Photosynthesis is an essential process for life that converts light into energy for higher-level plants and also phototrophic microbes. Even in the absence of light, these microbes can produce energy through other metabolic processes, such as aerobic and anaerobic respiration. In the case of the unicellular microalga Chlamydomonas reinhardtii, the cell’s swimming motility is affected by a...
Bacteria are able to migrate collectively over wet surfaces and form stable and highly motile aggregates, which are often referred to as biofilms. Collective locomotion of bacteria within aggregates is called swarming [1], and is affected by interactions between bacteria, their shape and the strength of propulsion, and the density of bacteria packing within a biofilm [2,3]. To better...
The ability of motile microorganisms to sense and migrate along due to a chemical or ligand gradient is known as Chemotaxis. This process is a key ingredient in some biological performances, like the acting of immune systems or tumoral migration in metastasis. This mechanism is used also for bacteria to find places to proliferate. The first relevant theoretical model to describe this...
Motile bacteria self-organize in numerous collective phases, such as orientationally ordered phase or swarming state. These collective phases result from properties and activities at the single cell scale, such as growth rate, swimming speed and cell-cell interactions. Understanding how individual properties can trigger emergence of long range order is a crucial aspect of biological and...
Many microorganisms form sessile communities, called biofilms, in self-secreted extracellular polymeric substances (EPS), which often attach to solid surfaces. Biofilm-associated infections have dramatic economic and societal impacts. Recently, slippery surfaces based on liquid infused surfaces have been developed to prevent biofilm formation [1]. However, their antibiofilm performance can...
Organisms such as Chlamydomonas reinhardtii use phototaxis to explore their environment and find the best conditions for photosynthesis. In our work, we study the phototactic behavior of suspensions of C. reinhardtii confined within shallow cylindrical wells, and exposed to directional light stimuli of various intensities. We recover known results: at low light intensities, the algae...
Magnetotactic bacteria (MTB) have garnered significant interest due to their unique ability to align with magnetic fields and respond to environmental stimuli. Currently, the well-studied species of MTBs exhibit complex cell morphologies and propulsion mechanisms, making them poorly suitable for physics modelling. In this project, we study the aerotaxis and magnetic alignment of the novel SS-5...
In this work, we investigate the synchronous swimming of a pair of self-propelled low Reynolds number swimmers in a chemical field [1]. We observe that the hydrodynamic interaction be- tween the swimmers helps them to reach the chemical target quicker than a single swimmer [1, 2].
We have used the chiral squirmer model [2, 4] to understand the dynamics of the swimmers. The former model...
Motility of microscopic entities is a central question in biology. In an idealization attempt, the problem could be reduced to four key elements: viscous flow, soft confinement, thermal fluctuations and activity.
To address this matter, a novel method based on Mie holography and stochastic inference was developed in the group [1]. In a nutshell, this method allows to track particles in 3...
Active fluids, like all other fluids, exert mechanical pressure on confining walls. Unlike equilibrium, this pressure is generally not a function of the fluid state in the bulk and displays some peculiar properties. For example, when activity is not uniform, fluid regions with different activity may exert different pressures on the container walls but they can coexist side by side in...
Hydrodynamic instabilities appear in E. coli suspensions at high enough concentrations. Controlling such instabilities could allow extracting energies at the microscales. We achieved control of the collective motion size in a sample confined between two parallel solid surfaces at a distance of H. By measuring the velocity correlation function in the center of the sample, we determined that the...
The navigation of swimming microorganisms, such as bacteria, is guided by rheotaxis, their reorientation with respect to flow gradients. While recent investigations focused on the con- trol of passive particles, such as red blood cells, in spatially modulated and time-dependent flows [1–3], less is known about the behavior of swimming agents in such flows. We show that bacteria modeled by...
In the laboratory, we built via soft lithography , geometrically controlled micro-fluidic environments of various complexity. We monitor trajectories of motile wild-type E.coli to characterize the mean transport and dispersion processes under flow. We show that the swimming activity of motile species and in particular their specific trajectories in a flow, their interaction with the walls and...
Micro-algae in relatively dense suspensions modify their environment by absorbing light, consuming and releasing chemical compounds or generating flows. Instabilities that appear in those systems can in turn give biological insight regarding the way this critically important class of micro-organisms navigate their environment.
Here we harness phototaxis to precisely control millions of...
Many motile bacteria, such as Escherichia coli, swim by rotating multiple flagella. These semi- flexible helical filaments are independently actuated by flagellar motors randomly distributed on the surface of the cell body. When all the motors rotate in the same direction, within a fraction of a second, this complex elastohydrodynamic system transforms into a straight swimmer in which all the...
Elastic Bistability and the Geometry of Cellular Neighbourhoods in Choanoflagellates and Green Algae
Raymond E. Goldstein
DAMTP, University of Cambridge, Cambridge, United Kingdom
E-mail: R.E.Goldstein@damtp.cam.ac.uk
This talk will describe two recent advances in understanding the physics of cellular organization in simple multicellular organisms. The first part concerns the...
Trypanosoma brucei, a eukaryotic parasite with a single flagellum, is transmitted by tsetse flies and thrives across a broad range of vertebrate species. These parasites cause several diseases in their hosts, exemplified by sleeping sickness in humans. Throughout their life cycle, these cells encounter diverse microenvironments with different physical attributes, such as viscosity 1. These...
Living organisms such as bacteria and algae often form biofilms at air-liquid and/or liquid-liquid interfaces. Therefore, it is important to understand the hydrodynamic interaction between the fluid-fluid interface and microorganisms. In this study, the flow field structures around a symmetrically trapped spherical microswimmer at an interface separating two fluids with different viscosities...
Excavates are phagotrophic flagellates characterized by a ventral groove and two flagella. The anterior flagellum is naked, and its beating creates a feeding current directed towards the groove, while the posterior flagellum is equipped with a vane and beats within the groove. We combined flow visualization and observations of prey capture in three clades of excavates with computational fluid...
The interplay of motility and geometry can lead to a variety of striking effects such as rectification of motion of motile particles and autonomous pumping of passive particles in a ratchet channel [1], trapping and release of motile and immotile particles assisted by motile species [2], guidance and transient rectification of motion of motile particles due to the directional locking in...
The flagellum is pivotal in the survival mechanisms of eukaryotic cells and measuring its three-dimensional shape is essential to understanding these key mechanisms. However, accurately assessing the intricate structure of flagella has been challenging due to the lack of a reliable method for determining the 3D position of individual points. Our digital holographic microscopy (DHM) method...
When many cilia are located on the surface of a microorganism, their beating can synchronise such that their phases form metachronal waves. To understand the process of synchronisation, we study a model where each cilium is represented as a spherical particle, moving along a tilted trajectory with a position-dependent active driving force and a position-dependent internal drag coefficient. The...
Bacteriophages - or "phages" for short - are viruses that can infect and kill bacteria. They are small particles that rely on thermal diffusion to find target cells, but are also advected in the flow-field generated by motile bacteria. We use coupled lattice-Boltzmann and coarse-grained molecular dynamics simulations to investigate the encounter between phages and bacteria. We find that while...
The impact of confinement on the self-organization of active particles has gained much attention1,2,3. Previous investigations have primarily focused on circular or square/rectangular geometric constraint. However, to explore more realistic scenarios, like particles enclosed in a membrane, confinement in curved geometry is essential 4. A suitable and simple geometry for such a case happens to...
Physical laws apply differently at the microscale than at the macroscale, therefore constraining microorganisms’ biological functions. A key instance of this is that microbes perceive their surrounding medium as extremely viscous, constraining them to adopt non-symmetric motion to be able to propel forward. Some bacteria (E. coli) and unicellular algae (C. reinhardtii) have evolved different...
Motility of sperm refers to its ability to swim in a forward direction to reach the oocyte and fertilize it. Normal sperm motility plays a crucial role in couple's reproductive health. Low motility of sperm cells could lead to issues with fertilization. Medical assisted reproduction (MAR) technologies are used to improve fertilization in this case. These imply selecting sperm with the highest...
A key feature of biological microswimmers is their ability to navigate away from or toward a certain stimuli, and this skill is termed as ‘taxis’. Tactic behavior can be induced by physicochemical changes in the environment such as light (phototaxis), chemical (chemotaxis), temperature (thermotaxis) and fluid flow (rheotaxis). In the last few years, a lot of research has been done for...
The ciliated epithelium of the human respiratory tract is lined by a thin stratified fluid. The airway surface liquid (ASL) serves as a protective barrier and is essential for maintaining normal respiratory mechanics. However, our understanding on how it is propelled by cilia and how flow is coupled between the two ASL compartments is still fragmentary. Mucus transport can be measured...
Antibiotic resistance is a growing issue in healthcare worldwide and is currently responsible for over one million deaths annually. We seek to develop a new method for antibiotic susceptibility testing (AST) using single-cell microscopy. The assay is based on a multi-pad agarose plate (MAP), where one or more bacteria samples are placed on 96 small agarose pads. Each pad provides a growth...
Run-and-tumble is a basic model of persistent motion and a widespread moving strategy in micro-organisms and individuals cells. In many natural settings, motion occurs in the presence of surfaces and confinement~[1]. While accumulation at the wall has been extensively studied~[2,3], the transport along the surfaces has received less attention. We consider a run-and-tumble particle confined in...
The microbial ecosystem is full of narrow constrictions that microorganisms need to learn to navigate in order to survive. Here, we study a Nature example of a "microorganism billiard": a system composed of a population of microorganisms packed in a closed space, with only a few narrow apertures to escape from. This situation occurs when the marine parasite Parvilucifera sinerae infects and...
The transport properties of colloidal particles in active liquids have been studied extensively. It has led to a deeper understanding of the interactions between passive and active particles. However, the phase behavior of colloidal particles in active media has received little attention. We have studied passive colloids dispersed in suspensions of active particles in experiments and...
Filamentous cyanobacteria are one of the oldest and today still most abundant lifeforms on earth, with manifold implications in ecology and economics. Their flexible filaments, often several hundred cells long, exhibit gliding motility in contact with solid surfaces. The underlying force generating mechanism is not yet understood. Here, we demonstrate that propulsion forces and friction...
E. coli is a multi-flagellated bacterium with a prolate spheroidally-shaped body and several left-handed helical flagella (typically between 2 and 5). The helical flagella are rotated by a motor, enabling the bacterium to propel forward. E. coli generally has two modes of swimming: (i) ‘run’ with a straight swimming direction, and (ii) ‘tumble’ during which the bacterium can change its...
E. coli is a multi-flagellated bacterium with a prolate spheroidally-shaped body and several left-handed helical flagella (typically between 2 and 5). The helical flagella are rotated by a motor, enabling the bacterium to propel forward. E. coli generally has two modes of swimming: (i) ‘run’ with a straight swimming direction, and (ii) ‘tumble’ during which the bacterium can change its...
Most of the bacterial biomass on Earth is found in three-dimensional communities, termed biofilms, which confer protection against many forms of physical, chemical, and biological stress. In this work we overcome previous limitations by developing a novel microfluidic high-throughput biofilm cultivation approach, as well as an automatized adaptive fluorescence microscopy screening procedure,...
The cooperative binding of molecular agents onto a substrate is pervasive in living systems, particularly stochastic processes inside cells. When the number of binding sites is small enough, we can rely on a fluctuation analysis of the number of substrate-bound units, an experimentally accessible quantity, to study whether a system shows cooperativity.
First, we present a general-purpose...
We report a type of marine, non-pathogenic bacteria, Flavobacterium Iridescent 1 (IR1), that grows into a colony in active liquid crystal phase with intense structural colour. We show that these rod-like gliding bacteria organize hierarchically, from bacteria clusters, to monolayer, multi-layers and finally into large scale chiral vortices up to one millimeter in diameter. We demonstrate that...
The natural habitat of microorganisms are non-Newtonian fluids, which besides shear viscosity also have an elastic response. Using a second-order model fluid, we present an analysis, how weak viscoelasticity affects the rheology of a dilute suspension of microswimmers [1]. Starting with modifications of the well-known Jeffery orbits and the orientational distribution due to tumbling and...
Cilia and flagella are cellular appendages that enable microorganisms to propel themselves and interact with surfaces. Particularly, the unicellular microalga Chlamydomonas reinhardtii swims by actuating its two flagella, but it can also use them to attach to surfaces and perform gliding motility. Its flagellar adhesiveness can be switched on and off by blue and red light, re- spectively...
Unraveling the motion of microorganisms in dilute and porous media is important for our understanding of both the molecular basis of their swim gait and their survival strategies in microbial habitats. First, I will show that by using renewal processes to analyze experimental measurements of wild-type E. Coli, we can provide a quantitative spatiotemporal characterization of their...
Passive particles immersed in an active bath of micro-swimmers, either artificial swimmers or living microorganisms, may be displaced due to the activity of the suspension [1]. This enhanced motion can lead to rich phenomena such as aggregation or phase separation. In this experimental work, we study how passive beads are moved by randomly swimming bacteria and directionnaly swimming...
Living cells improve their fitness by sensing noisy environmental signals and tuning their behavior in response in a seemingly modular fashion. Yet these two processes often occur simultaneously, and behavioral response affects future signal values. Whether a cell is aware of and able to efficiently use the information it itself generates, is an open question. A complete understanding of the...
How do sperm steer? How do they change their swimming direction? How is directional motion achieved in complex environments? These are important issues to be clarified in order to understand how sperm can navigate their tortuous journey towards the egg [1,2,3]. The sperm flagellum besides propelling the cell also acts as a sensory antenna, detects environmental cues, enabling steering...
