(398d) Enhanced Bacterial Motility in Complex Fluids

Understanding the motion of microorganisms in complex fluids is crucial for gaining insights into their behaviors in natural habitats and for their application as drug delivery agents. Here, we investigate the motility of E. coli, a flagellated bacterium, in colloidal media. We systematically vary the size of colloidal particles in the mixture from 20 nm to 1 μm and the volume fraction up to 20%. We image the motion of fluorescent bacteria using confocal microscopy and characterize the speed and wobble angle of bacteria. We find that bacteria in dilute colloidal suspensions display the quantitatively same motile behaviors as those in dilute polymer solutions, where a size-dependent motility enhancement up to 80% is observed accompanied by a strong suppression of bacterial wobbling. We reveal the microscopic origin of the speed enhancement via optical trapping of a colloidal particle and demonstrate the important role of discrete bacteria-particle interactions. By virtue of the well-controlled size and the hard-sphere nature of colloids, this striking similarity not only resolves the long-standing controversy over bacterial motility enhancement in complex fluids, but also challenges all the existing theories using polymer dynamics in addressing the swimming of flagellated bacteria in dilute polymer solutions. We further develop a simple hydrodynamic model incorporating the colloidal nature of complex fluids, which quantitatively explains bacterial wobbling dynamics and mobility enhancement in both colloidal and polymeric fluids. This work highlights the unusual swimming behavior of microorganisms in colloidal media and provides a unifying picture for understanding the effect of discrete interactions on bacterial locomotion in various complex fluids.