Interplay of motility and polymer-driven depletion forces in the initial stages of bacterial aggregation.

Michael K. Porter,Asher Preska Steinberg,Rustem F. Ismagilov

doi:10.1039/c9sm00791a

Michael K. Porter, Asher Preska Steinberg + Show 1 more

Open Access

https://doi.org/10.1039/c9sm00791a

Copy DOI

Abstract

Motile bacteria are often found in complex, polymer-rich environments in which microbes can aggregate via polymer-induced depletion forces. Bacterial aggregation has many biological implications; it can promote biofilm formation, upregulate virulence factors, and lead to quorum sensing. The steady state aggregation behavior of motile bacteria in polymer solutions has been well studied and shows that stronger depletion forces are required to aggregate motile bacteria as compared with their nonmotile analogs. However, no one has studied whether these same trends hold at the initial stages of aggregation. We use experiments and numerical calculations to investigate the polymer-induced depletion aggregation of motile Escherichia coli in polyethylene glycol solutions on short experimental timescales (∼10 min). Our work reveals that in the semi-dilute polymer concentration regime and at short timescales, in contrast to what is found at steady state, bacterial motility actually enhances aggregate formation by increasing the collision rate in viscous environments. These unexpected findings have implications for developing models of active matter, and for understanding bacterial aggregation in dynamic, biological environments, where the system may never reach steady state.

Highlights

Bacteria thrive in a wide range of biological and ecological contexts and play important roles in the human gut, soil, wastewater sludge, and other complex environments
We found that nonmotile bacterial aggregation in the presence of 1 MDa polyethylene glycol (PEG) (Fig. 2a) was qualitatively consistent with depletion-driven aggregation with similar trends observed at half (Fig. 2b) and double (Fig. 2c) the bacterial concentration
Bacterial aggregation often occurs in complex physiological environments such as the gastrointestinal tract or the lungs; in these settings it is important to understand the initial formation of aggregates before physiological factors such as peristalsis or swallowing play a role

Summary

Introduction

Bacteria thrive in a wide range of biological and ecological contexts and play important roles in the human gut, soil, wastewater sludge, and other complex environments. The depletion attractions increase as a function of polymer concentration due to the contribution of osmotic pressure, which is mirrored by the increasing aggregate size of the nonmotile bacteria in the dilute PEG concentration regime (eqn (3) and (4)). To investigate the interplay of swim and depletion forces that give rise to the differences in the observed aggregation behavior between motile and nonmotile bacteria, we began by using effective potentials to describe aggregation in the dilute polymer concentration regime.

Results

Conclusion