Flow and Transport in Bacterial Biofilms, Part II

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Philip Pearce, Mohit Dalwadi, Alys Clark, Igor Chernyavsky


Within complex biological systems, transport of nutrients, wastes, cells and signalling molecules is influenced by various physical mechanisms, including fluid flow, diffusion and active transport. To un- derstand the general relationships between structure and function in such systems, it is important to characterise the relative importance to transport of the dominant physical processes, as well as the sys- tem geometry. In recent work, experiments and theory have been used to delineate the effects of geometry, fluid flow, diffusion and active biological processes on transport in various idealised and realistic model systems; in growing systems, the geometry may be coupled to physical processes over certain timescales. This minisymposium brings together experimental and theoretical researchers to investigate the re- lationship between physics and geometry in several complex biological systems. In Part I we focus on bacterial biofilms, in which fluid flow is an important driver of nutrient availability, signalling via quorum sensing molecules, cell erosion, and growth-induced architecture. In Part II we focus on vascular networks and complex tissues, in which fluid flow is the primary mechanism for transport of red blood cells, oxygen and other solutes. The goal will be to discuss the state-of-the-art techniques used to characterise these different systems, including how the associated physical and biological processes interact across different spatial scales, and how to accurately parameterise models across scales. Such techniques are expected to be highly applicable between the systems. Therefore the minisymposium will promote collaboration and help advance research into the broad topic of flow and transport in biology. The minisymposium is intended for those studying specific or general complex biological systems, at any stage in their research careers. The talks will also be useful for any researcher who would like a broad introduction to the applications of fluid mechanics and transport phenomena in biology.

Mohit Dalwadi

"Using homogenization to determine the effective nutrient uptake in a biofilm from microscale bacterial properties"
In biofilm models that include nutrient delivery to bacteria, it can be computationally expensive to include many small bacterial regions acting as volumetric nutrient sinks. To combat this problem, such models often impose an effective uptake instead. However, it is not immediately clear how to relate properties on the bacterial scale with this effective result. For example, one may intuitively expect the effective uptake to scale with bacterial volume for weak uptake, and with bacterial surface area for strong uptake. I will present a general model for bacterial nutrient uptake, and upscale the system using homogenization theory to determine how the effective uptake depends on the microscale bacterial properties [1, 2]. This will show us when the intuitive volume and surface area scalings are each valid, as well as the correct form of the effective uptake when neither of these scalings is appropriate.

Alexandre Persat

"Cellular advective-diffusion drives the emergence of bacterial surface colonization patterns"
In the wild, bacteria predominantly live as multicellular aggregates called biofilms. In contrast to lab- oratory settings, natural biofilms commonly comprise multiple strains or species that interact with each other. The nature of such interactions depends on the relative proximity of different strains and thus on the spatial organization of the biofilm. However, little is known about how environmental factors affect this organization. Here, we demonstrate that biofilms of the freshwater bacterium /emphCaulobacter crescentus form distinct patterns of surface colonization depending on local hydrodynamic conditions. By imaging C. crescentus biofilms grown in microfluidic chambers under controlled flow, we observed that surface colonization rate decreases with increasing flow velocity. We also probed the effect of flow intensity on spatial lineage structure using two C. crescentus clones. Surprisingly, segregation increased with flow velocity, in contrary to the assumption that flow induces lineage mixing. To understand how these colonization patterns arise, we developed a theoretical model based on the balance between ad- vection and diffusion: planktonic cells released from the biofilm effectively diffuse as they swim with random trajectories, while they are transported unidirectionally by flow. As flow intensity increases, the residence time of planktonic cells in the channel decreases, reducing their spreading across the channel and decreasing encounters with the surface. To sum up, our work demonstrates that hydrodynamic forces impact biofilm architecture and lineage distribution in multi-strain biofilms. This indicates that social interactions within biofilms are not only dictated by biological factors, but also by mechanical conditions imposed onto the community.

Sara Jabbari

"Targeting bacterial adhesion as a novel therapeutic for Pseudomonas aeruginosa infections"
The rise in antibiotic resistance, combined with the shortage in discovery of new classes of these drugs, has led to the pursuit of creative ways to tackle bacterial infections that do not necessarily kill bacteria in conventional ways. Many of these approaches, however, are only partially successful when tested in infection models. We focus here on an approach that targets the ability of bacteria to bind to host cells – the first stage in infection and of biofilm formation. Through differential equation modelling and com- parison against experimental data, we investigate why the treatment is not fully effective. Furthermore, we use the model to predict how to improve the treatment in a variety of scenarios, including through changes in drug design and/or combination with alternative therapies. We illustrate when the therapy could replace or reduce antibiotic use, ultimately suggesting experimental pathways that should aid in accelerating the development of this novel therapy.

Philip Pearce

"The biofilm life cycle in high flow environments"
Bacterial biofilms represent a major form of microbial life on Earth. In their natural environments, ranging from human organs to industrial pipelines, biofilms have evolved to grow robustly under signifi- cant fluid shear. Despite intense practical and theoretical interest, it is unclear how strong fluid flow alters aspects of the biofilm life cycle including their formation, growth, and dispersal. In this talk, I will discuss how external flow affects biofilms across multiple scales at each of these stages of their life cycle, through transport of quorum sensing molecules, realignment of growing cells, and biofilm deformation and erosion.

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