Surface Display of Bacterial Metallothioneins and a Chitin Binding Domain on Escherichia coli Increase Cadmium Adsorption and Cell Immobilization

To increase the level of adsorption of cadmium ions to the surface of Escherichia coli, we fused cyanobacterial metallothioneins, SmtA (from Synechococcus elongatus PCC 3601) and MtnA (from Synechococcus vulcanus) to the E. coli cell surface using a Lpp′-OmpA-based display system. E. coli strains expressing Lpp′-OmpA–SmtA-linker-ChBD (chitin-binding domain from Bacillus pumillus SG2 chitinase S; chiS) and Lpp′-OmpA–MtnA-linker-ChBD on their surface adsorbed more cadmium compared to the E. coli cells expressing only the Lpp′-OmpA-linker-ChBD hybrid. These constructs also were bound to chitin through their chitin-binding domain, allowing them to be immobilized on a chitin matrix. We assessed surface presentation of Lpp′-OmpA–SmtA-linker-ChBD, Lpp′-OmpA–MtnA-linker-ChBD, and Lpp′-OmpA-linker-ChBD using immunostaining. The Lpp′-OmpA–SmtA-linker-ChBD chimera adsorbed metal and was bound to chitin with the highest efficiency compared to the other chimeras, suggesting that it is an effective bioadsorbent. This is the first example of coupling metal adsorption with cell immobilization using a whole-cell bioadsorbent.