Under a microscope, bacteria appear to wriggle and wobble to get to where they need to go. These bizarre motions are nevertheless effective: every second, a bacterium can swim tens of times the length of its body, the equivalent of a human swimming faster than 20 metres per second. Incredibly, in fluids such as those lining the lungs and stomach, bacteria can swim even faster. In fact, the “messier” the fluid, the straighter and faster bacteria travel. After decades of research, the physics of this phenomenon is still under debate.
How bacteria swim in fluids…maybe
Bacteria are single-celled organisms a few thousandths of a millimetre in size. To swim in fluids, they rely on flagella, flexible helical filaments that are connected to “motors” anchored to the cell surface. The thrust generated when the motors turn propels the bacteria forward. When all the motors are synchronized, the flagella appear as a single bundle that rotates uniformly. To balance this rotation, the bacteria’s cell body rotates, too. Imprecise alignments between the motion of the cell body and the flagellar bundle result in the helical, corkscrew-like trajectory seen as “wobbling” under a microscope.
This bacterial swimming looks different depending upon the fluid in which the bacteria are immersed. In complex fluids, such as those lining some organs, bacteria swim much faster than in simple fluids such as water – an observation that has intrigued scientists for over sixty years.
“Previous studies have shown that bacteria swim faster in polymer solutions of low concentrations than in pure water. But exactly why they show such an unusual behaviour was not known,” says Xiang Cheng, from the University of Minnesota.
Theories put forth to date about how bacteria swim in complex fluids focused on the dynamics introduced by the presence of polymers (molecules made by linking many small units together to form larger molecules). Yet, a new study finds similar enhanced bacterial swimming behaviours in the presence of colloids, polymers that are distributed evenly throughout a fluid.
A new model for bacteria swimming
The study, published in Nature, is the result of a collaboration between Cheng’s team and scientists at Beijing Computational Science Research Centre and Beijing Normal University. The researchers analysed how individual bacteria swim by studying low concentrations of fluorescently tagged E. coli injected in a colloid solution (at high concentrations, bacteria such as E. coli show collective swimming, like bird flocking or fish schooling, that changes swimming behaviours). The team noted seven features, such as swimming speed and tumbling rates, that agreed with bacterial swimming behaviours observed in polymer solutions and concluded that dynamics induced by the presence of polymers alone cannot explain why bacteria travel faster in complex liquids.
The researchers realized that each particle, whether a colloid or a polymer, appears as a solid surface to a travelling bacterium. Bumping into the surface of an object induces a torque, bending the flagella and decreasing the misalignment between the flagellar bundle and the cell body, resulting in straighter, faster swimming overall. To combine their experimental results mathematically, the researchers then developed a mathematical model of bacterial swimming. Their parameter-free model introduces a new way to think about bacterial swimming by combining the rigid-body rotation of the bacteria with the angular velocity of the cell body.
Physicists show how bacteria swim towards regions of higher viscosity
Cheng says that while micro-organism movements are relevant to many microbiological processes, such as disease infection, fertility and reproduction, and ecosystem health, his motivation for engaging in this field of research is a passion for how small swimmers such as bacteria live in their natural environments.
“I am simply interested in how such a small creature swims and moves around in their normal life,” he says.
Fortunately – for Cheng at least – this study is not the last word on bacterial swimming. His team is now investigating how bacteria behave in the presence of high concentrations of colloids and how bacteria interact with large solid boundaries.
