they give piliated bacteria significant advantages in rapidly changing fluidic environments.

Characterization of the Biomechanical Properties of T4 Pili Expressed by Streptococcus pneumoniae—A Comparison between Helix-like and Open Coil-like Pili

Bacterial adhesion organelles, known as fimbria or pili, are expressed by gram-positive as well as gram-negative bacteria families. These appendages play a key role in the first steps of the invasion and infection processes, and they therefore provide bacteria with pathogenic abilities. To improve the knowledge of pili-mediated bacterial adhesion to host cells and how these pili behave under the presence of an external force, we first characterize, using force measuring optical tweezers, open coil-like T4 pili expressed by gram-positive Streptococcus pneumoniae with respect to their biomechanical properties. It is shown that their elongation behavior can be well described by the worm-like chain model and that they possess a large degree of flexibility. Their properties are then compared with those of helix-like pili expressed by gram-negative uropathogenic Escherichia coli (UPEC), which have different pili architecture. The differences suggest that these two types of pili have distinctly dissimilar mechanisms to adhere and sustain external forces. Helix-like pili expressed by UPEC bacteria adhere to host cells by single adhesins located at the distal end of the pili while their helix-like structures act as shock absorbers to dampen the irregularly shear forces induced by urine flow and to increase the cooperativity of the pili ensemble, whereas open coil-like pili expressed by S. pneumoniae adhere to cells by a multitude of adhesins distributed along the pili. It is hypothesized that these two types of pili represent different strategies of adhering to host cells in the presence of external forces. When exposed to significant forces, bacteria expressing helix-like pili remain attached by distributing the external force among a multitude of pili, whereas bacteria expressing open coil-like pili sustain large forces primarily by their multitude of binding adhesins which presumably detach sequentially.

A fast and robust circle detection method using isosceles triangles sampling

Circle detection using randomized sampling has been developed in recent years to reduce computational intensity. However, randomized sampling is sensitive to noise that can lead to reduced accuracy and false-positive candidates. To improve on the robustness of randomized circle detection under noisy conditions this paper presents a new methodology for circle detection based upon randomized isosceles triangles sampling. It is shown that the geometrical property of isosceles triangles provides a robust criterion to find relevant edge pixels which, in turn, offers an efficient means to estimate the centers and radii of circles. For best efficiency, the estimated results given by the sampling from individual connected components of the edge map were analyzed using a simple clustering approach. To further improve on the accuracy we applied a two-step refinement process using chords and linear error compensation with gradient information of the edge pixels. Extensive experiments using both synthetic and real images have been performed. The results are compared to leading state-of-the-art algorithms and it is shown that the proposed methodology has a number of advantages: it is efficient in finding circles with a low number of iterations, it has high rejection rate of false-positive circle candidates, and it has high robustness against noise. All this makes it adaptive and useful in many vision applications. A new circle detection algorithm based on randomized sampling of isosceles triangles (ITs) is presented.Sampling of isosceles triangles provides distinctive probability distribution for circular shapes.A low amount of iterations are needed by using a simple clustering approch.The proposed algorithm has; short execution time, high accuracy and high robustness against noise.

Rigid multibody simulation of a helix-like structure: the dynamics of bacterial adhesion pili

We present a coarse-grained rigid multibody model of a subunit assembled helix-like polymer, e.g., adhesion pili expressed by bacteria, that is capable of describing the polymer’s force-extension response. With building blocks representing individual subunits, the model appropriately describes the complex behavior of pili expressed by the gram-negative uropathogenic Escherichia coli bacteria under the action of an external force. Numerical simulations show that the dynamics of the model, which include the effects of both unwinding and rewinding, are in good quantitative agreement with the characteristic force-extension response as observed experimentally for type 1 and P pili. By tuning the model, it is also possible to reproduce the force-extension response in the presence of anti-shaft antibodies, which dramatically changes the mechanical properties. Thus, the model and results in this work give enhanced understanding of how a pilus unwinds under the action of external forces and provide a new perspective of the complex bacterial adhesion processes.

UmUTracker: A versatile MATLAB program for automated particle tracking of 2D light microscopy or 3D digital holography data

We present a versatile and fast MATLAB program (UmUTracker) that automatically detects and tracks particles by analyzing video sequences acquired by either light microscopy or digital in-line holog ...

Detecting Bacterial Surface Organelles on Single Cells Using Optical Tweezers

Bacterial cells display a diverse array of surface organelles that are important for a range of processes such as intercellular communication, motility and adhesion leading to biofilm formation, infections, and bacterial spread. More specifically, attachment to host cells by Gram-negative bacteria are mediated by adhesion pili, which are nanometers wide and micrometers long fibrous organelles. Since these pili are significantly thinner than the wavelength of visible light, they cannot be detected using standard light microscopy techniques. At present, there is no fast and simple method available to investigate if a single cell expresses pili while keeping the cell alive for further studies. In this study, we present a method to determine the presence of pili on a single bacterium. The protocol involves imaging the bacterium to measure its size, followed by predicting the fluid drag based on its size using an analytical model, and thereafter oscillating the sample while a single bacterium is trapped by an optical tweezer to measure its effective fluid drag. Comparison between the predicted and the measured fluid drag thereby indicate the presence of pili. Herein, we verify the method using polymer coated silica microspheres and Escherichia coli bacteria expressing adhesion pili. Our protocol can in real time and within seconds assist single cell studies by distinguishing between piliated and nonpiliated bacteria.

Refining particle positions using circular symmetry

Particle and object tracking is gaining attention in industrial applications and is commonly applied in: colloidal, biophysical, ecological, and micro-fluidic research. Reliable tracking information is heavily dependent on the system under study and algorithms that correctly determine particle position between images. However, in a real environmental context with the presence of noise including particular or dissolved matter in water, and low and fluctuating light conditions, many algorithms fail to obtain reliable information. We propose a new algorithm, the Circular Symmetry algorithm (C-Sym), for detecting the position of a circular particle with high accuracy and precision in noisy conditions. The algorithm takes advantage of the spatial symmetry of the particle allowing for subpixel accuracy. We compare the proposed algorithm with four different methods using both synthetic and experimental datasets. The results show that C-Sym is the most accurate and precise algorithm when tracking micro-particles in all tested conditions and it has the potential for use in applications including tracking biota in their environment.

Hermitian structure for linear internal waves in sheared flow

Motivated by the problem of linear mode conversion between ducted internal gravity waves in a sheared flow, we construct a variational principle based on a Hermitian evolution operator, obtained from Larssons Hamiltonian formalism.

Excitation of zonal flows by electron convective cells in a nonuniform dusty magnetoplasma

It is shown that the ponderomotive force of electron convective cells can generate zonal flows (or dust convective cells) in a nonuniform magnetoplasma. For this purpose, two fluid equations are us ...

DSeg: A dynamic image segmentation program to extract backbone patterns for filamentous bacteria and hyphae structures

Motivation Quantitative image analysis of growing filamentous fungi and prokaryotes are important to detect and evaluate morphological effects of growth conditions, compounds and mutations. However, analysis of time-series image data is often limited by the ability of the algorithms to accurately segment structures that are complicated or if an organism is within a crowded population. To overcome these issues we present DSeg; an image analysis program designed to process time-series image data as well as single images to find multiple filamentous structures e.g., filamentous prokaryotes, yeasts and molds using a dynamic segmentation approach. DSeg automatically segments and analyzes objects, includes drift correction, and outputs statistical data such as persistence length, growth rate and growth direction. Availability and implementation DSeg is a free open-source program written in MATLAB. DSeg can be downloaded as a package from https://sourceforge.net/projects/dseg-software. Contact magnus.andersson@umu.se and hanqing.zhang@umu.se Supplementary information Supplementary data are available at online.