Speaker: Xiang Cheng, PhD
Department of Chemical Engineering and Materials Science, University of Minnesota
Topic: Locomotion of flagellated bacteria: the influence of complex fluids and the role of multiflagellarity
A flagellated bacterium inhabits and swims in fluids of low Reynolds number, a world, though foreign to us, is of ultimately importance to many aspects of our daily lives ranging from food production, disease prevention to environmental health. In this talk, I discuss two recent experimental works in my group on the fascinating swimming behaviors of a prominent example of flagellated bacteria, Escherichia coli. First, we study the motility of E. coli in colloidal suspensions of varying sizes and volume fractions. 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. 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. Second, we explore the role of multiflagellarity in maintaining the constant swimming of E. coli of different lengths. By synergizing experiments of immense sample sizes with quantitative hydrodynamic modeling and simulations, we reveal how bacteria utilize the increasing number of flagella to regulate the flagellar motor load, which leads to faster flagellar rotation neutralizing the higher fluid drag on their larger bodies. Without such a collective balancing mechanism, the swimming speed of uniflagellar bacteria generically decreases with increasing body size. As such, our study reveals a long-sought-after selective advantage of multiflagellarity as a ubiquitous cellular feature of bacteria. The uncovered difference between uniflagellar and multiflagellar swimming is also important for understanding environmental influence on bacterial morphology and useful for designing artificial flagellated microswimmers.