Modelling of surfactant-driven front instabilities in spreading bacterial colonies

Abstract

The spreading of bacterial colonies at solid-air interfaces is determined by the physico-chemical properties of the involved interfaces. The production of surfactant molecules by bacteria is a widespread strategy that allows the colony to efficiently expand over the substrate. On the one hand, surfactant molecules lower the surface tension of the colony, effectively increasing the wettability of the substrate, which facilitates spreading. On the other hand, gradients in the surface concentration of surfactant molecules result in Marangoni flows that drive spreading. These flows may cause an instability of the circular colony shape and the subsequent formation of fingers. In this work, we study the effect of bacterial surfactant production and substrate wettability on colony growth and shape within the framework of a hydrodynamic thin film model. We show that variations in the wettability and surfactant production are sufficient to reproduce four different types of colony growth, which have been described in the literature, namely, arrested and continuous spreading of circular colonies, slightly modulated front lines and the formation of pronounced fingers.

A multiphase theory for spreading microbial swarms and films

“…Natural next steps of our approach include (i) adding three-dimensional effects by allowing for spatial variations in the mechanical stresses, flows and nutrient fields in the vertical direction, (ii) accounting for orientational order in the bacterial swarms and films, and (iii) accounting for interfacial tension on the stability of the growing swarm/biofilm-fluid interface, especially in the context of fingering instabilities in microbial colonies Trinschek et al (2018).…”

“…For example, B. subtilis secretes the lipopeptide surfactin, whereas P. aeruginosa secrets rhamnolipids as the wetting agent. Consequently, existing thin-film models to describe bacterial swarming assume that gradients in wetting agent activity generate Marangoni stresses that drives swarming motility (Fauvart et al, 2012; Trinschek et al, 2018). However, E. coli exhibits swarming behavior despite the absence of lipopeptides or other agents that act as surfactants.…”

The quorum sensing system NprR-NprRB contributes to spreading and fitness in colony biofilms ofBacillus thuringiensis

“…Active dispersal strategies depend on the presence of flagella, while passive dispersal is flagellum-independent (63)(64)(65). One example of a passive dispersal strategy is sliding, which depends on the secretion of surfactants that reduce surface tension and facilitate the bacterial translocation (47,66,67). Bacteria from the genus Serratia, Pseudomonas, Legionella, Sinorhizobium, Salmonella, Mycobacterium and Bacillus spread by the sliding strategy, which is allowed by the secretion of lipopeptides, glycopeptidolipids and exopolysaccharides (55,68).…”

Active modulation of surfactant-driven flow instabilities by swarming bacteria

“…The height of this water film created is h(x,y,t) and the surface of the film is covered by insoluble surfactant molecules (area density) ( ) that the bacteria produces. The horizontal driving potential of this film has contribution from wetting energy [1,2] and capillary effect ( effect of gravity can be neglected) as follows…”

“…To fully couple the height of the water film and surfactant concentration, we relate the net surface tension to the surfactant concentration [1] as given below.…”