Issue 8, 2021

An expanding bacterial colony forms a depletion zone with growing droplets

Abstract

Many species of bacteria have developed effective means to spread on solid surfaces. This study focuses on the expansion of Pseudomonas aeruginosa on an agar gel surface under conditions of minimal evaporation. We report the occurrence and spread of a depletion zone within an expanded colony, where the bacteria laden film becomes thinner. The depletion zone is colocalized with a higher concentration of rhamnolipids, the biosurfactants that are produced by the bacteria and accumulate in the older region of the colony. With continued growth in population, dense bacterial droplets occur and coalesce in the depletion zone, displaying remarkable fluid dynamic behavior. Whereas expansion of a central depletion zone requires activities of live bacteria, new zones can be seeded elsewhere by adding rhamnolipids. These depletion zones due to the added surfactants expand quickly, even on plates covered by bacteria that have been killed by ultraviolet light. We explain the observed properties based on considerations of bacterial growth and secretion, osmotic swelling, fluid volume expansion, interfacial fluid dynamics involving Marangoni and capillary flows, and cell–cell cohesion.

Graphical abstract: An expanding bacterial colony forms a depletion zone with growing droplets

Supplementary files

Article information

Article type
Paper
Submitted
23 Jul 2020
Accepted
12 Jan 2021
First published
15 Jan 2021

Soft Matter, 2021,17, 2315-2326

An expanding bacterial colony forms a depletion zone with growing droplets

H. Ma, J. Bell, W. Chen, S. Mani and J. X. Tang, Soft Matter, 2021, 17, 2315 DOI: 10.1039/D0SM01348J

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