Study Of The Chemotaxis Behavior Of Flagellated Bacteria In Different Physical Environments

The ability to seek benefits and avoid harm is one of the most basic attributes of living organisms to survive,and it is also the most important difference from inanimate objects.It is meaningful to delve into the chemotaxis behavior of bacteria,one of the simplest life forms.Our researches about bacterium chemotaxis,on the one hand,are helpful to understand the pathogenic mechanism of bacteria,since the ability of bacteria to infect hosts and induce diseases is closely related to chemotaxis,on the other hand,enhance our comprehension for the conservation law of life evolution and the physiological behaviors of complex life systems,such as forging and migration.The research on the chemotactic behavior of bacteria has been going on for more than half a century,during which the previous studies focused on the chemotactic behavior of peritrichous bacteria in a three-dimensional free liquid environment,and had a deeper understanding of the internal mechanism of the chemotactic behavior.However,the distribution of flagellated positions of bacteria is not uniform,and the physical environment is also diverse,so it is important to explore the chemotactic behavior of various types of flagellated bacteria in different physical environments.We have combined a series of experimental techniques,including the 3D tracking technique,microfluidic technology,flagella staining technique of living bacteria,fluorescence resonance energy transfer technique,and so on to carry out the experimental studies.This work,which could be regarded as a useful complement to previous work and deepen the understanding of microbial chemotactic behavior,involved the movement pattern and chemotactic behavior study of Escherichia coli,the model strain of peritrichous bacteria,in a two-dimensional crowded environment and Pseudomonas aeruginosa,the model strain of monotrichous bacteria,in a three-dimensional free environment.In a dilute liquid environment where cell-cell interaction is negligible,peritrichous bacteria,such as Escherichia coli,perform chemotaxis by biased random walks alternating between run and tumble.In a two-dimensional crowded environment such as a bacterial swarm,the typical behavior of run-and-tumble is absent due to the interface effect and steric repulsion,and this raises the question whether and how bacteria can perform chemotaxis in a swarm.Here,by tracking single bacterial cells harvested from the swarm colony,we found that they can still perform a "run-tumble"movement pattern in a three-dimensional free environment,and respond to the stimulus spatial gradient.This revealed that chemotaxis is one of the inherent properties of bacteria and robust in different bacterial forms.For the first time,we experimentally observed the macro chemotactic behavior in a swarm of peritrichous bacteria by eatablishing a gradient filed of stimulus in the swarm agar.Then,by examining the chemotactic behavior as a function of the cell density,we showed that chemotaxis is surprisingly enhanced as the cell density increases in a bacterial swarm,and this enhancement is highly correlated with the increase of local alignment of cells along long axes due to cell crowding.After theoretical analysis,we found that cells tend to form clusters that move collectively in a bacterial swarm because of the hydrodynamic interaction,which can increase the effective run length and drift velocity toward attractants.Therefore,collective motion in a bacterial swarm enhances chemotaxis by enhancing the local alignment of cells along long axes.This finding can not only enhance our comprehension for the collective phenomena in the self-propelled particle system and active matter ensemble,but help us understand the bacterial migration and biofilm formation in the intestinal microenvironment.The selection of bacterial chemotactic strategies mainly depends on the differences in the external physical environments and morphological characteristics of the bacterial cells.Here,we focused on the research about the chemotactic behavior of Pseudomonas aeruginosa,the model strain of monotrichous bacteria.As an opportunistic pathogen,Pseudomonas aeruginosa was recently found to swim in a "run-reverse" pattern,which was inefficient for bacteria to seek for an excellent habitat and avoid adverse factors due to the limited change of swimming direction during flagellar motor switching.We speculated that there is a more effective strategy to carry the chemotactic movement for Pseudomonas aeruginosa.Here,by flagella staining and single cell tracking,we discovered for the first time a new swimming mode-the wrap mode,during which the flagellar filament wrapped around the cell body and kept rotating.We measured the behavior characteristics of the wrap mode,and found that it randomized the swimming direction,thereby allowing the bacterium to explore its neighborhood efficiently.We confirmed by stochastic simulations of Pseudomonas aeruginosa chemotaxis that the wrap mode enhanced its chemotaxis performance.This study not only filled a gap in the research field about the chemotactic behavior of Pseudomonas aeruginosa,but also was important for the understanding of the mechanisms of invasion and pathogenesis for this bacterium.Our researches,including the movement pattern and chemotactic behavior of Escherichia coli in the bacterial swarm and Pseudomonas aeruginosa in 3d environment,had deepened our comprehension of some bacterial physiological processes,including the bacterial migration,biofilm formation,host infection and disease induction in mammals.This possessed a medical significance for investigating and monitoring the gastrointestinal tumors in human body,and also provided some bionic insights for designing the highly sensitive biosensors and micro-robots.