Exopolymeric substances

Cold denaturation is an interesting phenomenon that results from hydration of polar and nonpolar proteins and weakening of hydrophobic forces, all of which have a significant effect on protein folding and stability. Intracellular enzymes from psychrophiles are protected from cold denaturation by compatible solutes, such as potassium glutamate and trehalose (67). Psychrophiles also have intracellular cold shock proteins that act as chaperones, cryoprotectors, and antifreeze molecules. There is no explanation yet for protection from cold denaturation for extracellular psychrophilic enzymes, but exopolymeric substances may be involved (67). 

Microorganisms tend to adhere to surfaces [48,56,106,141]. It is believed that more than 90% of all microorganisms grow in this way [14]. Concomitantly, exopolymeric substances are excreted. This may result in a slimy superficial layer on materials and/or, in the case of porous materials, in total clogging of die free pore volume [60]. The exopolymers are usually hydrated and may contain ionic groups favoring water inclusion. Thus, one result may be an increase in water content in die case of porous materials (a consequence is freeze-diaw attack see the following). 

Whereas the mediated electron transfer, for example, via the oxidation of primary metabolites, can be realized with cell suspensions (see later) without any direct interaction of the living cell and the electrode material, an attachment of the bacterial cells (or its appendices) to an electrode surface is compulsory for the direct electron transfer. This attachment is not necessarily permanent, some bacterial species possess a directed motility toward prospective electron acceptors such as electrodes allowing a DET of suspended cells [24]. Furthermore, there is growing evidence that the matrix of exopolymeric substances (EPS) may also play an important role in the microbial electron transfer [25]. 

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