Patra Pintu, Kissoon Kimberley, Cornejo Isabel, Kaplan Heidi B, Igoshin Oleg A
Department of Bioengineering, Rice University, Houston, Texas, United States of America.
Department of Natural Sciences, Del Mar College, Corpus Christi, Texas, United States of America.
PLoS Comput Biol. 2016 Jun 30;12(6):e1005010. doi: 10.1371/journal.pcbi.1005010. eCollection 2016 Jun.
Myxococcus xanthus, a model organism for studies of multicellular behavior in bacteria, moves exclusively on solid surfaces using two distinct but coordinated motility mechanisms. One of these, social (S) motility is powered by the extension and retraction of type IV pili and requires the presence of exopolysaccharides (EPS) produced by neighboring cells. As a result, S motility requires close cell-to-cell proximity and isolated cells do not translocate. Previous studies measuring S motility by observing the colony expansion of cells deposited on agar have shown that the expansion rate increases with initial cell density, but the biophysical mechanisms involved remain largely unknown. To understand the dynamics of S motility-driven colony expansion, we developed a reaction-diffusion model describing the effects of cell density, EPS deposition and nutrient exposure on the expansion rate. Our results show that at steady state the population expands as a traveling wave with a speed determined by the interplay of cell motility and growth, a well-known characteristic of Fisher's equation. The model explains the density-dependence of the colony expansion by demonstrating the presence of a lag phase-a transient period of very slow expansion with a duration dependent on the initial cell density. We propose that at a low initial density, more time is required for the cells to accumulate enough EPS to activate S-motility resulting in a longer lag period. Furthermore, our model makes the novel prediction that following the lag phase the population expands at a constant rate independent of the cell density. These predictions were confirmed by S motility experiments capturing long-term expansion dynamics.
黄色粘球菌是研究细菌多细胞行为的模式生物，它仅利用两种不同但相互协调的运动机制在固体表面移动。其中一种，即社会（S）运动，由IV型菌毛的伸展和收缩提供动力，并且需要相邻细胞产生的胞外多糖（EPS）的存在。因此，S运动需要细胞间紧密靠近，孤立的细胞不会移位。先前通过观察沉积在琼脂上的细胞的菌落扩展来测量S运动的研究表明，扩展速率随初始细胞密度增加，但其中涉及的生物物理机制在很大程度上仍然未知。为了理解S运动驱动的菌落扩展的动态过程，我们开发了一个反应扩散模型，描述细胞密度、EPS沉积和营养物质暴露对扩展速率的影响。我们的结果表明，在稳态下，群体以行波的形式扩展，其速度由细胞运动性和生长的相互作用决定，这是费舍尔方程的一个众所周知的特征。该模型通过证明存在滞后阶段——一个扩展非常缓慢的瞬态期，其持续时间取决于初始细胞密度，解释了菌落扩展对密度的依赖性。我们提出，在低初始密度下，细胞需要更多时间来积累足够的EPS以激活S运动，从而导致更长的滞后阶段。此外，我们的模型做出了一个新的预测，即在滞后阶段之后，群体以与细胞密度无关的恒定速率扩展。这些预测通过捕捉长期扩展动态的S运动实验得到了证实。
