Microbiology Laboratory, Department of Botany, University of Calcutta, 35, Ballygunge Circular Road, Kolkata, 700 019 India.
Indian J Microbiol. 2008 Mar;48(1):49-64. doi: 10.1007/s12088-008-0006-5. Epub 2008 May 1.
Extracellular polymeric substances (EPS) of microbial origin are a complex mixture of biopolymers comprising polysaccharides, proteins, nucleic acids, uronic acids, humic substances, lipids, etc. Bacterial secretions, shedding of cell surface materials, cell lysates and adsorption of organic constituents from the environment result in EPS formation in a wide variety of free-living bacteria as well as microbial aggregates like biofilms, bioflocs and biogranules. Irrespective of origin, EPS may be loosely attached to the cell surface or bacteria may be embedded in EPS. Compositional variation exists amongst EPS extracted from pure bacterial cultures and heterogeneous microbial communities which are regulated by the organic and inorganic constituents of the microenvironment. Functionally, EPS aid in cell-to-cell aggregation, adhesion to substratum, formation of flocs, protection from dessication and resistance to harmful exogenous materials. In addition, exopolymers serve as biosorbing agents by accumulating nutrients from the surrounding environment and also play a crucial role in biosorption of heavy metals. Being polyanionic in nature, EPS forms complexes with metal cations resulting in metal immobilization within the exopolymeric matrix. These complexes generally result from electrostatic interactions between the metal ligands and negatively charged components of biopolymers. Moreover, enzymatic activities in EPS also assist detoxification of heavy metals by transformation and subsequent precipitation in the polymeric mass. Although the core mechanism for metal binding and / or transformation using microbial exopolymer remains identical, the existence and complexity of EPS from pure bacterial cultures, biofilms, biogranules and activated sludge systems differ significantly, which in turn affects the EPS-metal interactions. This paper presents the features of EPS from various sources with a view to establish their role as central elements in bioremediation of heavy metals.
微生物来源的胞外聚合物（EPS）是一种由生物聚合物组成的复杂混合物，包括多糖、蛋白质、核酸、糖醛酸、腐殖质、脂质等。细菌的分泌、细胞表面物质的脱落、细胞裂解物以及环境中有机成分的吸附都会导致各种自由生活细菌以及微生物聚集体（如生物膜、生物絮体和生物颗粒）中 EPS 的形成。无论来源如何，EPS 可能松散地附着在细胞表面，或者细菌可能嵌入 EPS 中。从纯细菌培养物和异质微生物群落中提取的 EPS 在组成上存在差异，这些差异受微环境中有机和无机成分的调节。从功能上讲，EPS 有助于细胞间聚集、附着在基质上、形成絮体、防止干燥和抵抗有害外源物质。此外，外聚物作为生物吸附剂，通过从周围环境中积累营养物质，同时在重金属的生物吸附中也起着至关重要的作用。由于 EPS 具有聚阴离子的性质，它与金属阳离子形成复合物，导致金属在聚合物基质内被固定。这些复合物通常是金属配体与生物聚合物中带负电荷的成分之间的静电相互作用的结果。此外，EPS 中的酶活性也通过在聚合物质量中的转化和随后沉淀来协助重金属的解毒。尽管使用微生物外聚物结合和/或转化金属的核心机制保持不变，但来自纯细菌培养物、生物膜、生物颗粒和活性污泥系统的 EPS 的存在和复杂性有很大的不同，这反过来又影响了 EPS-金属相互作用。本文介绍了各种来源的 EPS 的特征，以期确定其作为生物修复重金属的核心要素的作用。
