Entrapment of nanobiofiber CS3 pili in Ca-alginate gel beads

Bacterial surface display of metal-binding peptides has become the purpose of many recent researches [1, 2]. Removal of the toxic metals from polluted water and wastewater can be improved by using surface-engineered bacteria [1]. Expression of histidine-rich and cysteine-rich metal-binding peptides on outer membrane proteins, fimbriae and flagella has been used to improve bioadsorption of divalent metals [3]. Fimbriae are 20 nm-thick proteins hallow tubes composed of hundreds of identical subunits which are presented in large numbers on bacterial surface. Therefore, they are readily accessible to the environment. The CS3 fimbriae/pili are known as suitable bacterial display systems for metaloadsorption because they tolerate the insertion of metal-binding peptides at permissive sites [4].The most obvious reasons for protein immobilization are stabilization of the proteins’ tertiary structure and their easily use for several times. This is more important when the proteins are used in industrial or environmental applications in continuous process [5]. The proteins are modified to a water-insoluble form by different immobilization techniques such as covalent attachment to solid supports, entrapment to polymeric gels, encapsulation in membranes and cross linking with bi-functional reagents. The gel entrapment method involves entrapping the protein within the interstitial spaces of cross-linked water-insoluble polymer gels. Alginate, which is a polysaccharide found in nature, is used extensively as biomaterial support due to its mild gelling and non-toxicity properties [5].In the previous study, a novel cell surface display system based on the CS3 pili with surface-exposed domains was developed which is accumulated high concentrations of Cd2+ and Ni2+. It is clear that there is a direct relation between the number of active and stale engineered pili and their metal adsorption. This study has been carried out to investigate the possible entrapment of the nanobiofibers in Ca-alginate gel beads and their capability for adsorption of Cd2+, for the first time.