Abstract
It is generally accepted that planktonic bacteria in dilute suspensions are not mechanically coupled and do not show correlated motion. The mechanical coupling of cells is a trait that develops upon transition into a biofilm, a microbial community of self-aggregated bacterial cells. Here we employ optical tweezers to show that bacteria in dilute suspensions are mechanically coupled and show long-range correlated motion. The strength of the coupling increases with the growth of liquid bacterial culture. The matrix responsible for the mechanical coupling is composed of cell debris and extracellular polymer material. The fragile network connecting cells behaves as viscoelastic liquid of entangled extracellular polymers. Our findings point to physical connections between bacteria in dilute bacterial suspensions that may provide a mechanistic framework for understanding of biofilm formation, osmotic flow of nutrients, diffusion of signal molecules in quorum sensing, or different efficacy of antibiotic treatments at low and high bacterial densities.
Highlights
It is generally accepted that planktonic bacteria in dilute suspensions are not mechanically coupled and do not show correlated motion
With the development of biofilm, the extracellular matrix is gradually fortified by different components of extracellular polymeric substances (EPS) which enable a transition from viscoelastic liquid to viscoelastic solid network[4, 5]
The correlated motion of neighboring bacteria in dilute bacterial suspensions using a single particle active microrheology was determined in the early exponential phase
Summary
It is generally accepted that planktonic bacteria in dilute suspensions are not mechanically coupled and do not show correlated motion. We employ optical tweezers to show that bacteria in dilute suspensions are mechanically coupled and show long-range correlated motion. The fragile network connecting cells behaves as viscoelastic liquid of entangled extracellular polymers. With the development of biofilm, the extracellular matrix is gradually fortified by different components of EPS (i.e., polysaccharides, proteins, eDNA) which enable a transition from viscoelastic liquid to viscoelastic solid network[4, 5]. We have reported on the physical interactions that facilitate self-assembly of B. subtilis cells at high cell densities into a mechanically coupled network employing dynamic rheology, small-angle X-ray scattering, dynamic light scattering, microscopy, densitometry, and sound velocity measurements in model viscoelastic polymer mixtures[17]. We probe viscoelasticity of dilute bacterial suspensions to clarify the link between extracellular polymer production and mechanical coupling of bacteria in the planktonic state. The phenomenon may have wide implications for understanding bacterial behavior and physiology in dilute suspensions
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