Institute of Structural and Molecular Biology, School of Biological Sciences, University of Edinburgh, Darwin Building, Mayfield Road, Edinburgh EH9 3JR, UK.
J R Soc Interface. 2012 Dec 7;9(77):3490-502. doi: 10.1098/rsif.2012.0498. Epub 2012 Aug 15.
In bacteria, the production of exopolysaccharides--polysaccharides secreted by the cells into their growth medium--is integral to the formation of aggregates and biofilms. These exopolysaccharides often form part of a matrix that holds the cells together. Investigating the bacterium Sinorhizobium meliloti, we found that a mutant that overproduces the exopolysaccharide succinoglycan showed enhanced aggregation, resulting in phase separation of the cultures. However, the aggregates did not appear to be covered in polysaccharides. Succinoglycan purified from cultures was applied to different concentrations of cells, and observation of the phase behaviour showed that the limiting polymer concentration for aggregation and phase separation to occur decreased with increasing cell concentration, suggesting a 'crowding mechanism' was occurring. We suggest that, as found in colloidal dispersions, the presence of a non-adsorbing polymer in the form of the exopolysaccharide succinoglycan drives aggregation of S. meliloti by depletion attraction. This force leads to self-organization of the bacteria into small clusters of laterally aligned cells, and, furthermore, leads to aggregates clustering into biofilm-like structures on a surface.
在细菌中，胞外多糖（细胞分泌到生长介质中的多糖）的产生对于聚集物和生物膜的形成至关重要。这些胞外多糖通常构成将细胞聚集在一起的基质的一部分。在研究根瘤菌（Sinorhizobium meliloti）时，我们发现一种过度产生胞外多糖琥珀聚糖的突变体表现出增强的聚集性，导致培养物的相分离。然而，聚集物似乎没有被多糖覆盖。从培养物中纯化的琥珀聚糖被应用于不同浓度的细胞，观察相行为表明，发生聚集和相分离的聚合物浓度极限随着细胞浓度的增加而降低，这表明存在一种“拥挤机制”。我们认为，正如在胶体分散体中发现的那样，以胞外多糖琥珀聚糖的形式存在的非吸附聚合物通过耗尽吸引作用驱动根瘤菌的聚集。这种力导致细菌自我组织成横向排列的细胞小簇，并且进一步导致聚集物在表面上聚类成类似生物膜的结构。
