Science and Technology News

China University of Science and Technology finds a new pattern of bacterial swimming

2022/4/1 Viewed：

Source: China University of Science and Technology News

The research group of Yuan Junhua and Zhang Rongjing from the Department of Physics of the University of Science and Technology of China discovered the new swimming pattern of Pseudomonas aeruginosa by using the three-dimensional tracking technology of bacteria and the dynamic fluorescence observation technology of flagellum filament. The findings were published in PNAS on March 29, 2022 [PNAS 119,e2120508119 (2022)].

Bacterial movement is key to their survival and to infecting their hosts. Bacteria explore their environment by alternating between swimming patterns. Unlike E. coli with multiple flagellates throughout its body, Pseudomonas aeruginosa is a typical polar monomastigote bacteria with a single flagellate located at one end of the rod-like cell body. Driven by a rotatable flagella motor, Pseudomonas aeruginosa switches its swimming mode in the liquid: the flagella rotates counterclockwise to push the cell forward, and the flagella rotates clockwise to drag the cell backward. Traditionally, Pseudomonas aeruginosa achieves environmental exploration by alternating "forward and backward" patterns, perhaps with short pauses in between. However, the change of swimming direction of bacteria in this way is mainly due to the fluctuations of cell body direction caused by Brownian rotational diffusion, so the efficiency of environmental exploration is not high. Could bacteria have evolved over eons of time in a more efficient way to explore their environment?

Figure 1. Bright field and fluorescence images of bacteria swimming. The dashed line represents the bacterial cell body, the bright color represents the fluorescently labeled flagellar filament, and the white arrow represents the swimming direction of the bacteria.

By means of gene editing, our research group improved the fluorescent labeling efficiency of P. aeruginosa flagellar filaments, realized three-dimensional tracking of swimming and synchronous observation of dynamic behavior of flagellar filaments in this bacterium, and thus found a new swimming mode (FIG. 1, called "wrap" mode), and further revealed the physical mechanism of this mode. When the flagellum rotates clockwise, the towing cell body recels; when the flagellum turns counterclockwise, the connecting parts (hook) of the flagellum near the cell body will undergo mechanical buckling and instability under the action of pressure at both ends, causing the flagellum to wrap around the cell body and form a wrap state. In this state, the orientation of the cell is unstable and it is easy to turn. The two ends of the buckled hooked sheath in Wrap state are subjected to tension, and after a short gap (average 1 second), the flagellar filament dissociates from the cell body and returns to the forward state (FIG. 2). Therefore, the wrap state occurs in the process of switching from the backward state to the forward state, and the statistical probability of its occurrence is about 40%. By comparing the statistical distribution of changes in swimming direction under the two modes of "backward-forward" and "backward-wrap-forward", the research group found that the wrap state made the changes in swimming direction of bacteria distributed randomly and evenly in the 4pi three-dimensional space, thus greatly improving the efficiency of bacteria exploring the environment. In addition, the effect of wrap state on enhancing the chemotactic level of bacteria was confirmed by the stochastic dynamics simulation of bacterial chemotactic swimming.

Figure 2. A new model of Pseudomonas aeruginosa. (Left) FIG. 1 Schematic diagram of bacterial swimming tracks, with different colors representing different swimming patterns. (Right) The state of the flagellar filament in three swimming modes.

There are abundant species of polar flagellate bacteria in nature, and the new swimming pattern discovered by our team may be widespread in polar flagellate bacteria. The physical mechanism to realize the change of swimming direction by mechanical buckling instability of hooked sheath found here is also enlightening for the design of artificial micro-nano machines.

Tian Maojin and Wu Zhengyu are the co-first authors of this paper. The above research was supported by the National Natural Science Foundation of China, the Ministry of Science and Technology, and the Collaborative Innovation Training Fund of Hefei University Science Center.

Paper link:

https://www.pnas.org/doi/10.1073/pnas.2120508119

(Department of Physics, Department of Scientific Research)

Go Back: Print