Exopolysaccharide-Independent Social Motility of <i>Myxococcus xanthus</i>

Social motility (S motility), the coordinated movement of large cell groups on agar surfaces, of Myxococcus xanthus requires type IV pili (TFP) and exopolysaccharides (EPS). Previous models proposed that this behavior, which only occurred within cell groups, requires cycles of TFP extension and retraction triggered by the close interaction of TFP with EPS. However, the curious observation that M. xanthus can perform TFP-dependent motility at a single-cell level when placed onto polystyrene surfaces in a highly viscous medium containing 1% methylcellulose indicated that “S motility” is not limited to group movements. In an apparent further challenge of the previous findings for S motility, mutants defective in EPS production were found to perform TFP-dependent motility on polystyrene surface in methylcellulose-containing medium. By exploring the interactions between pilin and surface materials, we found that the binding of TFP onto polystyrene surfaces eliminated the requirement for EPS in EPS- cells and thus enabled TFP-dependent motility on a single cell level. However, the presence of a general anchoring surface in a viscous environment could not substitute for the role of cell surface EPS in group movement. Furthermore, EPS was found to serve as a self-produced anchoring substrate that can be shed onto surfaces to enable cells to conduct TFP-dependent motility regardless of surface properties. These results suggested that in certain environments, such as in methylcellulose solution, the cells could bypass the need for EPS to anchor their TPF and conduct single-cell S motility to promote exploratory movement of colonies over new specific surfaces.社会运动（Social motility，S运动）指黄色粘球菌（Myxococcus xanthus）在琼脂表面上大型细胞群的协同运动，该过程依赖于IV型菌毛（type IV pili，TFP）与胞外多糖（exopolysaccharides，EPS）。此前的模型提出，仅在细胞群中发生的此类运动行为，需要IV型菌毛与胞外多糖紧密相互作用所触发的IV型菌毛伸-缩循环。然而，一项令人费解的观测结果显示：当黄色粘球菌被置于含1%甲基纤维素的高粘性培养基中的聚苯乙烯表面时，可在单细胞水平上实现依赖IV型菌毛的运动，这表明“S运动”并非仅局限于群体运动。进一步对此前S运动研究结论的挑战在于，研究人员发现胞外多糖合成缺陷的突变株，在含甲基纤维素的培养基的聚苯乙烯表面仍可进行依赖IV型菌毛的运动。通过探索菌毛蛋白与表面物质间的相互作用，我们发现：将IV型菌毛结合至聚苯乙烯表面，可消除胞外多糖缺陷型细胞对胞外多糖的需求，从而使其能够在单细胞水平上实现依赖IV型菌毛的运动。但在粘性环境中，通用锚定表面无法替代细胞表面胞外多糖在群体运动中的作用。此外，研究发现胞外多糖可作为一种自主产生的锚定底物，被释放至表面，从而使细胞能够不受表面性质影响，进行依赖IV型菌毛的运动。上述结果表明，在某些环境（如甲基纤维素溶液）中，细胞可无需依赖胞外多糖来锚定其IV型菌毛，进而开展单细胞水平的S运动，以促进菌落对新的特定表面的探索性移动。