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Abstract Bacterial colonies growing on solid surfaces can exhibit robust expansion kinetics, with constant radial growth and saturating vertical expansion, suggesting a common developmental program. Here, we study this process forEscherichia colicells using a combination of modeling and experiments. We show that linear radial colony expansion is set by the verticalization of interior cells due to mechanical constraints rather than radial nutrient gradients as commonly assumed. In contrast, vertical expansion slows down from an initial linear regime even while radial expansion continues linearly. This vertical slowdown is due to limitation of cell growth caused by vertical nutrient gradients, exacerbated by concurrent oxygen depletion. Starvation in the colony interior results in a distinct death zone which sets in as vertical expansion slows down, with the death zone increasing in size along with the expanding colony. Thus, our study reveals complex heterogeneity within simple monoclonal bacterial colonies, especially along the vertical dimension. The intricate dynamics of such emergent behavior can be understood quantitatively from an interplay of mechanical constraints and nutrient gradients arising from obligatory metabolic processes.
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Fluctuations and freezing of biofilm-air interfaces
The study of interfacial roughening is common in physics, from epitaxial growth in the lab to pio-neering mathematical descriptions of universality in models of growth processes. These studies led to the identification of a series of general principles. Typically, stochastic growth produces an interface that becomes rougher as the deposit grows larger; this roughening can only be counteracted by mechanisms that act on the top of deposit, such as surface tension or surface diffusion. However, even when relaxation mechanisms are present, interfaces that continue to grow stochastically continue to change; new peaks and troughs emerge and disappear as stochastic growth produces a constantly changing, dynamic interface. These universal phenomena have been observed for bacterial colonies in a variety of contexts. However, previous studies have not characterized the interfacial phenomena at the top surface of a colony, i.e., the colony-air interface, when activity is only present at the bottom surface, i.e., the colony-solid interface, where nutrients are available, over long times. As traditional interfacial roughening models primarily focus on activity occurring at the top surface it is unclear what phenomena to expect over long times. Here, we use white light interferometry to study the roughening of bacterial biofilms, from many different species. We find that these colonies are remarkably smooth, suggesting that a mechanism of interfacial relaxation is at play. However, colonies remain remarkably smooth even after growing large. We discover that topographic fluctuations “freeze” in place, despite the fact that growth continues for hundreds of microns more. With simple simulations, we show that this emergent freezing is due to the dampening of fluctuations from cell growth by the cells between the growing zone and the surface. We find that the displacement field caused by a single perturbation decays exponentially, with a decay length ofδL. In line with that observation we also show that the topography ceases to change when perturbations are a distanceδLaway from the surface. Thus, over-damped systems in which activity occurs at the bottom surface represent a distinct class of interfacial growth phenomena, capable of producing frozen topographies and remarkably smooth surfaces from spatially and temporally stochastic growth.
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- PAR ID:
- 10537449
- Publisher / Repository:
- bioRxiv
- Date Published:
- Format(s):
- Medium: X
- Institution:
- bioRxiv
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Posted Content:
- https://doi.org/10.1101/2024.05.14.594239
