Dogsa Iztok, Kriechbaum Manfred, Stopar David, Laggner Peter
Laboratory for Biophysics, Jozef Stefan Institute, Ljubljana, Slovenia.
Biophys J. 2005 Oct;89(4):2711-20. doi: 10.1529/biophysj.105.061648. Epub 2005 Aug 5.
Extracellular polymeric substances (EPS) play an important role in cell aggregation, cell adhesion, and biofilm formation, and protect cells from a hostile environment. The EPS was isolated by trichloroacetic acid/ethanol extraction from broth culture of a marine bacterium isolate. The EPS was composed of glucose and galactose as determined by HPLC and TLC; the protein content was on average 15 +/- 5% of EPS dry mass. The solution structure of EPS at different values of pH was revealed by small-angle x-ray scattering. Scattering curves of EPS solutions (0.4%, w/v) consistently showed two nearly linear log-log regions with slopes a and b in the q-ranges from 0.06 nm(-1) to 0.26 nm(-1), and from 0.27 nm(-1) to 0.88 nm(-1), respectively. Slope a was sensitive to pH changes whereas slope b was not. The observed sensitivity to pH was not a consequence of ionic strength variation with pH, as checked by salt addition. The pH variation causes major rearrangements of EPS structure mainly at length scales above 24 nm. To get a better understanding of the pH effect on EPS structure, the original model proposed by Geissler was refined into a mathematical model that enabled fitting of the experimental scattering curves in the pH range from 0.7 to 11.0. The model describes EPS structure as a network of randomly coiled polymeric chains with denser domains of polymeric chains. The results obtained from the model indicate that dense domains increase in average size from 19 nm at pH 11.0 to 52 nm at pH 0.7. The average distance between the polysaccharide chains at pH 0.7 was 2.3 nm, which indicates a compact EPS structure. Swelling was found to be at a maximum around pH = 8.8, where the average distance between the chains was 4.8 nm.
胞外聚合物（EPS）在细胞聚集、细胞黏附及生物膜形成过程中发挥着重要作用，并能保护细胞免受恶劣环境的影响。通过三氯乙酸/乙醇萃取法从一株海洋细菌分离株的肉汤培养物中分离出EPS。经高效液相色谱（HPLC）和薄层色谱（TLC）测定，EPS由葡萄糖和半乳糖组成；蛋白质含量平均占EPS干质量的15±5%。通过小角X射线散射揭示了不同pH值下EPS的溶液结构。EPS溶液（0.4%，w/v）的散射曲线在q范围分别为0.06 nm-1至0.26 nm-1和0.27 nm-1至0.88 nm-1时，始终显示出两个斜率分别为a和b的近似线性的对数-对数区域。斜率a对pH变化敏感，而斜率b则不敏感。如通过添加盐所检测的那样，观察到的对pH的敏感性并非离子强度随pH变化的结果。pH变化主要在长度尺度大于24 nm时引起EPS结构的重大重排。为了更好地理解pH对EPS结构的影响，将Geissler提出的原始模型细化为一个数学模型，该模型能够拟合pH范围为0.7至11.0的实验散射曲线。该模型将EPS结构描述为随机卷曲的聚合物链网络以及聚合物链的致密区域。从该模型获得的结果表明，致密区域的平均尺寸从pH 11.0时的19 nm增加到pH 0.7时的52 nm。在pH = 8.8左右发现肿胀最大，此时链间平均距离为4.8 nm。
