Enhanced bacterial motility in colloidal media

Understanding the locomotion of microorganisms in complex fluids is crucial for gaining insights into their behaviors in natural habitats. 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. A substantial increase in bacterial speeds (up to 81%) is observed as the colloid volume fraction increases to 3%, followed by a decrease at higher volume fractions. The speed enhancement coincides with a decrease in bacteria’s wobble angle. Larger sized particles lead to higher speeds and smaller wobble angles. We reveal the microscopic origin of the speed enhancement via optical trapping of a colloidal particle and demonstrate the important role of discrete flagella-particle interactions. 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.