Description
In nature, many biological fluids that host or block bacterial populations, such as mucus, exhibit non-Newtonian rheology. To investigate the spatial exploration of Escherichia coli, a model multiflagellated bacterium, in such environments, a tunable motility medium based on Carbopol was employed. Increasing the concentration of soft carbomer grains transitioned the medium from a Newtonian fluid to a yield-stress gel. Bacterial motion was tracked using a novel 3D Lagrangian tracking system within a simple microfluidic chamber, enabling the collection of extensive individual trajectories for both a wild-type and a smooth-swimming mutant. Key motility properties—swimming velocities, persistence time, and diffusivity—were characterized up to the point where a complete motility barrier forms at high Carbopol concentrations, arresting bacterial swimming via a biobarrier (Urra et al., 2025).
Our results demonstrate that local mechanical heterogeneity and resistance to penetration can override the innate run-and-tumble navigation strategy. This leads to a “medium-assisted” exploration scenario, characterized by forced directional switching and stop-and-go kinematics, which is closely linked to the mechanical flexibility of the flagellar bundle. These dynamics, precisely quantified using microfluidic platforms, provide a fundamental framework for understanding bacterial motility in biologically relevant complex fluids like mucus. Mucus, a biopolymer hydrogel primarily composed of mucins, shares critical rheological features with carbopol gels, notably viscoelasticity and a microscopic yield stress. The transition observed in our controlled microfluidic environments—from altered swimming to a motility barrier—directly mirrors the challenge pathogens and commensals face when penetrating the mucosal layer. The emergent stop-and-go motion and medium-assisted reorientations are likely fundamental mechanisms for navigating the poroelastic network of mucins, where flagella constantly interact with obstacles and regions of varying stiffness.
Consequently, this work establishes that the physical properties of the environment are as crucial as biological steering in determining bacterial dispersal in structured fluids. The critical dependence on flagellar bundle flexibility suggests that adaptation to mucus may involve not only biochemical sensing but also the evolution of motility apparatuses with specific mechanical proficiencies to overcome the rheological barrier.
By providing a tunable model system, this work also offers a versatile platform for in vitro microfluidic studies. The ability to precisely control rheological properties via Carbopol enables the recreation of key tissue-like conditions—such as the mucosal layer—within microfluidic chips. This facilitates the controlled observation of bacterial motility, barrier penetration, and response to mechanical gradients under conditions that mimic biologically relevant complexity. This approach can enhance the design of experimental platforms that simulate hostile or protective environments, with direct applications in infection studies, probiotic research, and the development of strategies to modulate bacterial colonization at mucosal interfaces.