| 摘要: | 群游行為是一種細菌藉由轉動鞭毛馬達產生推進力，進而在物體表面上移動的運動行為。在近二十年內，科學家們對群游細菌的研究已經展示出了群游細菌多樣化且迷人的集體行為。在菌落中較厚的區域，細菌形成類紊流、邊界波浪、邊界湧流等運動結構。但是由於無法追蹤單一細菌，只能針對集體性質做探討。然而在菌落中有一單層結構，可以清楚見到細菌間的體積排斥力使細菌形成群集，並排列成有序結構。這樣的單層結構菌落是一個良好的實驗系統，可以提供我們來檢驗主動系統的行為。
雖然科學家們對群游細菌性質及集體行為的研究已經進行數年了，但很多細菌運動的細節還是不清楚的，尤其實在高密度細菌的狀態下。例如細菌是如何擴張菌落？多鞭毛細菌如何控制它的鞭毛形成鞭毛束並產生推進力？
這篇論文主要探討的是：1. 藉由觀察群游溶藻弧菌在單層菌落中的運動行為，研究二維自我推進粒子的性質與集體運動。2. 利用共軛焦顯微術研究細菌如何擴張菌落。
藉由追蹤每一個單一細菌在單層菌落的運動，我們展示了群游溶藻弧菌在不同密度下的集體行為。我們發現，在密度介於0.2~0.6的區間，巨型數量震盪現象(Giant number fluctuation)會發生在我們的系統中。而在更高的密度時，我們觀測到細菌可以形成區域有序結構，且結構的有序性會隨著細菌長度和密度增加而被增強。我們也發現群游溶藻弧菌的一些特殊性質。其一：在壅塞環境下，多數細菌都已無法移動時，有少數的細菌可以在菌落中進行長距離的位移。我們認為這個現象是源自於細菌間產生推進力能力的差異。另一個特殊性質為：我們發現，群游溶藻弧菌具有反轉速度方向的能力。這個現象的機制與已知的大腸桿菌速度反轉的機制不同，在本論文中，我們量測出在三個不同密度下，速度反轉的頻率都約為0.7次每秒，但細菌反轉其速度方向的機制還不清楚。
另外我們發現，在菌落擴張時，細菌的運動會使得菌落前沿的表面有明顯的形狀變化。實驗結果表示，在菌落擴張期，菌落的表面張力是會隨著細菌的集體運動而改變的。
;Bacterial swarming motility is a rapid and collective translocation of a bacterial population on the semi-solid surface by rotating their flagellar motors to generate thrusting force. In the last decades, people have demonstrated that there are many fascinating collective behaviors and pattern formations in the swarming colony of bacterial cells. In multiple layer region of swarming colony, turbulence state, edge waving, and edge streaming were observed in the swarm of Vibrio alginolyticus. People can only studied the collective behavior of bacterial cells due to the technical difficulty of individual cell tracking of high density region. However, the capability of single cell tracking can provide further insight to the collective behavior of the complex bacterial swarm. It is clear to see that cell alignment due to the excluded volume effect in the single layer region. Therefore, the swarming bacterial cells in single layer region is an ideal experimental system to study both bacterial properties and dynamics in active system.
People have studied the properties and collective behaviors of swarming bacterial cells for many years, but there are still many questions left unsolved especially in high cell density system. For example, how does the bacterial cells expand the colony, how does bacterial cells with peritrichous flagella form flagella bundle and generate propulsion. In this work, we studied the mechanism behind colony expansion with the use of confocal microscope, and demonstrate both the properties and collective behaviors of self-propelled particles in two dimensional systems by observing the swarming motion of Vibrio alginolyticus in single layer region.
By tracking the motion of each bacterial cell in single layer region of the swarm. We demonstrated the collective behaviors of swarming Vibrio alginolyticus with different cell density. Giant number fluctuation was observed in the system when the cell density is in the range of 0.2 to 0.6. At high cell density, bacterial cells were able to form a local order nematic structure, and the order is enhanced when both the average cell length and cell density increased. Two interesting properties of swarming Vibrio alginolyticus were observed in the system. First, some of the swarming bacterial cells were able to reverse their velocity direction with a frequency 0.7 hertz in three different densities. The mechanism of these velocity reversal events are different from the known mechanism of velocity reversal event in E. coli swarm. Second, some bacterial cells are able to travel long distance in high cell density system. Here, long range displacement events are considered as a result of wide distribution in force generation of cells. But the mechanism behind the velocity reverse events are not clear.
We also show that during the expansion, the motion of bacterial cells could change the local surface curvature of the colony, and make the surface tension a variable during the expansion. |
